Malignant catarrhal fever is a highly fatal disease of cattle. The causative virus is carried by, but does not cause disease in, sheep and can be spread to cattle by these silent carriers of disease. In the UK, the virus that causes malignant catarrhal fever is ovine herpesvirus 2 (OHV-2). The means of transmission of OHV-2 is not known, although it is known to be spread by neonatal lambs in particular (although all ages can be infectious). It is thought that fairly close contact between susceptible animals and sheep is necessary for spread of disease. It appears that the virus does not spread easily between individual cattle, and while cases of cattle to cattle transmission have been documented, this seems to be rare. The virus may take anywhere from 9 to 200 days from infection to development of clinical signs in the host animal. Some animals do not develop disease unless subjected to periods of stress that allow the virus to invade.
Typical infections present as
- Sudden death
- Fever with clear, watery discharge from eyes and nose, which may become thicker and yellow in colour (mucopurulent)
- Mucous membranes and skin may become ulcerated and/or necrotic
- Corneal opacity starting at the rim of the cornea and moving inwards
Other signs that may be observed include
- Disseminated intravascular coagulation, seen as areas of bruising or bleeding into the skin and mucous membranes (gums, vulva etc.)
- Dyspnoea (difficulty breathing)
- Increased salivation
- Horn and hoof covering may become loosened and fall away from the underlying skin
- Swollen joints
- Reduced milk yield
- Neurological signs such as lowered pain threshold, head pressing, incoordination, etc.
Post Mortem Signs
- Epithelial surfaces are haemorrhagic and/or ulcerative
- Lymphoid tissues and the liver may be enlarged
- Histology shows damage to epithelium and blood vessels
Treatment is often unsuccessful and the prognosis is poor. The disease is fatal in almost all animals that show clinical signs. Disease can be prevented by not co-grazing sheep and cattle.