Sheep Health & Production

Arthritis | Viral diseases associated with lameness | Minor bacterial dermatoses | Major bacterial dermatoses | Footrot | Recommended reading

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Arthritis

A non-suppurative polyarthritis caused by Erysipelothrix rhusiopathiae, is often seen in recently marked lambs. The marking and mulesing wounds probably provide a portal of entry and spread to joints occurs haematogenously. Chlamydia psittaci also causes a non-suppurative arthritis in lambs and weaners. The route of infection is not known although it is possibly oral.

Many infected lambs recover spontaneously from infection, possibly after an illness lasting a few days which is rarely observed. A proportion of lambs, perhaps as high as 20% of those infected, develop chronic arthritis and are culled or killed because of their poor performance and inability to graze. Arthritis is usually diagnosed post mortem at abattoirs and in one recent study E rhusiopathiae was found to be present in approximately 50% of infected joints.

Methods of prevention may include hygienic precautions at marking and the use of a vaccine against E rhusiopathiae (prepared for pigs). The success of these strategies has not been proven.

Treatment

Treatment is only likely to be successful if commenced early - before chronic joint changes occur. C psittaci is susceptible to tetracyclines, E rhusiopathiae to penicillin.

Suppurative arthritides can be caused by a number of organisms, including Fusobacterium necrophorum, Actinomyces pyogenes, Histophilus ovis, Staphylococcus spp and Streptococcus spp. In very wet years, post-mulesing infections with Fusobacterium necrophorum result in a high prevalence of arthritis in some flocks.

Viral diseases associated with lameness

Contagious pustular dermatitis (CPD, contagious ecthyma, scabby mouth)

CPD usually causes lesions on the lips, adjoining skin, muzzle and oral mucosa. Occasionally, similar lesions occur on the legs, around the coronary band and palmar surface of pastern and the interdigital skin (IDS), particularly between the bulbs of the heel. Lesions can also extend to the tarsal or carpal areas and be accompanied by a painful cellulitis and secondary infection[1].

Foot and mouth disease (FMD)

The disease is characterised by vesicles in the mouth and on the feet and teats, although oral lesions are not prominent in sheep. Feet lesions commonly occur on the coronet, interdigital skin (IDS) and the bulbs of the heel. FMD foot lesions can resemble footrot, particularly if there is secondary bacterial infection. Lameness is severe, the morbidity is high.

Bluetongue

During the initial stages of infection with the virus of bluetongue there is hyperaemia of the mucous membranes of the mouth and of the skin of the feet around the coronet. Coronitis is severe and prominent and linear haemorrhages may be visible in the hooves. Lameness is severe.

Osteodystrophies

Osteodystrophies include all diseases in which there are failures of normal bone development or normal bone maintenance. Clinically, there may be distortion, predisposition to fracture or disturbance of gait or posture.

Nutritional deficiencies

Calcium, phosphorus and vitamin D are the major nutritional components responsible for normal bone development. Deficiency of copper is also an important and common cause of osteoporosis and susceptibility to bone fractures of lambs and weaners in some areas. Deficiency of dietary protein can also lead to osteoporosis in young sheep.

Vitamin D deficiency can lead to ill-thrift, lameness, bone fragility, rickets and recumbency in lambs and weaners. It is a significant cause of lameness and rickets in spring-born lambs grazing green oats in winter. It is also associated with hypocalcaemia and osteoporosis in autumn-born lambs weaned onto green oats in winter[2][3]. The probable presence of a rachitogenic property of green oats plus (often) the absence of sun- and sky-shine in winter contribute to the vitamin D deficiency. A useful diagnostic test of osteoporosis in young sheep is to press with the thumbs on the frontal bones. In affected animals the bones will fracture (harmlessly) with little pressure.

Mineral excess

Excessive dietary intake of calcium, fluorine and chronic lead poisoning can lead to a variety of osteodystrophic conditions.

Plant poisoning

Bentleg (or Bowie), with extreme outward bowing of the forelimbs, occurs as a result of ingestion of Trachymene spp (Wild Parsnip).

Minor bacterial dermatoses

Post-dipping lameness

This condition is caused by an infection with Erysipelothrix rhusiopathiae. The disease follows plunge or shower dipping in a dip fluid contaminated with faeces and other organic material in sheep which have skin wounds. Multiple skin punctures caused by grass seeds in the fleece are one of the worst predisposing factors. If the wounds are not given sufficient time to heal after shearing, infection enters through the skin abrasions and causes a cellulitis which extends to a laminitis.

Clinical signs

There is usually a sudden onset two to seven days post-dipping with a high morbidity. Affected animals are depressed and usually persistently febrile for some days. The sheep are observed to be grazing less than expected and slow to rise and move off when disturbed. Weight loss, or failure to recover weight following shearing, is marked. Lameness occurs in one to four legs. The affected legs are hot and slightly swollen. There is not usually joint involvement. A history of recent dipping is important in establishing a diagnosis.

Prevention

Prevention involves the correct use of a bacteriostat in the dip, a reduction in faecal contamination of dip solutions and, most importantly, delaying dipping until shearing wounds heal (at least seven days post-shearing). Dip bacteriostats cannot be relied upon to prevent post-dipping lameness in sheep dipped soon after shearing. Bacteriostats are most effective when added to dip solutions when a day’s dipping is finished, in order to exert a bacteriostatic or bactericidal effect in the dip solution overnight.

Treatment

Penicillin is effective. Most untreated cases recover but may lose considerable bodyweight before recovering fully if not treated.

Strawberry footrot

This disease is a proliferative dermatitis of the lower limbs associated with the production of scabs and open bleeding areas (strawberries) when the scabs are removed.

The causative agent is Dermatophilus congolensis (which also causes lumpy wool). The lesions resemble those caused by CPD virus and many cases of ’Strawberry footrot’ diagnosed in Australia may in fact be CPD.

Major bacterial dermatoses

Five further bacterial dermatoses cause significant concern to sheep producers. These are ovine interdigital dermatitis (OID), benign footrot (BFR), virulent footrot (VFR), foot abscess and toe abscess (see Table 13.1).

Of these, VFR, foot abscess and toe abscess are economically important and have very adverse effects on the welfare of affected animals.

Toe abscess

Toe abscess is an acute, purulent infection usually involving only one digit of a sheep. Unlike foot abscess, the infection is confined to the sensitive laminae of the hoof, usually in the toe region. The infection probably enters through cracks in the hoof. Lameness is acute and severe but responds quickly to drainage.

Table 13.1 : Major bacterial dermatoses of sheep

Foot abscess

Foot abscess is an acute, often purulent infection usually involving one digit of the foot. In most cases infection begins in the interdigital space and extends into the deeper structures of the digit to involve the distal interphalangeal joint, associated ligaments and tendons.

Clinical signs

The predominant sign is acute lameness, often to the extent of complete disuse of the affected foot and extended periods of recumbency. Once infection becomes established in the joint some permanent damage is inevitable. Initially there is marked oedema and inflammation of the interdigital area with necrosis in some areas. Sinus tracts, which are continuous with the joint and the IDS, form along the abaxial coronary border. The discharge becomes increasingly purulent and less necrotic as the disease progresses. In some cases the axial collateral ligaments rupture. Usually, the digit heals within 8 weeks to a stage where no infection remains, but some permanent deformity and disability usually remain.

Economic importance

The prevalence is generally low and seldom rises above 10%. Much of the importance of the disease arises because of the severity of lameness and because the disease primarily affects rams and ewes close to lambing. Ewes with foot abscess in late pregnancy are predisposed to pregnancy toxaemia. Rams with foot abscess during joining will have reduced serving capacity.

