Pathogenesis of quadritypic bluetongue virus infection in cattle

Abstract

The present study examines clinical, serological, and virological responses in cattle to quadritypic bluetongue virus (BTV) inoculation. Eight cattle were inoculated with 1 x 10('4) sheep infectious doses of each of four virulent BTV international types (BTIT 10, 11, 13, and 17). Two animals were used as contact controls. The cattle selected for this study were negative serologically for bluetongue by the agar gel precipitin(AGP) test but positive by the enzyme-linked immunosorbent assay (ELISA) test. The study lasted 182 days, during which time specified samples and data were gathered on a daily, weekly, and biweekly basis. Inoculated cattle did not develop clinical signs of disease. Those which developed viremia did exhibit variable pyrexias, some exceeding 40.0 C. Hematological examinations, done at weekly intervals, revealed variable leukocyte responses and an overall depression in hematocrit and hemoglobin values. Serum chemistry values showed no deviation from physiological limits. Bluetongue virus was isolated from buffy coats and whole blood of virus inoculated cattle. Virus isolations were made at 7 days post inoculation (PID 7) through but not beyond PID 21. Virus was recovered more frequently from buffy coats than whole blood. No viral isolations were made from the tissues of BTV inoculated cattle at necropsy, though BTV-specific immunofluorescence was detected in spleen sections of an animal killed on PID 21. No gross or microscopic pathologic changes, directly attributable to BTV infection, were seen. Transmission electron microscopy studies of glutaraldehyde fixed buffy coat material showed the intracytoplasmic presence of virus-like particles within membrane bound vacuoles. These electron dense particles were approximately 60 nm in diameter and occurred only in agranulocytic leukocytes of viremic calves. Precipitin antibodies were detected on PID 14 in the virus inoculated animals and persisted through the respective animal's termination date. Virus neutralization and enzyme-linked immunosorbent assay results showed that BTV antibodies were detectable in all ten cattle on the day of virus inoculation. A definite increase in log neutralizing indexes was seen in all viremic cattle, whereas ELISA absorbancy values only showed slight variations. According to the neutralization data, 3 calves (2 virus inoculated and 1 control) experienced a significant immunological response to four BTV international types.