Epizootiological study of bluetongue virus infection in California livestock: an overview.

An epidemiologic program was undertaken in California to study bluetongue virus (BTV) infection in domestic livestock. The study was designed to determine: a) prevalence of BTV infection, b) serotypes of BTV actively causing infection, c) seasonality of infection and d) species infected. A total of 8,751 cattle, 14,639 sheep and 4,785 goats were tested over the 3 1/2 year study. Serologically, 41% of the cattle, 42% of the sheep and 21% of the goats were positive. Virologically, 2.4% of the cattle, 1.4% of the sheep and 0.7% of the goats were viremic. One BTV isolation was made in April (sheep) and 3 in June (1 each from sheep, cattle and goats); the remainder of the BTV isolations (a total of 359) were made in the months of July through early December. No isolations were made from January through March. Four serotypes of BTV (10, 11, 13 and 17) were isolated from all species tested (sheep, goats and cattle) and Culicoides variipennis. The serotypes isolated from C. variipennis correlated with the serotypes isolated from livestock in given areas. Multiple serotypes were isolated from single herds, flocks and individual animals on single given days. In addition to multiple serotypes being isolated, extensive heterogeneity in the electrophoretic mobility of the RNA genome segments was observed. These different migration patterns (electropherotypes) were observed between and within serotypes. No single serotype could be identified by a given pattern. No clinical disease was associated with BTV infection of cattle. Clinical disease was observed in infected sheep; however, BTV was also isolated from flocks with no overt clinical signs of disease. No reproductive problems could be associated with BTV in cattle in this endemic study area; however, BTV infection of pregnant sheep appeared to be associated with abortion and birth of dummy lambs in certain flocks.