Predisposing factors

Some prior damage to the IDS is necessary to allow the bacterial invasion of the subcutaneous tissues. Experimentally, the disease has been reproduced by housing the sheep with moist dirty conditions underfoot following a treatment to cause damage to the IDS. Naturally occurring predisposing factors include stones and prickly or abrasive vegetable matter. The disease is most common in heavy ewes in wet conditions, particularly if the ewes are driven on roads or in stony areas.

Footrot

Footrot is a contagious dermatitis of sheep and goats caused by an infection with the bacterium Dichelobacter nodosus in association with other bacteria and under particular environmental conditions. Three forms of the disease are recognised; benign footrot (BFR), intermediate footrot (IFR) and virulent footrot (VFR) although the differences between them are not distinct. Virulent footrot, the most severe form of the disease, causes large economic losses in sheep flocks in the medium and high rainfall zones of Australia, losses which provide a strong incentive for flock owners to eradicate the disease from their properties. Veterinarians in practice, with a firm basis in the scientific understanding of disease generally and footrot in particular, and understanding the complexities of sheep management, are well equipped to develop and apply footrot plans for individual producers.

Necessary causes of footrot

Footrot is an infectious disease but no identifiable agent fulfils all of Koch’s postulates. Under farm conditions, a particular sequence of conditions must occur before the foot is predisposed to infection:-

• exposure of the feet to wet pasture
• hydration and hyperkeratosis of the stratum corneum of the IDS
• bacterial growth in the IDS
• invading bacteria, including aerobes and Fusobacterium necrophorum
• development of an interdigital dermatitis

Complication of the environmental and bacterial dermatitis by D nodosus results in a necrotising infection of the interdigital skin (IDS). New infections of footrot always start as infections of the IDS. If the strain of D nodosus is sufficiently virulent, and the host sufficiently susceptible, the interdigital lesion will extend to become the severe and chronic infection of the sensitive laminae underlying the horn of the hoof which is characteristic of footrot. The cells which generate horn are destroyed and the hoof separates from its underlying structures. This separation is termed underrunning[4]. The development of the footrot lesion, influenced by host, environmental and bacterial factors, is termed the expression of the disease.

Thus, the development of new cases of virulent footrot requires :-

• exposure to wet pastures and faeces - the source of F necrophorum
• environmental temperatures which favour bacterial growth
• introduction of virulent strain or strains of D nodosus

Footrot lesion scoring system

The advance of a footrot infection in a sheep’s foot is a consequence of a number of factors, which are discussed further below. Because the amount to which the infection has advanced, or its severity, are used in diagnostic procedures, some simple repeatable scoring systems have been devised to give a semi-objective measure of the severity of footrot lesions, such as that shown in Table 13.2. Other more complex systems subdivide score 3 lesions and define the most severe, chronic lesions as score 5.

Table 13.2 : Classification of footrot lesion severity

The essential organism

D nodosus (formerly Bacteroides nodosus) is effectively a parasite in that its only habitat is an infected foot. It does not survive in the environment for more than 7 days; a few hours is probably more usual[5][6]. The organism is a Gram-negative anaerobe. Under a light microscope it appears as a rod with polar caps, areas at each end which stain more intensely than the remainder of the cell, a characteristic most pronounced in organisms taken from footrot lesions rather than cultures[7]. Electron microscopy has revealed the presence of filamentous appendages or pili (also called fimbriae) which are known to carry the important antigens on which sero-classification is based and which stimulate effective immunity after vaccination[8][9].

In culture, D nodosus is strictly anaerobic and is usually grown in an atmosphere containing 10% hydrogen and 10% carbon dioxide. The organism is fastidious in growth requirements on media, requiring either 10% horse serum[5] or ground hoof material[10] in agar (hoof agar), or trypticase, arginine and serine (TAS agar) as additives to agar or broth[11].

Concentrations of agar above 3% restrict the size and spreading of bacterial colonies other than D nodosus[12]. Consequently 4% agar is currently recommended for primary isolation of the organism from lesion material, and 2% agar is used for sub-cultures[13]. The organism grows best at 37EC and, at that temperature, colonies appear on agar plates in four to six days.

Two serogrouping systems exist. One describes 10 serogroups (Table 13.3), some of which are divided into subtypes making a total of 19 serotypes altogether[14][15][16]. A system described in UK[17][18] classifies the organism into 17 serogroups (A to R with no I) with no sub-types. At least some of the serotypes in the Australian system, particularly B₂, B₃ and B₄, are classified as distinct serogroups in the alternate system.

Table 13.3 Serogroups and serotypes of D nodosus currently recognised in Australia

Serogrouping is particularly important in vaccine production because cross-protection across serogroups is limited. In addition, the existence of a relatively simple classification system based on serological reactions has led to serogrouping being used as an epidemiological marker to identify infecting strains of D nodosus across time within flocks or between flocks. Its use, however, is limited because there are only relatively few (10) serogroups and because there is evidence that the serogroup of a strain can change, through genetic recombination, in the course of an outbreak if a mixed-serogroup infection is present[19][20].

The presence of mixed-serogroup infections in sheep flocks is probably normal[3][21][15][22]. Up to six serogroups[23] and nine serotypes have been reported in any one flock and it is noted that the more sheep that are sampled within a flock, the greater the diversity of serogroups.

Environmental factors

Long-term exposure to moisture and a particular temperature range are required to initiate new cases of footrot. For example, for an outbreak of footrot to occur in spring, a sustained and reasonably consistent amount of rainfall, averaging about 50 mm or more per month, must fall over the preceding four months(2). Then, as mean daily temperatures start to consistently exceed 10EC, spread from infected sheep to uninfected sheep will commence. Typically, in districts suitable for footrot, these conditions occur in late August to early October, but not usually in every year[24]. Further, conditions suitable for sustained spread and, therefore, an outbreak of footrot, will only be maintained for three to four weeks unless there is continuing rain.

Environmental temperatures consistently below a mean daily temperature of 10EC, such as occurs in winter, will prevent transmission of infection between sheep. High temperatures do not directly inhibit transmission but lead to drying of pastures and, therefore, the feet of the sheep. While footrot lesions, particularly underunning ones, may persist in some sheep as conditions become drier, spread from sheep to sheep will cease.

For transmission of footrot to occur, pasture is a key environmental factor. In the absence of rain, provided the sheep are predisposed by long periods of prior exposure to wet pastures and conditions are cool, heavy dew can keep pastures wet all day and provide conditions suitable for spread. This is more likely to occur if pastures are long and dense[25], and clover has a reputation for encouraging footrot spread, probably by providing a wet environment at the base of the sward even when rain is relatively infrequent.

Outbreaks occur less commonly in late summer and autumn and only in years when the average monthly rainfall for the four or five months preceding the outbreak exceeds 70 mm per month and exceeds 75 mm in the month of the outbreak. Footrot does not spread following isolated periods of heavy rain or irrigation of pastures during a hot, dry period.

Warm moist conditions which with high levels of soil moisture accumulated over months or rainfall and low evaporation rates favour both pasture growth and footrot spread, if D nodosus is present. Thus footrot outbreaks are typically seen in ’good’ springs, usually in August, September and October in winter rainfall areas and one to two months later in summer rainfall districts. Outbreaks at other times of the year are uncommon but will occur if seasonal conditions are suitable. In flocks where footrot has not been previously recognised, diagnosis may not be made until late in an outbreak or even until transmission has ceased and most of the sheep affected have relatively advanced and chronic infections.

