Sheep Health & Production

Fleece rot | Dermatophilosis (Dermo, Lumpy wool) | Fly strike (Cutaneous myiasis) | Psorergates ovis (itchmite) | Chorioptes bovis | Other external parasites of sheep | Recommended reading

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Fleece rot

This is a superficial dermatitis caused by prolonged wetting of the skin and the multiplication of bacteria, particularly Pseudomonas aeruginosa. Many of the organisms produce a pigment, typically bright green but also blue, orange, brown or pink. 'Weather stain' is a yellow discolouration of wool caused by bacterial products and pigment.

Some of the colouration of the wool is non-scourable. Consequently, the wool is lowered in value. More important economically is the fact that fleece rot is a major predisposing condition for fly strike. In one recent study, fleece rot was estimated to cost approximately $1.00 per hogget in moderately affected flocks. Approximately half of this cost was associated with fly strike control measures and half with discounts applied to coloured wool.

Fleece rot is endemic predominantly in the high and medium rainfall areas of Australia, particularly in spring, autumn and following rains in summer. In low rainfall areas fleece rot can be a serious problem following prolonged rain because it predisposes sheep to the devastating effects of fly strike, the management of which is often difficult in pastoral areas. Within flocks, the highest incidence of fleece rot is generally in young sheep.

Prolonged wetting - over several days - and warm temperatures are necessary for the disease to commence. P aeruginosa multiplies rapidly in moist, well-aerated fleeces and within three days is able to establish itself as the dominant if not sole species of bacteria present. It grows profusely on the fleece constituents, wax, suint and insoluble nitrogenous material and can hydrolyze wool wax which normally forms a hydrophobic layer on the wool fibres and skin surface.

Association with wool length

Sheep are more susceptible to fleece rot when they have 4 to 6 months wool than at any other time off-shears[1] (Figure 10.1) and this may have significant influence on some management decisions. For example, shearing lambs in spring (tipping) can increase their susceptibility to fleece rot in the following autumn. In flocks where fleece rot is a problem or where time of shearing is under review, the association must be considered. Fleece rot occurrence, however, is just one of several factors which influence the determination of shearing time and is not necessarily one of the most important. (See Chapter 3.)

Figure 10.1 The effect of wool length on the susceptibility of sheep to flece rot

Resistance to fleece rot

There is genetic variation in resistance to fleece rot both within and between flocks. Between strains of Merino, strong wools are more susceptible than fine wools. Within strains, some bloodlines are more susceptible than others. Within flocks there is genetic variation in susceptibility; the heritability of fleece rot incidence and severity score is of the order of 0.2 to 0.4, the underlying liability to fleece rot has a heritability near 0.4.[2]

The variability in expression of fleece rot between seasons and between different environments has encouraged the search for indirect measures of resistance. Of these, fibre diameter variability and greasy wool colour are the best[3]. For greasy wool colour to be a useful indirect selection criterion, it is necessary for it to be measured objectively with a colorimeter rather than subjectively[4]. Although the use of fleece characteristics (like colour, 'character' and 'handle') are potentially useful as indirect selection criteria for fleece rot resistance, the gains from performing any selection in commercial flocks which buy rams from an external source are very small and genetic improvement in the trait is more profitably left to the stud ram breeder[5].

Prevention of fleece rot

The following steps can be considered to reduce the incidence of fleece rot within a flock.

• Selection of strains and bloodlines which perform best in the particular environment
• Selection of shearing time
• Minimizing the yarding of wet sheep
• Vaccination may be available within a few years[6]

Dermatophilosis (Dermo, Lumpy wool)

This is a dermatitis caused by Dermatophilus congolensis which affects sheep, particularly young sheep, after an episode of wetting and delayed drying. The predominant clinical sign is the formation of a hard scab in the wool. It causes economic loss to producers through deaths, lowered production, lowered wool values[7][8], treatment costs and restriction of management options. The prevalence of the disease is very high in some areas of Australia. A survey in Western Australia indicated a flock prevalence over 60% and a prevalence within flocks of 0% to 75% - with hoggets being the most frequently affected. The disease is more common on strong wool properties in medium and high rainfall areas but can occur in medium and even fine wool flocks[9]. It is, with fleece rot, one of the very important predisposing causes of fly strike[10].

Investigations into the heritability of resistance to dermatophilosis are continuing. At this stage there are no useful indirect selection criteria to facilitate breeding for resistance in Merino sheep.

