Bluetongue Virus Infection in Cattle
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FIG. 1. Cow demonstrating mild ulceration of the muzzle due to bluetongue virus infection.
FIG. 2. Cow demonstrating more progressed ulceration of the muzzle due to bluetongue virus infection.
Bluetongue viruses are infectious, noncontagious viruses in the family Reoviridae and genus Orbivirus that are transmitted to domestic and wild ruminants (sheep, cows, goats, deer) by biting midges (no-see-ums) in the genus Culicoides (Fig. 3) (6). The viruses are nonenveloped and the icosohedral 85-nm-diameter capsid contains the 10 segments of the linear double-stranded RNA genome of approximately 25 kilobases (4, 5). The disease is found worldwide in sheep, goats, and cattle (ruminant species). Due to the seasonal increase in the vector, disease is most prevalent mid summer to early fall.
Bluetongue virus infection in cattle is very similar to infections in sheep, although morbidity and clinical signs of disease in adult animals are much lower, usually about 5% versus 50 to 80% in sheep. Factors such as severe weather, infection with parasites, poor diet, and stress are known to exacerbate mortality in sheep. Disease signs include fever (usually 104 to 106oC), depression, and lassitude beginning 6 to 8 days postinfection. The disease progresses to redness and swelling of the mucous membranes of the mouth (Fig. 1 and 2), eventually leading to ulceration of the mouth and nose (Fig. 2), sometimes giving a burnt appearance. Signs as severe as those seen in Fig. 2 are rare. Mild infections often go unnoticed. Although the virus is infectious in cattle and most animals in infected herds show serologic indication of infection, few show disease signs. Full recovery usually occurs within 10 to 14 days. Milk production decreases as a result of infection can occur (1