Host effects

Merino sheep are more susceptible to footrot than British breeds and their crosses. Variation in susceptibility is expressed as variation in severity and duration of lesions. There is also genetic variation in susceptibility within breeds of sheep and some sheep fail to respond to treatment, whether treatments are topical, parenteral or immunological. The variation in host response to infection, combined with the variability associated with environmental influences, means that diagnosis of footrot, particularly classification into BFR, IFR or VFR, requires examination of a large sample of an infected flock.

Relapses of infection

Relapses of infection from the sub-clinical state probably occur and may require less environmental moisture than is required for the development of new infections. The expression of the disease in an infected foot is less influenced by environmental conditions than is transmission[26]. Development of an infection, proceeding from a sub-clinical state to a clinical interdigital lesion or from a limited interdigital lesion to one involving underrunning, can occur with relatively minor changes in environmental conditions favouring multiplication of the organism, even if the conditions are not suitable for transmission of the disease to occur.

Alternate hosts

Cattle, goats and deer are suitable alternate hosts for D nodosus. Goats suffer severely from footrot. Some D nodosus isolates which appear to be virulent for goats are virulent for sheep, others are benign. Until we have any information to the contrary, deer should be considered potential reservoirs of infection for sheep. D nodosus occurs naturally in the feet of cattle and is associated with interdigital lesions and probably contributes to the severity of those lesions[27][28][29][30]. The interdigital lesions vary from mild, superficial erosions to chronic hyperkeratotic lesions with thickened folds of skin in the interdigital space and deep, cracked fissures. Rarely, the horn is underrun[31][32][33]. Lameness is uncommon. The bacterial flora of the disease in cattle appears very similar to that of ovine footrot, with spirochaetes and F necrophorum.

The strains of D nodosus which infect cattle feet appear to be the same as those which infect sheep although, generally, strains isolated from cattle will only cause benign footrot in sheep. Indeed, in 1 investigation, it appeared that cattle and sheep on the same farm were infected with different strains[34]. This is, however, unlikely to be always so and cattle isolates which cause BFR in sheep are known to be transferable from cattle to sheep both artificially and under natural field conditions[35][36].

One attempt to transfer a virulent sheep strain (originally isolated from sheep) to cattle has been unsuccessful. There is one report of a cattle isolate producing severe lesions following artificial transfer[27] and 2 reports of properties on which cattle appeared to be a reservoir of infection of ovine VFR and led to the re-infection of a newly purchased, uninfected flock after a flock with VFR had been removed from each property[37][38]. In these 2 cases it was clear that the infection in cattle was not transient but persistent and reasonably difficult to eradicate from the cattle herd despite topical and systemic therapy. It is likely that most strains of D nodosus which occur naturally in cattle will only cause BFR in sheep but this is not always the case. VFR has been eradicated from many properties despite the continuing presence of cattle so it seems reasonable to conclude that cattle are not often a risk to eradication programs for ovine footrot. Nevertheless, there is a risk of VFR being introduced with cattle or persisting in cattle during an eradication program. Cattle should be considered a small but significant threat to management or exclusion of VFR in sheep flocks.

Agent factors - variation in virulence

There are many genetic variants of D nodosus and consequently a spectrum of virulence amongst isolates of the organism. The virulent strains are those at the top end of a spectrum of virulence and it is these that cause a disease of sufficient economic impact to warrant intervention and eradication. It must be appreciated, however, that within infected flocks there is normally a mixture of D nodosus isolates present. This mixture will probably contain strains which vary in serotype and in innate virulence.

Virulent footrot is defined as that disease which results in underrunning of the hard horn (score 4) of the hoof of more than 10% of sheep that are exposed to infection[39]. This level of severity will cause enough lameness and reduction in feed intake to measurably affect productivity.

Benign footrot is that form of the disease in which infections are restricted to the IDS in all or nearly all affected sheep. Some sheep in a flock, however, may be particularly susceptible to infection. As a general rule, it is accepted that fewer than 1% of the flock will develop score 4 lesions with benign strains of D nodosus.

Intermediate footrot typically causes 1% to 10% of an exposed flock to develop score 4 lesions[40][41][42]. With the exception of a small proportion of sheep, underrun lesions are qualitatively less severe than those of virulent footrot, with less necrosis evident. Overall, intermediate footrot is a milder disease than virulent footrot on a flock basis and all but the few severely affected sheep recover spontaneously when climatic conditions become dry. Score 3 lesions occur at a low prevalence in benign footrot, but may occur at a high prevalence in intermediate footrot. Fewer than 5%[43] or 10%[44] of sheep are expected to have score 3 or score 4 lesions in benign footrot, while 25% of sheep may have score 3 lesions with intermediate footrot

Laboratory tests are available to distinguish between isolates of different virulence but these tests are not readily available - nor generally necessary for diagnosis. The diagnosis of footrot, both clinically and bacteriologically, should be based on the examination of a representative sample of the flock.

There is no relationship between virulence and serogroup.

Differential diagnosis of D nodosus infections in flocks

Traditionally in veterinary medicine, diagnosis of disease has been based on recognition of a characteristic clinical case of the disease in question. In managing footrot this approach is far less applicable because of the occurrence naturally of a number of different expressions of the disease. Benign, intermediate and virulent outbreaks can be identified and at present these outbreaks are believed to be due to genetically stable variants of D nodosus. Owners generally, and some veterinarians find it difficult to comprehend the concept of a spectrum of expression of disease severity.

Responsible diagnosis of footrot as a flock disease must be based on an accurate assessment of the severity of disease in each flock because the form of the disease present will determine the extent of intervention which is justified. Benign infections may not warrant any intervention whereas the more severe forms certainly cause production loss and justify programmes designed either for control or eradication.

Responsible flock diagnosis entails making a quantitative assessment of the prevalence of the disease in the flock under investigation and the prevalence of severe infections. This quantitative assessment requires a count of the number of sheep affected in an adequate sample of the flock and recording the severity of the disease in those sheep.

The first sign of footrot in a flock is usually lameness but there are many other causes of lameness in sheep. In addition, footrot may occur concurrently with those other diseases. This possibility reinforces the need to examine representative samples of affected flocks.

The virulence classification of the disease should be based on the prevalence of sheep with score 4 lesions relative to the number of sheep exposed to footrot.

The epidemiology of footrot

In an infected flock, footrot prevalence shows a strongly seasonal variation (Figures 13.1 and 13.2). While environmental conditions favour the transmission of the disease from sheep to sheep the prevalence rises until most of the susceptible sheep are affected. As environmental conditions become drier the prevalence declines as some sheep recover from infection and become free of the disease. Generally, within any flock of sheep, there are some individuals who remain infected throughout the following non-transmission period and these sheep are a source of re-infection for the flock in the next transmission period.

In footrot endemic areas, seasonal conditions do not necessarily allow transmission in every year but there are generally some chronically infected sheep which remain so for very long periods and will be a source of reinfection in the next favourable season. If conditions remain unfavourable for several years, the disease may disappear from the flock completely. Thus, sheep from low rainfall areas of the country are unlikely to have footrot. On the other hand, in high rainfall districts which have relatively short hot, dry periods, footrot can become endemic and difficult to control or eradicate.

For VFR, the chronically infected sheep which maintain the flock infection over non-transmission periods have advanced and chronic underrunning lesions of the hard horn of the toe or wall of the hoof. In some cases, these lesions are obvious and cause deformity of the foot, severe lameness and may be fly-struck. In other cases, the lesions may be restricted to small pockets of infection in the horn of the hoof with only small amounts of deformity.

For BFR, infection is not maintained in underrun horn because few, if any, sheep develop underrunning with BFR. Consequently, it is believed that infection persists in sub-clinical infections of the IDS.