Close contact following episodes of wetting such as dipping[11][12], jetting, transport or yarding (eg for marking) in wet weather are the most effective methods of spreading the disease from infected to uninfected sheep. Zoospores are harboured in lesions on affected or 'carrier' sheep - in the latter the lesions may be confined to the ears and face. The addition of some bacteriostats (for example, zinc sulphate at 0.5%) to dips will reduce the transmission of dermatophilosis in dips but will have no curative effect on existing lesions.

The disease can be treated by the administration of penicillin/streptomycin mixtures (70mg of each per kg will cure approximately 95% of cases). Approximately 8 weeks after treatment sufficient wool grows under the scabs ('lifts' the scabs) to allow a shearing comb to shear the sheep and remove the scabs. Treatment should be restricted to the most severe cases. Most mild or moderately affected sheep will cure themselves and develop some resistance to the disease - a process which may be delayed by antibiotic treatment.

Fly strike (Cutaneous myiasis)

Primary flies are those which can initiate a strike, secondary flies are those which normally do not initiate but can extend a struck area. Lucilia cuprina is the most important primary fly. Strikes also occur by Lucilia sericata and Calliphora spp. Secondary flies include Chrysomyia spp and Sarcophaga spp. Each year there is normally a double wave of primary flies, their prevalence rising to a peak in spring, declining in summer and rising again in autumn. Secondary flies are most active in summer[13].

Lucilia cuprina is attracted to sheep by an inherent attractiveness of sheep to the fly and, additionally, the odour of putrefaction which accompanies the moist exudate from fleece rot or dermatophilosis lesions. Moisture and a bacterial skin infection are essential for a successful strike. All flies which cause myiasis of sheep can and do breed in carrion.

There are 5 main forms of fly strike :

• Breech strike. The major predispositions to breech strike are urine soiling of the skin and wool below the vulva of ewes, excess skin wrinkle in ewes and diarrhoea in sheep of either sex but which is more common in young sheep. Two common sheep management operations reduce the prevalence of this form of strike from disastrous proportions to a more tolerable level. These are crutching, which shortens the wool which collects urine and faeces, and mulesing which removes some woolled skin from the edge of the perineum and extends the width of the bare area. Mulesing is of particular importance because it confers lifelong protection to breech strike. The length at which the tail is amputated is very important in conferring susceptibility to breech strike.
• Body strike. Strike anywhere other than poll, pizzle or breech and most common around the withers and shoulders. The sheep are usually rendered attractive and susceptible to flies by fleece rot or dermatophilosis.
• Poll strike. Usually confined to rams and resulting from the accumulation of 'sweat' at the base of the horns or from wounds suffered from fighting.
• Pizzle strike. Wethers or rams with belly wool stained by urine or by discharges from posthitis, particularly if the wool is matted with burrs, are susceptible to strike in the area around the preputial orifice.
• Wound strike. This can occur in infected wounds - such as discharging abscesses and, less often, in clean wounds such as shearing wounds. Mulesing wounds of lambs are sufficiently susceptible in early spring to warrant protective treatment of winter or spring born lambs.

Prevention of body strike

Jetting (strictly, hand jetting) is the application of insecticidal chemical to sheep through a wand with approximately 6 nozzles which are combed through the wool of the dorsum of the head, neck, back and rump under sufficiently high pressure to wet the sheep to the skin. Insecticides can also be applied through automatic races, fire-hoses or plunge or shower dips. The former two are quicker but less effective than hand jetting, the latter two are effective but more expensive because more chemical is applied per sheep than is necessary to prevent strike.

Insecticides used

• Organophosphates (OP's). These chemicals are widely used for lice control and were used extensively for fly strike prevention before resistance to them became widespread. They can now be used for short-term (2 to 3 weeks) protection against fly-strike and are an effective treatment for strike. In addition to diazinon, coumaphos and propetamphos which are used for lice treatment, chlorphenvinphos is also used as a jetting fluid and hand dressing treatment for flystrike.
• Synthetic pyrethroids. These chemicals are widely used for lice treatment and some are registered for use against flystrike. They act by preventing oviposition.
• Insect growth regulators (IGRs). Cyromazine (Vetrazin, Ciba Australia and Jetcon, Nufarm Animal Health) is registered only for fly control in sheep but diflubenzuron (Fleececare, Hoechst Agrivet and Strike, Coopers Animal Health) is registered for lice eradication and flystrike prevention. Cyromazine, diflubenzuron and ivermectin, are the three most effective chemicals for fly strike protection conferring up to 14 weeks protection in sheep with three or more months of wool when applied by hand jetting. The IGRs are not larvicidal, their action is to interrupt moulting, so they are slightly less efficacious as a treatment for active strikes than OP's. The IGRs have extended withholding periods before shearing.

