Bluetongue and related orbivirus diagnosis in the United States.

The serologic test for bluetongue (BT) that was used in the US from 1968 to 1980 to qualify animals for export was the modified direct complement-fixation (MDCF) test. The MDCF test was replaced by the immunodiffusion (ID) test in 1980. In January 1984, there were 70 laboratories approved by the USDA to conduct BT ID export testing. Both tests are used at the National Veterinary Services Laboratories (NVSL) for the serologic diagnosis of suspected cases of BT and/or epizootic hemorrhagic disease (EHD). Serologic surveys were conducted on bovine samples collected from 18 northeastern states in the winters of 1977/1978, 1979/1980, 1982/1983 and 1983/1984. The percents of positive animals were 0.3, 0.3, 5.4 and 0.9. In 1977/1978 samples were collected from all states; 18.5% were positive. The primary technique used for the isolation of BT virus (BTV) at the NVSL is intravenous inoculation of embryonating chicken eggs with subpassage into Vero-M cell line cultures. Baby hamster kidney (BHK) cell line cultures are used for isolation of EHD virus (EHDV). Each virus is identified by staining with specific fluorescent antibody conjugate.