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Bovine viral diarrhoea virus (BVDV), which exists as two genotypes (BVDV1 and BVDV2), has been associated with a complex of disease syndromes in cattle (Brock 2004). BVDV causes acute (transient) and persistent infections in cattle. Cattle persistently infected with BVDV often develop, as a late sequel to foetal infection, a syndrome termed mucosal disease (Brownlie and others 1984). Persistent infection occurs in animals that are specifically immunotolerant as a result of early gestational infection (Radostits and others 2007a), and is characterised by widespread distribution of BVDV antigen including CNS neurones (Hewicker and others 1990, Montgomery 2007, Bielefeldt-Ohmann and others 2008, Montgomery and others 2008), in contrast with later gestational and postnatally acquired infections in which the distribution of BVDV antigen is more limited and does not include CNS neurones (Ellis and others 1998, Bielefeldt-Ohmann and others 2008, Montgomery and others 2008). Most acute BVDV infections in cattle are subclinical, but there are a number of reports of acute infections causing severe disease (Corapi and others 1989). Acute uncomplicated BVDV2 infection has been reported as the cause of a haemorrhagic syndrome in cattle in North America (Ridpath and others 2000), which has been reproduced experimentally (Corapi and others 1989). BVDV-induced haemorrhagic syndrome in calves is characterised by pyrexia, diarrhoea, leukopaenia, thrombocytopenia and haemorrhages (Corapi and others 1989). BVDV1 has also been isolated from cases of haemorrhagic disease in cattle in Europe, although experimental infection with these isolates did not result in clinical illness or thrombocytopenia (Hamers and others 2000), and the relationship between these observations of haemorrhagic disease and BVDV1 isolation is unclear. Induction of a sporadic haemorrhagic syndrome by BVDV1 may require the presence of a number of cofactors (Hamers and others 2000), for example, host and environmental factors may be involved (Odeon and others 1999).
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