Bovicola (Damalinia) ovis

The biting louse of sheep is distributed widely throughout Australia. The flock prevalence in NSW and, probably, in Vic and SA is approximately 30% to 40%. Approximately 60% to 70% of flocks are, therefore, completely free of lice at any one time but do not necessarily remain so for long periods, due to the unwitting introduction of sheep infested with lice. A recent survey in WA estimated that 27% of flocks become infected with lice each year and 35% of properties with lice at any one time will fail to eradicate lice within the next 12 months[14].

B ovis is an obligate parasite of sheep. (It can reproduce on goats which can, therefore, remain as vectors for lice from sheep to sheep.) B ovis cannot survive off sheep for more than a few hours unless protected from temperature extremes and light. In shorn wool in sheds it can survive for up to 5 days. Female lice lay eggs singly at the rate of approximately 1 every two to three days. Eggs hatch after 10 days. There are 3 nymphal stages and adult females commence egg-laying 24 to 25 days after hatching. In suitable conditions lice survive 30 to 60 days on sheep, and possibly some survive up to 5 months.

Transmission of lice occurs principally by direct contact - when lousy sheep are introduced to an uninfested mob or when sheep are mixed in yards. Transmission can occur if uninfested sheep follow an infested mob through yards at a very short interval. Lice numbers build up relatively slowly on newly infested sheep and the pattern of population increase depends on the time of year the introduction occurs.

Seasonal pattern of lice numbers

Solar radiation, temperature and rainfall all have a profound effect on lice numbers. Shearing has the most dramatic effect in that many lice are physically removed with the wool and the exposure of lice to climatic conditions leads to the death of many more. Lice numbers may decline by up to 90% of lice following shearing.

Solar radiation affects skin and wool temperatures. Wool tip temperatures can be as high as 75EC in direct sun on hot days while skin surface temperatures are 40E to 45EC. Temperatures over 42.5EC reduce oviposition and at temperatures over 45EC some adults and nymphs die - and do so at increasing rates with higher temperatures and more sustained high temperatures. Lice will distribute themselves along the wool fibre and around the body to find a suitable temperature. In freshly shorn sheep, the ventral neck may provide shelter, particularly if there are tufts of long wool missed in shearing.

Saturation of the fleece by heavy and prolonged rain also kills many lice. Eggs exposed to a relative humidity over 90% fail to hatch.

There is a general pattern of increase in lice numbers from late autumn to early winter and decline over summer but this pattern is influenced strongly by the timing of shearing. Long wool can provide a relatively stable micro-climate in either winter or summer. Thus sheep shorn in late spring/summer will remain with very low lice numbers until winter; sheep shorn in autumn may have lice numbers increasing throughout summer.

Cost of lice infestation

The presence of moderate and heavy lice infestations causes a reduction in wool production (by one estimate) of 0.3 to 0.8kg of clean wool weight. In addition to lowered productivity, the infested wool has a lower sale value due to lowered yield and yellow discolouration. Lice control costs include the expense involved in maintaining freedom from infestation and treatment of infested sheep. Treatment costs include the cost of chemicals, the cost of owning and maintaining plunge and shower dips and the losses of sheep and production of sheep as a result of dipping.

Clinical signs

B ovis is a biting louse and feeds on the stratum corneum as well as scurf, sebaceous secretions and skin bacteria. Irritation of the sheep is probably derived from the biting activities of the lice and possibly some immune based sensitivity. Infested sheep rub against objects like fences and fence posts and bite and chew their fleeces. The fleeces therefore appear deranged and have a 'pulled' and ragged appearance, particularly the areas on the sides behind the shoulder which they can reach with their mouths.

Diagnosis

The diagnosis of lice infestation is usually carried out under one of 2 circumstances. Sometimes it is necessary to determine if lice are present to determine the cause of fleece derangement. On other occasions, fleece derangement may be absent but a diagnosis of lice infestation or freedom from lice is desired so that the need for off-shears treatment can be determined. In the latter case, a diagnosis at the time of shearing is preferred, so that the most effective forms of treatment can be given soon after shearing.