For IFR, sheep with chronically underrun infections do occur, although there are relatively few of them compared to VFR. IFR can persist in a flock through a non-transmission period in chronic lesions in the horn of the hoof and, possibly, in sub-clinical lesions of the IDS like benign strains are believed to do.

The sheep in which infections persist are the most susceptible sheep in the flock. The most resistant sheep in the flock are generally the last to become infected, develop the mildest lesions and heal spontaneously soon after conditions suitable for transmission and lesion expression have passed.

Figure 13.1 Climatic data for Tarcutta, on the south west slopes of NSW, in 1993 and 1994. The anticipated transmission period, predicted from monthly rainfall and daily mean temperature data, is indicated. Mean daily temperature (the average of daily maximum and daily minimum) consistently exceeded 10ºC from 24 August 1993.

Figure 13.2 Prevalence of footrot in a 400 sheep flock at Tarcutta, NSW, subject to the climate shown in Figure 13.1. There was a low prevalence of footrot in the flock during winter (less than 5%) following an outbreak in the previous year. Transmission commenced in late August or early September and rapidly spread to most of the flock such that 80% had footrot by mid-December when hot, dry climatic conditions led to a decline in prevalence. The following season was a drought and footrot did not spread

Laboratory aids to diagnosis

A number of laboratory-assessed characteristics of D nodosus isolates have been associated with virulence. These include colony morphology, twitching motility[45][46][47][48] and the presence and nature of extracellular proteases. It is the latter characteristic, first described in 1962[49], which has received the most attention for its perceived ability to predict, in vitro, the in vivo virulence characteristics of strains of D nodosus[50]. The tests that have been developed to measure protease characteristics include the proteolytic index[451], the degrading proteinase test[52] and its derivatives, the elastase test[53], the zymogram test[54][55] and the protease ELISA[56].

The degrading proteinase test was based on the tendency of the proteinases from virulent strains of D nodosus to remain stable during incubation in culture at 37EC, while the enzymes produced by benign isolates were less likely to do so. It later became clear[57] that the differences in stability of the proteases from virulent and benign strains were detectable more quickly at higher temperatures and the stability also varied with the concentration of calcium ions. Subsequently, the test incorporating these developments became known as the protease thermostability test[34][58][35].

The protease thermostability test was further enhanced[59] by substituting gelatin for hide powder azure as the substrate used to detect protease activity. Gelatin, compared to hide powder azure, was cheaper, soluble and more easily standardised between batches. The gelatin-gel protease thermostability test is now frequently referred to as the ’gelatin-gel’ test and the results are reported as stable (S) proteases, inferring virulence, or unstable (U) proteases, inferring benign characteristics.

The elastase test uses a solid culture medium containing elastin to compare the elastase activity of strains of D nodosus isolated from cases of virulent or benign footrot. Elastase-positive isolates produced a clearing of elastin particles in six to seven days of culture or slightly longer in some cases. Elastase-negative isolates produced no clearing within 21 or, in some cases, 28 days. There is generally close agreement between the elastase test result and the degrading proteinase test of the same isolates, and a strong agreement (but less than 100%) between the elastase test result and the reported clinical virulence of the outbreak from which each strain is isolated[41][42]. At the time when these tests were developed intermediate strains were not recognised[60]. Subsequently, it was shown that the elastase test could also distinguish at least some intermediate strains which showed rates of elastin-clearing between those of virulent and benign strains[61].

The zymogram test distinguishes between strains of D nodosus on the basis of the patterns produced by electrophoresis of their extracellular protease enzymes. Virulent strains produce some bands which benign strains do not produce, and vice versa[62]. The range of zymogram patterns was expanded to three for thermostable proteases (S1, S2 and S3) and six for unstable proteases (U1 to U6)[63].

The protease ELISA uses monoclonal antibodies against a virulent protease and a benign protease in an ELISA system on microtitre plates to demonstrate the presence or absence of the protease characteristic of virulent or benign strains.

A number of reports have compared the results from a range of laboratory tests of virulence. In general, there is good agreement between the tests, particularly in classification of benign or virulent strains. Agreement is less clear for isolates which are reportedly intermediate in virulence[64]. There are several problems associated with reliance on protease-based laboratory tests[39]. These include

• the absence of a gold-standard determination of virulence. Definitions of virulence differ between states and clinical expression of virulence can be modified by environmental conditions
• the isolates tested in a laboratory may not be from the dominant strain causing footrot in the field
• the absence of controlled studies of in vivo virulence in sufficiently large groups of sheep in most reported evaluations of protease-based tests
• a small, but significant level of disagreement between the results of the various tests used
• a small, but significant level of disagreement between test results and reported field virulence

Genetic tests for virulence

A region of the chromosome of D nodosus which occurs at a high frequency in virulent strains but at a low frequency in benign strains has been identified and named the vap region (virulence associated protein)[65][66]. The vap regions are repeated in the genome of many of the strains that have been examined and, within each region, there are a number of vap genes[67][68]. The function of any of the products of the vap genes has not been determined.

The virulence-related locus or vrl[69] is a DNA sequence also present in some strains of D nodosus. The presence of the vap region and/or the vrl is a reasonable predictor of virulence, where virulence is determined by clinical evidence, elastase activity, protease thermostability, zymogram pattern or colony morphology[70]. Using the presence or absence of these genomic regions, isolates of D nodosus can be placed into three major categories. Category 1 isolates contain both vap region and vrl, category 2 isolates contain only vap region and category 3 isolates contain neither loci. In one study, 88% isolates in category 1 were classified as virulent or high intermediate, 18% of category 2 isolates were classified as virulent but 70% were classified as intermediate. Of category 3 isolates, 83% were classified as benign or low intermediate. No isolates were detected with the vrl region only. Possibly, the presence of the vap region is essential either for the insertion or the maintenance of the vrl locus.

A technique using D nodosus genomic clones and dot-blot hybridisation[71][72] has been applied to panels of D nodosus isolates to differentiate strains of virulent, intermediate and benign virulence. The specificity of the virulent probe is less than 100% because 15% (3/20) of strains, determined to be benign by the elastase test, reacted with the virulent probe as well as the benign probe. The benign-specific probe reacted only with strains which had been characterised as benign or intermediate by elastase tests, demonstrating high specificity, but some intermediate strains did not hybridise with the benign-specific probe. The virulent probe is known to be derived from the vap region or vrl region[73].

Economic effects of footrot

Production losses associated with virulent footrot are due to :-

• reduced wool production
• reduced wool quality
• increased incidence of body strike
• reduced weaning percentages
• reduced value of sale sheep
• increased culling rates

It is likely that footrot interferes with the productivity of affected sheep by reducing food intake. Severely affected animals can be observed in flocks spending increased amounts of time recumbent and grazing while kneeling, rather than standing. The lowered wool production and bodyweight losses of footrot-affected sheep in pen trials where feed was available in troughs supports this view[74].

There is no doubt that the presence of uncontrolled infections with virulent footrot reduces the wool production of sheep[75]. It is likely that the estimate of 8% loss of annual wool weight by Marshall et al[76] is the best available estimate of the effect of virulent footrot on average annual wool production in an infected flock but there are two reasons why this estimate is probably an underestimate of the losses which occur in field outbreaks. First, the comparison in their experiment was made between treated and untreated sheep. Footrot did occur in the treated sheep and so one can presume that these sheep would have produced more wool had they been completely free of footrot. Second, welfare concerns led to treatment of the infected sheep on two occasions during the experiment, and the withdrawal of one animal which was severely affected. It can therefore be assumed that the ’untreated’ sheep would have produced even less had no treatment whatsoever been administered.

