Exotic Diseases

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Species Infected

affects sheep

reported in goats

Clinical Signs

• incubation 2 years or more
• insidious onset
• early signs
• sheep lag behind when driven
• slightly aberrant gait when trot
• weight loss pronounced, without loss of appetite
• later signs
• stumbling, weakness of hindlimbs, knuckling of fetlock
• facial muscle trembling
• paresis and paraplegia
• no pruritis
• disease progresses up to 1 year
• most cases die - few recover
• prolonged course of debilitating disease - other diseases supervene, e.g. Pasteurellosis

Lesions

• none grossly except muscle atrophy
• demyelinating leucoencephalitis
• maedi lesions may be concurrent

Aetiology

• Lentivirus (Retroviridae) identical to the virus causing maedi
• reproduction cycle results in virus insertion in host genome and persistent infection
• little virus production but sufficient to enable infection to progress
• virus has ability to undergo antigenic drift

Pathogenesis

• concept of slow virus diseases evolved from study of visna
• pathogenesis not completely understood
• after infection a leucocyte-associated viraemia occurs and virus is distributed to a wide range of organs
• lungs, lymphoid, bone marrow, kidney, chorioid plexus
• replication appears to occur in the lymphocyte following infection and subsequent insertion into host genome of target cells, e.g. neurones and choroid plexus
• this occurs despite host immune response which is mounted early
• viral replication appears to be inhibited by host factors
• as yet not completely known but does involve a lipoprotein inhibiting factor
• sufficient replication does recur to allow disease progression
• lesions in visna appear to be mediated by the host's immune system
• antigenic drift by virus allows persistence of infection in face of host immune response

Epidemiology

• dam to lamb transmission postnatally most likely in a direct contact as well as milk
• adult to adult is as important
• confined housing very important in spread, e.g. winters
• little evidence for indirect transmission via contaminated yards etc (within 2 weeks affected premises are safe to restock)
• may be some genetic factors involved in influencing susceptibility to infection

Differential Diagnoses

     1.  scrapie - pruritis, no loss of condition - advancing hindquarter paralysis is not evident
     2.  listeriosis
     3.  brain abscess (nonspecific)
     4.  parasitic invasion of CNS
     5.  plant toxicities