The definitive diagnosis is based on the presence of lice. Sheep should be examined for lice in a good light and the fleece parted to the skin in a number of areas of the upper flank, mid-flank and under the neck. Standard counting techniques include a systematic parting of the wool along the flanks (20 partings per side, and the fingers are used to create an exposed line of skin 10 cm long). The number of lice seen per parting are averaged and the level of infestation categorised according to the values in Table 10.1. Light infestations are hard to detect and it may be necessary to examine several sheep thoroughly to find any lice at all. It is virtually impossible to declare a group of sheep free of lice with one inspection.

Table 10.1 : Relationship between average lice counts and degree of infestation

A lice detection test (LDT) is offered to commercial wool growers by the Australian Wool Testing Authority (AWTA) for a small fee. To perform the test, wool samples which are collected from bales of wool for fibre diameter and vegetable matter testing are also examined for the presence of louse exoskeletons. The sensitivity of the LDT has been estimated to be only 30% when conducted on one line of wool but increased to 75% when conducted on an average of 5.5 lines of wool from each flock[15]. The specificity, however, is also less than 100% owing to the possible cross-contamination of samples in the laboratory[16], and decreases by about 1% for each line tested. A significant drawback with the LDT is the time between shearing and test reporting. Core testing of wool is often not performed until some weeks after shearing has commenced.

A lamp test, which relies on the negative phototaxis and negative thermotaxis of lice in wool samples, and a table locks test have been compared to pre-shearing fleece parting as flock tests for lice which can be performed at the time of shearing[17]. Fleece parting of 1 side of 10 sheep had a sensitivity of 58%. Lamp tests on 10 fleeces, read after 10 minutes, was 63% but the test accuracy is sensitive to the temperature obtained under the lamp. The table locks test had a sensitivity of 87% but involved a greater degree of experience than the other two tests.

The presence of fleece derangement does not imply necessarily the presence of lice. Other important causes of fleece derangement include itchmite and grass seed irritation primarily and, to a lesser extent, dermatophilosis and fly-strike.

Treatment of lice infestation

The principal ectoparasiticides used to treat lice infestations in sheep fall into 3 categories based on their chemical structures. These are SPs, OPs and IGRs. One other chemical in use but not included here is magnesium fluosilicate.

Synthetic pyrethroids (SPs)

There are 4 SPs in current usage. These are deltamethrin, alphamethrin, cyhalothrin and cypermethrin. They are available as short wool 'pour-on' or 'back-line' treatments for sheep immediately off­shears, long-wool pour-on treatments for sheep with up to 10 months wool and as dip chemicals for plunge or shower dipping or jetting.

Organophosphates (OPs)

There are 3 OPs in current usage for lice. These are diazinon, coumaphos and propetamphos. They are all sold as saturation dip chemicals except for 2 products ­ Blaze, (Coopers) which includes diazinon with cypermethrin as a pour­on treatment, and Eureka Gold OP Spray-On (Young's Animal Health) which contains diazinon as the only lousicidal ingredient. This last product is an off-shears treatment requiring a special applicator and only registered for use in sheep over 23 kg.

Insect growth regulators (IGRs)

There are two IGRs registered for lice treatment of sheep ­ diflubenzuron (FleeceCare, Hoechst Agrivet) for use in saturation dips, and diflumuron (Zapp, Bayer Australia) which is a back-line off-shears treatment[18]. IGRs work by adversely affecting the maturation of the juvenile stages of the insect life cycle, consequently live lice may be detected in decreasing numbers for up to 12 weeks after treatment. For both products, the label warns that a lice infestation could be transfered from treated, infested sheep (eg pregnant ewes) to untreated sheep (eg their lambs) for up to 14 weeks after treatment. A recent study[19] has shown that adult lice persisting after treatment are either unable to transfer to untreated sheep or, if they do transfer, do not lead to patent infections. Long quarantine procedures after treatment, therefore, may not be warranted.

SPs as short wool pour-ons

A very high concentration of SP is applied along the back, from neck to rump. The chemical translocates around the body, probably dissolved in wool grease, and achieves a gradient of concentration over the skin with the lowest concentration furthest from the site of application. The position of the line is important in the diffusion of chemical around the body. All short wool treatments must be applied within 24 hours of shearing.

