Bluetongue virus in sheep and cattle and Culicoides variipennis and C. stellifer (Diptera: Ceratopogonidae) in Louisiana.

The transmission of bluetongue virus (BLU) to sheep and cattle relative to Culicoides abundance and infection was studied in two areas of the Louisiana State University Agricultural Center, Baton Rouge, LA. The seroprevalence of BLU in beef cattle was 70.5% in 1989 and 37.0% in 1990; 2 of 37 animals tested in both years became positive, indicating active transmission. The prevalence of BLU in sheep decreased from 52.2% in 1989 to 34% in 1990; seroconversion was not detected. Serotypes of positive sheep were as follows: 13 sheep were positive for BLU-13, 6 for BLU-17, 2 for BLU-2, and 2 for both BLU-13 and BLU-17. The seroprevalence of BLU virus in dairy bulls increased from 15.4% in 1989 to 47.4% in 1990; 0 and 7 seroconversions occurred in 1989 and 1990, respectively. Serotypes of BLU from the bulls were as follows: five bulls for BLU-13; two for BLU-17; one for BLU-13 and BLU-17; and one for BLU-13, BLU-17, and BLU-2. Peaks in the abundance of Culicoides variipennis (Coquillett) and C. stellifer (Coquillett) collected in New Jersey light traps corresponded with the suspected transmission dates during 1990. Bluetongue viral RNA was detected in one of 381 pools (6,072 flies) of C. variipennis; no virus was detected in 125 pools (884 flies) of C. stellifer.