Exotic Diseases

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Species Infected

primarily of sheep, occasionally goats

• one outbreak in Australia, 1951; New Zealand 1950's and 1970's; both were in imported sheep
• eradication successful

Clinical Signs

• incubation period 6 months to 5 years
• clinical course 2 weeks to 6 months
• initial signs are transient and sporadic
• become hyperexcitable and charge dogs/gates
• high stepping gait/ataxic
• fine tremor, convulsions when handled
• when rubbed over loins, licking, chewing movement is elicited
• after development of pruritis animals rub, bite skin and fleece
• appetite not impaired and little loss of condition till terminal
• difficulty in swallowing, easily fatigued, severe incoordination, sternal recumbency terminally
• morbidity up to 40%; 100% mortality

Lesions

• apart from skin lesions, no other gross lesions
• histopathology: characteristic bilateral spongiform change apparent as neuronal vacuolation and consequent Wallerian degeneration

Pathogenesis

• may have proliferation in lymphoid tissues
• severity of CNS signs are related to duration of infection and are not dose related

Aetiology

• infectious agent but debate exists whether it is a virus
• was considered to be a "viroid"
• another hypothesis is that it is a "prion", an infectious protein not associated with nucleic acid
• infection and replication may involve a complex interaction with a normal cellular protein and its gene
• no immune response stimulated

Epidemiology

• maternal transmission predominant method
• if prenatally not known whether it is via gametes or occurs early in embryonic life
• placental membranes have high levels of agent and are a source of infection
• contact transmission from infected animals and contaminated pasture
• agent remains viable in environment for several years
• evidence suggests that before extraneural infection can occur the animal needs to be immunologically mature
• special features of scrapie include:
• long incubation period, therefore dams produce infected lambs long before onset of clinical signs
• susceptibility is inherited

Differential Diagnoses

     1.  itchmite
     2.  lice
     3.  sheep scab
     4.  grass seed infestation
     5.  maedi or visna
     6.  pseudorabies and other viral encephalidites
     7.  certain neurotoxins
     8.  pregnancy toxaemia
     9.  listeriosis
     10.  certain degenerative CNS diseases reported in Australia including Murrurundi disease and Yass ataxia