Isolation and identification of bluetongue virus: a serotype new to the U.S.

There are 5 known serotypes of bluetongue (BT) virus (BTV) in the US: 2, 10, 11, 13 and 17. The most recently discovered US serotype, BTV 2, was isolated from blood collected from cattle at Ona, Florida in 1982. Isolations were made from the September through November bleedings and from 1 pool of Culicoides insignis Lutz collected in October. Seventeen viral isolates were obtained by injecting the samples into embryonated chicken eggs (ECE) followed by serial passage onto BHK-21 cells. A single isolate was made from inoculation of pooled bovine blood into sheep. The isolates were shown to be Orbivirus-like particles by electron microscopy and were identified as BTV by indirect immunofluorescence tests. All the isolates were found to be serotype 2, however genome analysis using polyacrylamide gel electrophoresis (PAGE) identified 2 distinct electropherotypes. There were major differences between the 2 electropherotypes at least in genome segments 1, 5, 7, 8 and 9. The electropherotypes of viral isolates from bovine blood collected in September, 5 of those from the October bleedings, the insect isolate, and that from pooled blood were identical by PAGE and were designated Ona A. This electropherotype was also found to be indistinguishable from the prototype of the African serotype 2. The 2nd electropherotype, Ona B, included 3 of the viral isolates from the October bleedings and the single isolate from November. Ona A appeared to be closely related to the African prototype while Ona B may have resulted as a variant of Ona A after being subjected to a different environment. Both electropherotypes, however, appear to be stable forms of BTV serotype 2.