Marshall et al also found a significant relationship between the duration of severe lesions (score $3) and number of infected feet and fibre diameter, staple length and tensile strength. While the effect of footrot on fibre diameter may have a positive effect on wool price, lower fleece weights and lower staple strength may have a strong negative effect on fleece value.

Footrot-affected sheep are more likely to become flystruck. First, their infected feet themselves are very attractive to blow flies (L cuprina). Second, the necrotic material from the feet, often accompanied by maggots, can be deposited in the fleece when the animals are recumbent and the deposited material can initiate a body strike[56].

Virulent footrot also reduces the rate of bodyweight gain, increases the loss of bodyweight or leads to the maintenance of lower body weights in affected sheep compared to uninfected sheep[77][54][55][78][56]. Estimates of the effect of footrot on bodyweight range from a relative loss of 6.7% of bodyweight during the 8 week period of infection[54], maintenance of bodyweight at least 6 kg lower than that of uninfected adult Merino wethers during a 16 week period of infection[55], to a difference of 3.5% to 7% between infected and uninfected Downs breed-sired weaners[57]. Marshall et al[11] related the duration and severity of footrot lesions to the change in bodyweight and found that the more infected feet that a sheep had and the longer the period of active infection, the greater was the effect on bodyweight. They concluded that for each foot continuously affected with footrot (score $3) for 2 years, the bodyweight of Merino wethers which weighed 54 kg before infection would fall by 12.3 kg. In fact, most affected sheep have periods of remission from infection during the year when conditions are dry or unsuitable for footrot.

It is clear from these experimental studies that the greatest changes in bodyweight occur during periods when footrot is spreading within the flock and lesions are actively developing. At times when lesions are regressing or following treatment, previously affected sheep may regain some lost weight. It is possible for sheep to regain much of the lost bodyweight rapidly after curative treatment, with compensatory gain. On the other hand, lowered wool production over the period of active infection will not be fully recovered following treatment but, presumably, the rate of woolgrowth will return to normal about the same time that lesions heal. Thus the effect of virulent footrot on annual wool production will depend on the duration of the infection as well as the severity of lesions.

The effect of footrot on reproductive rates is a result of the effect of footrot on bodyweight and food intake. Lower body weights in ewes will lead to lower ovulation rates during joining and lowered lambing percentages, increased risk of pregnancy toxaemia, neonatal mortality and reduced weaning rates and weaning weights of lambs.

The effect of footrot on sale value of cull and cast-for-age sheep results from the restricted markets available to footrot-declared producers and the low body weights of affected sale sheep. Both lower reproductive rates and higher culling rates due to chronic footrot lesions reduce the amount of culling for productive traits that producers with affected flocks can practise.

Intangible social effects arise from :-

• concern about sheep welfare
• reputation of the property
• effects on relationships with neighbours

Estimated losses associated with the presence of virulent footrot on a property where no control measures are practised are between $6.75 and $14.35 per sheep per annum (1990 dollars), depending on the suitability of the environment for footrot transmission and development[79] and the impact of footrot on the market options for cull and cfa sheep. Where control measures are implemented, the costs are reduced to between $5.45 and $9.50[80].

The severe and chronic lameness that can occur in uncontrolled outbreaks of virulent footrot may at times lead to concerns on animal welfare grounds. There are times when such concerns justify treatment or slaughter regardless of any economic justification.

Infection with less virulent strains of D nodosus causes less severe effects on wool weight and bodyweight than virulent strains[58][55]. In a field study, Glynn[81] found that infection with a strain classified as intermediate in virulence led to a difference of 5% in greasy fleece weight between untreated and sheep treated by footbathing to reduce the severity and prevalence of footrot lesions. There are no published estimates of the effect of benign footrot on bodyweight. Glynn found that uncontrolled benign footrot decreased bodyweight of sheep in some parts of the year compared to sheep treated to reduce the effect of footrot, but that there were no significant differences at the end of the footrot spread season. Uncontrolled intermediate footrot did lead to significantly lower bodyweight than treated sheep, although the effect of the footrot was presumably exacerbated by grass seeds penetration of the interdigital skin. One report estimated the cost of uncontrolled intermediate footrot in a high risk environment was $3.90 per sheep per annum, $0.20 for benign footrot, compared with $14.35 for virulent footrot.

Treatment

Topical treatments

Topical treatments are used to achieve both cure of affected individual sheep and control of transmission. Curative treatment is generally attempted during a non-spread period, usually when pastures are dry, while control measures are implemented during times when transmission is expected. The solutions most commonly used are formalin (5%) and zinc sulphate (10%). Other products made specifically for footrot in sheep include a zinc sulphate/sodium lauryl sulphate solution (Footrite^R - Nufarm Animal Health)[82][83] Malecki and Coffey 1987) and a 10% solution of CHF-1020 (copper nitrate trihydrate/copper chloride dihydrate in water) (Radicate^R - Colbert Holdings)[84].

Traditionally, curative treatment has involved paring all affected feet and applying the antibacterial chemical in a footbath. The paring necessary to achieve good cure rates is labour-intensive and arduous and excessive paring causes additional pain and discomfort to affected sheep.

To control spread without prior paring, sheep are walked through a six metre long footbath containing either 5% formalin or 10% zinc sulphate at weekly intervals, or held in footbaths of Footrite^R for one hour at three-weekly intervals, during the times of the year when transmission is expected to occur. These treatments will reduce the incidence of new infections. With the possible exception of Footrite^R, underrun lesions do not respond well to topical treatments without prior paring to expose infections in underrun horn so bathing must be sufficiently frequent to intercept lesion development.

The following factors affect the success of topical treatments.

• good cure rates of underrun lesions require extensive paring
• formalin is not very effective unless sheep are placed in a dry environment after treatment
• Footrite^R requires foot-bathing for one hour to achieve better results than zinc sulphate alone

Injected antibiotics

One parenteral treatment with penicillin is effective in curing a high proportion of sheep affected with footrot, but the dose required is higher than that recommended by manufacturers for use in other disease conditions. Procaine penicillin at the rate of 70 000 units/kg has been used successfully[85][86] while rates of 50 000 units/kg or 300 000 units per sheep[87] have failed.

Streptomycin is ineffective alone but, in combination with procaine penicillin, has been shown in a number of studies to be highly effective and more effective than penicillin alone. It has been widely used in the field but streptomycin is no longer available for this purpose in Australia.

Long-acting oxytetracycline is effective at 20 to 24 mg/kg[88]. Intramuscular erythromycin has been found to be highly effective at 12[89] and 20 mg/kg[64]. Combinations of lincomycin-spectinomycin (at 5 and 10 mg/kg of each antibiotic respectively, given intramuscularly) are effective[90] but the drugs are not registered for sheep in all states of Australia. No significant improvement in efficacy is achieved by repeating treatment with lincomycin/spectinomycin on two following days, nor by using a three times higher dose.

Antibiotic treatment has advantages of topical applications for curative treatment. Extensive foot paring is not required so treatment is faster. None of the antibiotics, however, is effective unless sheep are in a dry environment for at least 24 hours post-injection. Dry environments may be provided by pasture in hot dry seasons or by placing sheep on battens (in a woolshed, for example) after treatment. Antibiotic treatment is rarely appropriate for control of the disease during a spread period because it is expensive and gives no significant protection against re-infection. It is much more widely used as a treatment during a non-transmission period. Cure rates exceeding 90% can be expected and will frequently be as high as 95%. Care must be exercised when treating animals which could potentially be soon culled for slaughter; withholding periods must be observed.