SPs as long wool pour-ons

Two products are available to treat sheep with 1 to 9 or 10 months wool ­ Vanquish (alphamethrin, Pfizer) and Blaze (Coopers); the latter containing cypermethrin plus diazinon. Very high doses of insecticide are applied in a strip along the back.

Dip chemicals applied by jetting

All of the dip chemicals can be applied through jetting races or by hand jetting. These techniques, particularly jetting races, will only achieve a reduction in lice numbers, not eradication. Cyhalothrin (Grenade, Coopers) has achieved eradication by hand-jetting 2 to 4 litres of solution along the back at high pressure (>600 kPa) but such results are difficult to achieve reliably in the field.

Dip chemicals applied by plunge and shower dips

Total saturation of the sheep has, theoretically, the best chance of applying lethal concentrations of the insecticide to all parts of the sheep. Nevertheless, failures to eradicate lice are still common. Compared to off-shears pour-ons, dipping is time-consuming, involves additional musters and predisposes to a number of diseases. Dipping should be performed after shearing cuts have healed (7 days) but before wool has grown long enough to impede wetting and delay drying.

Failures to eradicate

When applied correctly to sheep infested with susceptible strains of lice, all the SP pour-ons will kill all the lice on the sheep. Failures to do so are generally caused by :

• incorrect dosage
• incorrect application (not central stripe or too short a stripe)
• treatment given too late off-shears
• some sheep omitted or lambs born soon after treatment
• excessively long wool left after shearing, particularly under neck
• some sheep affected by dermatophilosis
• resistance of the lice to the SP

Similarly, shower or plunge dipping is highly effective unless :

• dip concentration falls too low, usually through 'stripping'
• too short a time in the dip
• incorrect shower pressure (should exceed 450 kPa)
• badly set-up or maintained nozzles in the dip

Stripping is the selective removal of active insecticide from the dipping solution. Dipping fluid is recycled from draining pens (where the sheep stand on release from the dip) back into the sump. The solubility of dipping chemicals in wool grease leads to a depletion of concentration of the active ingredient as more sheep are dipped. OPs strip more readily than SPs. Mixing dip chemicals in cloudy water or dam water containing clay particles does not reduce the effectiveness of dip solutions provided the dip sump is thoroughly stirred at least once per hour and provided the clay-insecticide complex is not selectively filtered out of suspension by the sheep's fleeces[20]. The addition of zinc sulphate to such dip suspensions may however, reduce their effectiveness.

Synthetic pyrethroid resistance

A significant number of field strains of lice are now resistant to SPs. Resistance is measured in vitro by the ratio of the LC₅₀ of the test strain to that of a fully susceptible known strain of lice, often the Peak Hill strain. (These strains are usually named to the town or area where the sample of lice was collected.) While a number of strains have been identified with LC₅₀ resistance factors up to 20[21], some have been recently identified with LC₅₀ resistance factors of 98 (the Hartley strain)[22] and 684[23]. Pour-on treatments are likely to fail to eradicate lice with resistance factors greater than 4[24]; saturation dips are likely to fail against lice with resistance factors of 20 or greater[25] and there are documented reports of failure[26]. A recent survey of 30 lice-infested sheep flocks in a high rainfall district of SA (Kangaroo Island) found resistance factors (LC₅₀ relative to a local susceptible strain, Kersbrook) exceeding 4 in approximately half of the flocks[27]. Resistance may not occur so frequently in other sheep growing districts but it highlights the hazard of recommending pour-on SP treatment without knowledge of the susceptibility of the lice involved in an outbreak. A field test for lice SP-resistance has been described⁽¹⁹⁾ and, by one estimate, it will predict susceptibility correctly in 75% of cases⁽²⁵⁾. A more accurate laboratory bio-assay is used experimentally but is not commercially available.

Treatment strategies

In almost all circumstances, the objective of a lice treatment programme is the eradication of lice from the flock. There are occasions, however, when control (with perhaps 95% lice kill) will be a satisfactory goal - usually when sheep have long wool and eradication is planned for an off-shears treatment. Assuming eradication is the goal, the strategies most likely to achieve it are :

• shower or plunge dip with an SP, OP or IGR
• all sheep treated 2 to 4 weeks off-shears
• all sheep are shorn together; this is clearly not always possible and strict isolation of shorn and unshorn mobs becomes essential

Once lice are eradicated, maintaining freedom from lice becomes the new objective.

