Changes

Acute BVDV infection causes a significant leukopenia, hampering the host's defences against invading pathogens. This BVDV-associated immunosupression has a particularly important role in bovine respiratory disease: an association has been demonstrated between BVDV antibody titre and treatment for respiratory disease³⁶. BVDV is the virus most frequently isolated from pneumonic lungs and is often found in association with ''Pasteurella haemolytica''³⁵, causing severe fibrino-purulent bronchopneumonia with the total lesion area increased by 35-60% to that caused by pasteurellosis alson³⁴ Synergism is also displayed with parainfluenza, bovine rhino-tracheitis and respiratory syncitial viruses.

Acute BVDV infection causes a significant leukopenia, hampering the host's defences against invading pathogens. This BVDV-associated immunosupression has a particularly important role in bovine respiratory disease: an association has been demonstrated between BVDV antibody titre and treatment for respiratory disease³⁶. BVDV is the virus most frequently isolated from pneumonic lungs and is often found in association with ''Pasteurella haemolytica''³⁵, causing severe fibrino-purulent bronchopneumonia with the total lesion area increased by 35-60% to that caused by pasteurellosis alson³⁴ Synergism is also displayed with parainfluenza, bovine rhino-tracheitis and respiratory syncitial viruses.

Although BVDV infections in naive, non-pregnant animals are usually mild, outbreaks of a severe form of BVD have been known^{11, 37}. These were characterised by the acute onset of diarrhoea, pyrexia and milk drop, with some cases proving fatal. These oubtreaks were associated with genotype 2 viruses, and it transpired vaccintion with type 1 vaccines had not afforded cross-protection in these instances due to non-compliance with instructions. Generally, BVDV-2 infection is seen less frequently than disease related to type 1 virus, but is associated with haemorrhagic syndrome. Haemorrhagic syndrome is characterised by severe thrombocytopaenia leading to haematochezia, petechiation and epistaxis</sup>38</sup> and has been described in both Europe and North America.

Although BVDV infections in naive, non-pregnant animals are usually mild, outbreaks of a severe form of BVD have been known^{11, 37}. These were characterised by the acute onset of diarrhoea, pyrexia and milk drop, with some cases proving fatal. These oubtreaks were associated with genotype 2 viruses, and it transpired vaccintion with type 1 vaccines had not afforded cross-protection in these instances due to non-compliance with instructions. Generally, BVDV-2 infection is seen less frequently than disease related to type 1 virus, but is associated with haemorrhagic syndrome. Haemorrhagic syndrome is characterised by severe thrombocytopaenia leading to haematochezia, petechiation and epistaxis³⁸ and has been described in both Europe and North America. Severe disease is also possible with virulent type 1 infection, presenting as high fever, oral ulcerations, eruptive lesions of the coronary band and interdigital cleft, diarrhoea, dehydration, leukopenia, and thrombocytopenia. Thrombocytopenia may give petechiation of  the conjunctiva, sclera, nictitating membrane and the mucosal surfaces of the mouth and vulva, as well as prolonged bleeding from injection sites³⁹.

====Acute Infections: Pregnant Animals====

====Acute Infections: Pregnant Animals====