Vaccination

Natural infection with footrot produces a small antibody response in sheep which is not, however, protective and sheep can be repeatedly re-infected with homologous strains of D nodosus[91]. By contrast, sub-cutaneous injection of adjuvanted piliated whole cells of D nodosus will stimulate a strong protective response against homologous infection[92][93][94]. The protective, agglutinating immunogen in vaccines is associated with pili and vaccines of pure pili derived from D nodosus or from recombinant Pseudomonas aeruginosa will produce as good or better protection than whole cell vaccines[95][96][97][98].

There is very limited cross-protection between serogroups of D nodosus. In fact, within serogroups, particularly serogroup B, cross-protection between serotypes is also limited. Vaccination is, therefore, only effective against strains of D nodosus of the same serogroup (homologous challenge) as occurs in the vaccine.

As observed earlier, mixed-serogroup infections in natural footrot outbreaks are normal and it is not practical to attempt to identify which serogroups are present in a sheep flock experiencing an outbreak of footrot. Nor is it practical for manufacturers to market vaccines which offer protection only against specific serogroups - it would make vaccines too expensive and, should the wrong vaccine be used, perceived ’failure’ to protect sheep from footrot would bring vaccine into disrepute.

Consequently, vaccine manufacturers sell multivalent vaccine; for example, Footvax^R (Coopers Animal Health) contains 10 serotypes of D nodosus. Unfortunately, multivalent vaccines are much less effective than single strain vaccines, due to the phenomenon of antigenic competition.

Antigenic competition refers to the apparent competition for immune responses which occurs when multi-component vaccines are compared to any one component of the vaccine administered alone. Antigenic competition significantly reduces the response by sheep to vaccination with multistrain D nodosus vaccines, compared to the response to each component[99][100][101].

The titre of agglutinating antibody achieved following one vaccination is low and not sustained, so two doses of vaccine are necessary to achieve useful protection. For intervals up to one year, the longer the time period between the primary and secondary vaccination, the higher the titre achieved but the faster the titre declines after the second vaccination[102]. Whether the interval between vaccinations is 6 weeks or longer (up to one year), the titre 12 weeks after the second vaccination is similar.

Vaccines containing antigens to protect against eight to ten serogroups provide only a short duration of protection. In one report, two doses, nine weeks apart, of a multivalent (eight serogroup) vaccine protected Merino ewes for at least ten weeks during an outbreak of footrot[103]. A number of other reports suggest that multivalent vaccination can be expected to protect Merino sheep for up to 12 weeks after the second dose, although for some animals protection may be inadequate at eight weeks[104][105].

The use of multivalent vaccine in affected flocks has two important effects.

• Within a few weeks of the second vaccination, most unaffected sheep will be protected against footrot
• Sheep already affected will heal more quickly and the severity of disease in those remaining affected will, mostly, be reduced

There are some disadvantages of vaccination.

• At least 6 weeks must elapse between the first primary dose and the achievement of control. The timing of the first dose should take this into account so that the second dose is given shortly before transmission commences. Footbathing can be done to control the disease before immunity develops, if necessary
• Effective immunity lasts for at most 16 weeks; probably no more than 10 weeks in Merinos, 12 weeks in crossbred sheep
• Vaccine is expensive. Two vaccinations, plus labour, cost approximately $3.00
• There is often a reaction at the injection site

Despite these disadvantages, there are many conditions under which vaccination offers the simplest and cheapest effective control measure. It will not eradicate the disease, but will reduce prevalence by up to 80%.

Control of footrot in infected flocks

Control refers to strategies aimed at preventing or restricting the rate of transmission of footrot between sheep in an infected flock. Control measures, therefore, are normally applied immediately before or during a transmission period. In many Australian environments transmission occurs principally in spring and early summer although in some districts and in some seasons, conditions are suitable for transmission at other times of the year. For the sake of this discussion, however, it shall be assumed that transmission occurs in spring.

Broadly, two strategies are used for control and often they are used together. The first is footbathing, normally in 10% zinc sulphate solution or 5% formalin solution. Formalin is generally slightly cheaper; zinc sulphate is more pleasant to use and safer for sheep, dogs and operators. Alternative footbathing solutions have been mentioned above. The second strategy is vaccination.

When used to achieve control, footbathing does not require paring of the feet. Paring is not justified because control measures are aimed at limiting the establishment of infection - a process which occurs in the IDS and which therefore does not require reduction in the amount of horn tissue. Further, because of the tremendous increase in time required to pare feet, paring is not advisable.

Footbathing does not provide sustained protection against reinfection. If done effectively, footbathing will cure or ameliorate most early cases of footrot involving the IDS and superficial underrunning. Consequently, the bathing must be repeated frequently. It is not feasible, during a transmission period, to attempt to separate infected sheep from uninfected sheep because many cases will be inapparent or mild. There is perhaps a small advantage in attempting to provide a ’clean’ pasture to receive sheep after bathing, but successful control is not dependent on this and, in any event, because transmission will quickly resume on the new pasture from those infected sheep which do not respond to treatment.

Given these facts, the recommended procedures for footbathing during a transmission period in order to reduce transmission rates are to walk the sheep through a footbath every five days while transmission is occurring - perhaps 3 to 6 weeks under most circumstances. Bathing should commence as soon as transmission is likely to occur - perhaps in late August, September or early October depending on the season.

The foot bath should be at least 6 m long and the solution should be at least 50 mm deep. It is often preferable to have portable baths and to use them in portable yards, taking the facility to the sheep rather than droving sheep long-distances to central permanent baths, but the best approach will vary with each farm’s physical infrastructure.

Footrite^R, if used as recommended, may offer some advantages in cases where frequent footbathing is difficult. If sheep are held in a Footrite^R solution for one hour, superior penetration of the horn of the hoof will occur, providing better cure rates and longer protection periods and treatment frequencies may be reduced to once every two to three weeks.

The most obvious limitation for footbathing is for treatment of lambing and lactating ewes and their lambs, or on farms where labour is in short supply. Under these circumstances, vaccination may be the preferred option for control.

In sheep which have not previously been vaccinated against footrot, the first vaccination must be given at least seven weeks before the anticipated commencement of transmission. The second vaccination can then be given one week before transmission is expected to commence, and satisfactory protection can be expected for at least 10 weeks from a time about one week after the second vaccination.

Should the first vaccination be given too late, footbathing can be used to control transmission in the period preceding protection from the second vaccination.

Sheep which have been vaccinated in the previous year will need only a booster, given once, about a week before transmission is anticipated.

Should transmission continue for longer than the protective period (say, 10 weeks), a further vaccination may be necessary to prevent an increase in footrot prevalence late in the season.

Vaccination has several obvious management advantages over footbathing and, in addition, its ability to cure chronically-affected sheep without paring may be a distinct advantage over footbathing in cases where there are still significant numbers of sheep affected in this way - presumably following the previous season’s uncontrolled outbreak. The most obvious disadvantage of vaccination is its price but, for some farms, the cost of vaccination may be less than the cost of repeated footbathing.

Eradication of footrot

The process of control can be an end in itself but is often a part of the process of eradication of the disease from the flock and the farm. Eradication is based on the principle that elimination of cases of D nodosus infection eliminates the disease. The organism persists only in animal hosts, not in the environment.

Methods of eradication

Non-selective disposal and replacement and replacement of the flock means that every sheep on the farm or at risk of footrot is sold, and ’clean’ sheep purchased. The probability of success depends on a complete muster, the availability of footrot-free replacement sheep and the ability, through secure fencing, to prevent re-infection of the replacement sheep. Obviously, a period of at least one week must elapse between the departure of the last infected sheep and the arrival of the replacement flock.