Psorergates ovis (itchmite)

P ovis is an obligate parasite of sheep and all stages of the life cycle are completed on the host. The mites infest sheep of all ages; the size of the population generally increases with the age of the host. Young sheep are usually relatively free, but can develop heavy infestations after weaning, particularly if under nutritional stress. Peak numbers of mites on infested sheep generally occur in late winter/spring and are lowest in late summer. Rainfall does not have a pronounced effect on mite numbers. Spring shearing has little effect on mite numbers because mites are still reproducing actively and continue to do so until the hot weather arrives. The plane of nutrition of the host may also influence mite numbers - sheep with poor nutrition having more mites, more scurf and more fleece damage.

Adult mites mainly occur beneath the surface of the outer stratum corneum. It appears they feed on epidermal lipid, probably suctorially. It is unlikely that mechanical stimuli are the source of irritation, more likely that a mite product is a sensitising antigen for an immune mediated hypersensitivity response[28]. Itchmites are associated with a thickening of the stratum corneum and increased scurf - which is apparently a host response to itchmite infestation.

Transmission

Close contact, particularly at shearing, facilitates spread between sheep. Generally, spread between sheep is slow. Vertical spread occurs irrespective of wool length but may be facilitated by shearing ewes just after or just before lambing.

Prevalence

Itchmite infestation is a common cause of fleece derangement of sheep. In one NSW survey of 41 flocks, selected on the basis of the presence of sheep with deranged fleeces, itchmite infestation was considered the sole cause in 26 flocks (63%). The number of sheep with derangement associated with itchmite in each flock was, however, low[29]. This is a characteristic of itchmite infestation - they are present in many flocks, but the incidence of fleece derangement and the proportion of sheep with moderate or high (and therefore detectable) infestations, are both low. Another NSW survey of randomly selected flocks estimated the prevalence of infested sheep to be 5%[30], but it is not clear if that figure is low because some flocks are completely free of itchmite. It is clear from other studies[31] that the sensitivity of the diagnostic test for itchmite is significantly less than 100%. The other main causes of fleece derangement, B ovis and grass seeds, attain higher prevalences of affected sheep within flocks, as recorded by both deranged fleeces and detectable infestations, but in fewer flocks.

The cost of itchmite infestation

Itchmite infested sheep may produce less wool than uninfested sheep. The main cause of economic loss, however, is the lower value of wool due to fleece damage caused by rubbing. On a flock basis, the cost of the disease depends on the prevalence of serious fleece derangement.

Clinical signs

These range from a small area of the fleece which has a bleached appearance on the tips, to a generally ragged and tangled fleece with strings of wool hanging from the sides. The wool may be cotted and may have yellow discolouration. Sheep rub their fleeces particularly but also bite and chew their wool. The degree of fleece derangement appears to bear little relationship with mite numbers but the degree of scurfiness varies directly with mite numbers. The presence of both fleece derangement and excess scurf is associated with the detectable presence of itchmite in over 60% of sheep⁽²⁷⁾.

Diagnosis

When fleece derangement is the presenting sign, B ovis, grass seed infestation, dermatophilosis, fleece rot and a wool break should be considered as potential causative agents as well as itchmite. All of these can be detected, if present, by careful examination of several sheep with deranged fleeces. Scurf is the most consistent grossly visible evidence of itchmite infestation. Definitive diagnosis requires the positive identification of mites in skin scrapings but the presence of mites is not necessarily proof that they are causing the derangement. The presence of fleece derangement, excess scurf and mites and the absence of other causes is strongly suggestive that itchmite is at least partly responsible for the fleece derangement.

Skin scraping

Johnson (1990) describes the technique as :

Prepare 2 sites on the side between a line from the point of the ileum to the top of the shoulder and a line joining the bare skin areas of fore and hind limbs. The two sites are clipped with electric clippers with a size 40 blade. 2 ml of Shell Ondina medicinal oil is applied to the skin which is then scraped with a blunt, fixed blade scalpel until there is an erythematous reaction but no bleeding. Oil and debris are collected into vials and examined or deep frozen within 4 hours. The scrapings are examined at 25x magnification.

The test is highly specific but the sensitivity varies with the mite density. Mite densities in summer may be very low and they vary between sheep at any time of the year. At least 20 sheep should be examined before any estimate of the flock prevalence is made.

