Epidemiologic study of bluetongue virus on the Tejon Ranch, California: vector, host, virus relationships.

Culicoides gnats were monitored from April through November, 1981, on the Tejon Ranch, Kern County. Levels of gnat breeding were determined by quantitative sampling of substrate from intermittent and permanent water sources. Adult populations were measured by light trap collections taken around pens holding sentinel beef cattle, dairy calves, sheep, goats and deer. Adult gnats were collected in the environs of the study area, and blood samples were obtained from sentinel animals for laboratory diagnosis and isolation of bluetongue virus (BTV). C. variipennis was 1st observed in various water sources in April, with a continuous increase in density through October. Adult populations reflected this trend in density through October. Both immature and adult gnat populations rapidly decreased in November. BTV serotypes 11, 17 and 13 were isolated from C. variipennis from August through October. BTV serotypes 11 and 17 were initially isolated from nearly all sentinel sheep, goats and dairy calves from late July through September, whereas isolates of 11 and 17 were made from only 2 beef cows and 2 deer in August and September. Sheep were the only sentinel animals that showed clinical signs of BT disease.