This method of eradication is often the cheapest and most reliable, particularly when the owner is confident that infection is restricted to a small part of the flock, such as one recently purchased and isolated ’mob’. The ’change-over’ price is a critical cost in deciding the value of this approach. Sheep at risk of footrot must often be sold relatively cheaply while ’clean’ replacements may warrant a price premium.

By contrast, selective disposal requires the identification and sale or slaughter of only the infected animals from within a flock and the retention of sheep which, despite exposure to footrot, are clinically free of the disease. Clearly, this method requires the inspection of every foot of every sheep in the flock. Eradication based on selective disposal will be the preferred option in cases where the prevalence of footrot within the flock is low but when all or most of the flock is considered ’at-risk’.

The final steps in an eradication program based on inspection and selective disposal - the removal of the last infected sheep - are most likely to be successful during a non-transmission period. Both the financial outcome and the probability of eradication are enhanced if control measures applied during the previous transmission period have been effective. In that case, fewer sheep will be sold, reducing the ’change-over’ cost, and the odds of misdiagnosing an infected sheep as uninfected are lower.

A third method of eradication of footrot involves chemotherapy, which can be selective or can involve the whole flock. Selective treatment, like selective disposal, suffers from the risk of errors in identification of infected sheep (the sensitivity of the inspection).

Treatment is highly effective in dry, summer conditions (>90%). There is no necessity for more than light paring or for separation of ’clean’ and ’treated’ sheep. Treatment with parenteral antibiotics is not effective if sheep are returned to wet pastures. All sheep must be re-inspected 3 weeks after treatment. Non-responders should be culled.

General procedure for eradication

Experience has shown that eradication based on inspection and selective disposal, with or without chemotherapy, is more likely to succeed if

• at least two, and preferably three inspections of all sheep in the exposed flock are made during the non-transmission period, at intervals of 3 to 4 weeks
• the climate during the non-transmission period is hot and dry (usually summer), and the sheep’s feet are dry
• every infected sheep, or suspect sheep, is culled from the flock
• facilities for inspection are good and do not make the task difficult or excessively tiring for the operators; hence machines which invert the sheep, good lighting, good environment for operators (usually requires provision of shade and air movement) and pneumatic parers are all encouraged

At the first inspection, infected animals are culled or treated. At the second inspection, any affected animals are culled. This will include non-responders if treatment was used at the first inspection. At the third inspection, affected animals are culled although, hopefully, at this inspection, there will be no clinical evidence of footrot in most or all of the individual mobs.

Surveillance through the next period that pastures are moist following the inspection and disposal procedures is highly recommended. Surveillance generally involves careful inspection of each mob at rest at pasture and then the individual examination of any lame sheep. Surveillance potentially allows the removal of any sheep which ’breaks down’ with clinical footrot from an undetected focus of chronic infection in the foot before transmission occurs. Alternatively surveillance allows the isolation of any mob which ’breaks down’ and the prevention of transmission from that mob to other, uninfected mobs on the farm. If such breakdown occurs, control measures should be applied to the infected mob promptly pending a decision about the best eradication options in the next non-transmission period.

For many sheep farmers, eradication is not successful in the first year and it is important that the reasons for failure are determined before proceeding with another attempt to eradicate. In many cases failure occurs because the summer inspection and selective disposal activities are undertaken soon after footrot is diagnosed. Consequently, there is insufficient development of farm infrastructure to cope with the new activities (laneways, sheepyards, handlers, fence integrity) or inadequate control during the transmission period with, subsequently, a high prevalence of infection at the first inspection. Inevitably, these factors are better managed in the second year so the ’failure’ in the first year should be viewed as a preparatory step to final eradication rather than wasted effort.

Cost of eradication

In any eradication programme it is appropriate to present a realistic or even a maximum-possible list of all costs likely to be encountered. As a minimum list of costs, consider vaccine, antibiotic, mustering and handling time, veterinary consultative involvement, depreciation of equipment and low sale cost of cull sheep. It is unlikely that this cost will be less than $10.00 per head for a programme running in just one year.

Prevention of footrot

Managers of footrot-free flocks must take steps to prevent the introduction of footrot. Sound boundary fences are essential. Purchased stock can be inspected, preferably before delivery, and should be isolated from resident sheep until they have been through a transmission period without developing footrot.

It is important to consider the roles of goats particularly, but also cattle, in introducing footrot into sheep flocks.

Prevention of footrot should not, however, include control strategies if the disease is believed to be absent. Thus, footbathing or vaccination should not be unless footrot is known to be present. These strategies will not prevent the introduction of footrot from stray or purchased sheep but could delay its diagnosis. The cost of these activities is better directed at maintaining secure fencing, for example.

For most commercial producers, it is necessary to buy some sheep into the flock, even if these are only rams. To reduce the risk of introducing footrot with purchased sheep, it is important to ascertain the footrot-status of the vendor’s flock. Strategies such as footbathing purchased sheep will not prevent the introduction of the disease.

NSW Strategic Plan for eradication of footrot

In 1988, NSW Agriculture, The University of Sydney, private veterinarians and representative industry groups developed a Strategic Plan with the objective of reducing the prevalence of footrot-infected flocks to less than 1% in all Rural Land Protection Board (RLPB) areas by December 2000. Thus, all of NSW would be deemed a Protected Area for footrot.

Table 13.4 Definition of disease control zoning based on prevalence of affected flock

For the purposes of eradication in NSW, footrot has been defined as virulent and non-virulent. Under current legislation (Stock Diseases Act) owners are required to notify the presence of virulent footrot in their sheep in protected and control areas. Private veterinarians, therefore, should advise their clients accordingly. Footrot has been a notifiable disease throughout NSW since January 2000. It is illegal to travel flocks with VFR on public roads or to offer them for sale in public yards.

Several additional aids to footrot control have been introduced. These include vendor declaration forms - standard forms which enable owners of flocks free of VFR to make a formal declaration to that effect when selling sheep - and footrot free accreditation - a scheme for accreditation of flocks as free of VFR is a component of the strategic plan. Many studs and some commercial flocks have this status. Accreditation is based on a report submitted to NSW Agriculture by private veterinarians.

The division of footrot into two forms only (benign and virulent) ignores the continuity of the virulence scale. Increasingly, IFR is confusing the diagnosis of footrot in the field. The classification of isolates of D nodosus using the gelatin gel test into stable (S) and unstable (U) tends to include many intermediate strains as VFR (caused by S strains) despite field evidence that the footrot is mild and causing serious lesions only in a small proportion of the flock. While these sheep warrant treatment, it is doubtful if eradication can be justified on economic grounds. Further, there is doubt whether some forms of IFR can be eradicated using programs of inspection and selective disposal. It is likely that this is an issue which will require resolution over the next few years.

Figure 13.3 A schematic representation of the proposed relationship between the virulence of a form of footrot, the laboratory assessment of the strains of D nodosus isolated from the outbreak, the likelihood that eradication can be achieved with inspection and selective disposal methods and the economic justification of eradication of the disease. Points of overlap or distinction between characteristics are intended to be ill-defined, reflecting the lack of reliable information about the relationships between characteristics of the organism.