Treatment and control

A number of chemicals can be used for control of itchmite in sheep. Mites are most susceptible to acaricides when their reproductive activity is at its peak - usually between winter and spring. Where treatments are used in shower or plunge dips (all of those shown in Table 10.2 except ivermectin), treatment should be given within 4 weeks of shearing. Itchmite control after this time is much less effective.

Table 10.2 : Efficacy of acaricides for sheep itchmite[32]

Chorioptes bovis

C bovis causes an allergic, exudative dermatitis and is seen most frequently on the scrotum of rams. Rams are more heavily infested than ewes but ewes do become infested and transmit the mites to their lambs during suckling. In heavy infestations on rams there is a thick, yellow crust and skin thickening which may interfere with temperature regulation of the testes and cause some testicular degeneration. Treatments used to reduce the severity of infection include amitraz and ivermectin.

Other external parasites of sheep

Linognathus pedalis - the foot louse

A sucking louse which lives predominantly in the hairy parts of the legs, infestations with L pedalis are often unnoticed unless they are very heavy, in which case they appear as dark patches on the legs, belly, scrotum or crutch. Light infestations have little, if any, effect on sheep but heavy infestations cause stamping and biting of the affected areas. The lice are controlled by treatment in shower or plunge dips but re-treatment is necessary after 2 weeks to kill lice which have hatched since treatment. Diazinon, coumaphos and synthetic pyrethroids[33] are effective against adult stages, but must be applied by direct contact, not through pour-on backline treatments. L pedalis can survive off sheep for 13 to 18 days so it is necessary to depopulate the paddock of origin from the time of the first treatment until several days after the second treatment if eradication is to be achieved[34].

Linognathus ovillus - the face louse

These lice live in the hairy wool of the face - often near the wool-hair junction. Treatment is the same as for L pedalis but these lice can only survive 4 days off the sheep. Both the foot louse and the face louse have been seen more frequently since organophosphates have been used less frequently for fly and body lice control - cyromazine and synthetic pyrethroid pour-ons are ineffective against sucking lice. Ivermectin and closantel[35] have some activity against sucking lice.

Melophagus ovinus - the sheep ked

M ovinus is a wingless hypoboscid fly, 4 to 7 mm long. Keds are blood suckers and cause irritation, resulting in rubbing and biting. The sheep ked has declined in flock prevalence to very low numbers following the introduction of modern insecticides. It is killed by OPs, SPs including backliners and ivermectin and is now rare.

Ticks

Ticks which infest sheep include Ixodes holocyclus, Boophilus microplus (the cattle tick) and Haemaphysalis longicornis (the bush tick).

Mites

Psoroptes ovis is the cause of sheep scab which no longer occurs in Australia.

Tarsonomid mites occasionally infest sheep and cause mild irritation.

Table 10.3 : Products effective against lice

Table 10.4 Products registered for blowfly only

Recommended reading

Edwards JR (1988) Ovine dermatophilosis In Sheep Health and Production, University of Sydney Post-graduate Committee in Veterinary Science, Proceedings No 110, p 383

Raadsma HW (1988) Flystrike In Sheep Health and Production, University of Sydney Post-graduate Committee in Veterinary Science, Proceedings No 110, p 317

Johnson PW (1990) Ectoparasites In Shee Medicine, University of Sydney Post-graduate Committee in Veterinary Science, Proceedings No 141, p 157

Wilkinson FC (1988) Sheep Lice and Itchmite In Sheep Health and Production, University of Sydney Post-graduate Committee in Veterinary Science, Proceedings No 110, p 19

[1] Raadsma HW (1988) Flystrike In Sheep Health and Production, University of Sydney Post-graduate Committee in Veterinary Science, Proceedings No 110, p 317

[2] McGuirk BJ and Atkins KD (1984) Fleece rot in Merino sheep I. The heritability of fleece rot in unselected flocks of medium-wool Peppin Merinos Aust J Agric Res 35 p 423

[3] McGuirk BJ and Watts JE (1983) Associations between fleece, skin and body characteristics of sheep and susceptibility to fleece rot and body strike. In 2nd national symposium, The sheep blowfly and flystrike in sheep. p 367

[4] Raadsma HW and Wilkinson BR (1990) Fleece rot and body strike in Merino sheep IV. Experimental evaluation of traits related to greasy wool colour for indirect selection against fleece rot Aust J Agric Res 41 p 139

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