Abstract
Bluetongue virus serotype 20 (BTV20) (CSIRO 19 isolate) was compared with 17 other BTV serotypes using various serum neutralization (type antigen) tests to determine whether any serological relationships existed. Plaque-reduction neutralization tests employing 50% and 80% end-points could not clearly differentiate BTV20 from BTV4. Plaque-inhibition tests and quantal microtitre neutralization tests also showed a relationship between BTV20 and BTV4. Antisera against BTV20 and a Cyprus isolate of BTV4 (A SOT 1) showed a low level of cross-neutralization against BTV17. Investigation of plaque-reduction neutralization of virus—antiserum mixtures, by the calculation of regression curves and comparison of the area under the curves, showed that the BTV4 isolates studied could not be differentiated, and that BTV4 typing antiserum could not distinguish between BTV4 and BTV20, but that BTV20 antiserum could distinguish between BTV20 and BTV4. BTV20 did not show any significant type relationships with any of the other BTV types 1 to 17 using any of the neutralization tests. Our results suggest that BTV20 is closely related to, although not identical with, BTV4 and could be grouped as a subtype of BTV4. BTV17 appears to be distantly related to BTV20 and BTV4, but is clearly a distinct type.
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