Exotic Diseases

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Species Infected

affects sheep, but asymptomatic or mild infections of cattle and goats occur

Clinical Signs

• severity varies with virus strain and sheep breed
• incubation 4-7 days
• febrile period for about 1 week
• 24-48 hrs after onset of fever
• nasal discharge
• salivation
• hyperaemia of nasal buccal mucosa
• discharge may become mucopurulent and blood stained
• next few days
• oedema of lips, tongue, face, ears, intermandibular space
• hyperaemia may become more intense with petechiae developing
• cyanosis of tongue may occur in a few cases
• 5-8 days following onset of fever
• necrotic ulcers of cheeks, tongue, gums
• heal slowly with a diphtheritic membrane
• respiration may become laboured, dysentery may occur
• when mouth lesions begin to heal a coronitis may occur (at end of febrile period)
• loss of condition, muscle wastage
• torticollis may develop as a late sign
• death any stage up to a month post-infection or prolonged convalescence
• mortality 5-20%; some virus strains 70%
• abortions, foetal death or lambs born small with/without malformations

Lesions

• oral, nasal, labial ulceration
• coronitis, hoof may slough
• oedema of lips, face, tongue, etc.
• oedema, hyperaemia of various other tissues
• haemorrhage in tunica media at base of pulmonary artery is considered characteristic
• subendocardial haemorrhage
• catarrhal enteritis with petechiae in mucosa
• muscular petechiae, necrosis

Aetiology

• Orbivirus (Reoviridae)
• arthropod borne
• Culicoides spp. midges are essential for virus transmission
• at least 23 serotypes, none cross protective
• marked variations in virulence not related to serotype
• virus of sheep closely related to Ibaraki, epizootic haemorrhagic disease of deer and African horsesickness viruses
• at least 8 Australian serotypes independently evolved from African viruses
• can cause disease in sheep experimentally

Pathogenesis

• replication in local draining lymphoid tissue
• viraemia
• localisation in endothelium with development of microthrombi secondary to endothelial damage
• oedema, haemorrhage and epithelial necrosis
• cattle
• oedema due to type 1 immediate hypersensitivity to virus

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Epidemiology

• a number of serotypes present in Australia but field disease not seen
• disease patterns reflect behaviour of Culicoides spp.
• insect has preference for cattle and therefore cattle act as reservoir and amplifying host
• if sheep present with cattle
• rarely infected but remove cattle sheep become infected as midges bite second preference hosts
• midges bite at dusk/dawn, when low wind speeds present
• large midge populations are necessary for disease to spread
• factors such as temperature, humidity, rainfall also involved distribution and prevalence of midge and hence disease
• wind can blow insect up to 700 km

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Differential Diagnoses

     1.  vesicular diseases
     2.  sheep pox
     3.  contagious ecthyma
     4.  peste des petits ruminants
     5.  Rift Valley fever
     6.  photosensitisation
     7.  acute haemonchosis