Recommended reading

Radostits OM, Blood DC and Gay CC (1994) Table 38. Differential diagnosis of diseases of the musculoskeletal system In Veterinary Medicine Edition VIII p 508

Egerton JR (1985) Control and eradication of ovine footrot In Footrot in Ruminants. DJ Stewart, JE Peterson, NM McKern, DL Emery eds. Proceedings of a Workshop, Melbourne 1985. CSIRO Division of Animal Health and Australian Wool Corporation, Melbourne, p 35

Various authors (1985) 5. Vaccination against ovine footrot as above, p 173

Venning CM, Curtis MA and Egerton JR (1990) Treatment of virulent footrot with lincomycin and spectinomycin Aust Vet J 67 p 258

West DM (1989) In Footrot and Foot Abscess of Ruminants Egerton JR, Yong WK and Riffkin GG (eds) CRC Press, Florida. p 57

[1] CPD is discussed in more detail in Chapter 17, Diseases of the Skin and Eye

[2] FMD and bluetongue are discussed in more detail in Chapter 16, Diseases of the alimentary system

[3] Caple IW (1990) Vitamin D deficiency In Sheep Medicine, University of Sydney Post-graduate Committee in Veterinary Science, Proceedings No 141, p 381

[4] Graham NPH and Egerton JR (1968) Pathogenesis of Ovine Footrot: The role of some environmental factors Aust Vet J 44 p 235

[5] Beveridge WIB (1938) Investigations on the viability of the contagium of footrot in sheep Journal of the Council for Scientific and Industrial Research 11:4-13

[6] Laing EA and Egerton JR (1981) Aspects of Bacteroides nodosus infection of the feet of cattle. In: Ovine footrot, A report of a Workshop at University of Sydney, May, 1981. pp 195-199

[7] Beveridge WIB (1941) Footrot in sheep: a transmissible disease due to infection with Fusiformis nodosus (n.sp.) Journal of the Council for Scientific and Industrial Research Bull no. 140

[8] Stewart DJ (1973) An electron microscopic study of Fusiformis nodosus Res Vet Sci 13 p132

[9] Stewart DJ and Egerton JR (1979) Studies on the ultrastructural morphology of Bacteroides nodosus Res Vet Sci 26 p227

[10] Thomas JH (1958) A simple medium for the isolation and cultivation of Fusiformis nodosus Australian Veterinary Journal 34:411

[11] Skerman TM (1975) Determination of some in vitro growth requirements of Bacteroides nodosus Journal of General Microbiology 87:107-119

[12] Thorley CM (1976) A simplified method for the isolation of Bacteroides nodosus from ovine footrot and studies on its colonial morphology and serology Journal of Applied Bacteriology 40:301-309

[13] Stewart DJ and Claxton PD (1993) Ovine footrot clinical diagnosis and bacteriology In: Corner LA and Bagust TJ (eds) Australian Standard Diagnostic Techniques for Animal Diseases. Standing committee on agriculture and resource management. Subcommittee on animal health laboratory standards. CSIRO, Parkville, Australia pp1-27

[14] Claxton PD, Ribeiro LA and Egerton JR (1983) Classification of Bacteroides nodosus by agglutination tests Aust Vet J 60 p 331

[15] Claxton PD (1986) Serogrouping of Bacteroides nodosus isolates In : Footrot in ruminants. Proceedings of a workshop, Melbourne 1985, Stewart DJ, Peterson JE, McKern NM and Emery DL (eds) CSIRO Division of Animal Health/Australian Wool Corporation, Glebe, NSW 131-134

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[17] Thorley CM and Day SEJ (1986) Serotyping survey of 1296 strains of Bacteroides nodosus isolated from sheep and cattle in Great Britain and western Europe In : Footrot in ruminants. Proceedings of a workshop, Melbourne 1985, Stewart DJ, Peterson JE, McKern NM and Emery DL (eds) CSIRO Division of Animal Health/Australian Wool Corporation, Glebe, NSW 135-142

[18] Day SEJ, Thorley CM and Beesley JE (1986) Serotyping of Bacteroides nodosus : a proposal for 9 further serotypes (J-R) and a study of the antigenic complexity of B nodosus pili In : Footrot in ruminants. Proceedings of a workshop, Melbourne 1985, Stewart DJ, Peterson JE, McKern NM and Emery DL (eds) CSIRO Division of Animal Health/Australian Wool Corporation, Glebe, NSW 147-159

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[23] Egerton JR (1983) Footrot control in drought Australian Veterinary Journal 60:315

[24] Egerton JR, Ribeiro LA, Kieran PJ and Thorley CM (1983) Onset and remission of ovine footrot Australian Veterinary Journal 60:334-336

[25] Cummins LJ, Thompson RL and Roycroft CR (1991) Production losses due to intermediate virulence footrot Proceedings Australian Sheep Veterinary Society Annual Conference, Darling Harbour, 1991, pp 106-108

[26] Abbott KA and Egerton JR (2001) The epidemiology of intermediate footrot 1. Aust Vet J submitted.

[27] Shenman G (1962) A case of possible F nodosus infection in a cow Aust Vet J 38 p306

[28] Gupta RB, Fincher MF and Bruner DW (1964) A study of the etiology of footrot in cattle Cornell Vet 54 p66

[29] Thorley CM, Calder HAMcC and Harrison WJ (1977) Recognition in Great Britain of Bacteroides nodosus in foot lesions in cattle Vet Rec 100 p387

[30] Richards RB, Depiazzi LJ, Edwards JR and Wilkinson FC (1980) Isolation and characterisation of Bacteroides nodosus from foot lesions of cattle in Western Australia Aust Vet J 56 p517-521

[31] Morgan IR (1969) A survey of cattle feet in Victoria for Fusiformis nodosus Aust Vet J 45 264

[32] Laing EA and Egerton JR (1978) The occurrence, prevalence and transmission of Bacteroides nodosus infection in cattle Res Vet Sci 24 p300-304

[33] Toussaint-Raven E and Cornelisse JL (1971) The specific contagious inflammation of the interdigital skin in cattle Vet Med Review 2-3/71 p223-247

[34] Egerton JR and Laing EA (1978) Characteristics of Bacteroides nodosus isolated from cattle Vet Micro 3 p269-279

[35] Alexander TM (1962) The differential diagnosis of footrot in sheep Aust Vet J 38 p366-367

[36] Wilkinson FC, Egerton JR and Dickson J (1970) Transmission of Fusiformis nodosus from cattle to sheep Aust Vet J 46 p382-384

[37] Mitchell RK, Moir DC, Bowden MS, Westwood DJ, Palmer MA, Pitman DR, Richards RB, Depiazzi LJ and Hill N (1992) Cattle transmit virulent footrot to sheep - new implications for eradication policy Proceedings of Australian Sheep Veterinary Society p49-53

[38] Trengove CL, Riley MJ and Saunders PE (1993) The role of cattle in the spread of ovine footrot Proceedings of Australian Sheep Veterinary Society p74-78

[39] Egerton JR (1989) Control and eradication of footrot at the farm level - the role of veterinarians. Proceedings Second International Congress for Sheep Veterinarians, Massey University, NZ pp 215-218

[40] Roycroft CR (1986) Managing benign strains and intermediate strains of footrot In: Sheep Health and Welfare, Department of Agriculture and Rural Affairs, Victoria Conference Proceedings series Number 8:153-156

[41] Dobson KJ (1986) The impact of Bacteroides nodosus strains of intermediate virulence on footrot control In: Footrot in ruminants. Proceedings of a workshop, Melbourne 1985, Stewart DJ, Peterson JE, McKern NM and Emery DL (eds) CSIRO Division of Animal Health/Australian Wool Corporation, Glebe, NSW pp 23-25

[42] Stewart DJ (1989) Footrot in sheep In: Footrot and foot abscess of ruminants Egerton JR, Yong WK, and Riffkin GG (eds) CRC Press, Boca Raton, Florida, USA p 11

[43] Allworth MB (1995) Investigations of the eradication of footrot PhD thesis, University of Sydney

[44] Allworth MB and Egerton JR (1999) An objective clinical assessment method for footrot diagnosis Proceedings Australian Sheep Veterinary Society Annual Conference, Hobart, 1999, pp 74-76

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