Infection with bluetongue and related orbiviruses in the Sudan detected by the study of sentinel calf herds.

Using two sentinel calf herds, the seasonal incidence of bluetongue in central Sudan was shown to be a predictable event related to the rainy season and to vector population. Seroconversions to bluetongue virus (BTV) were detected only between July and December. Neutralizing antibodies to 16 BTV serotypes (1-10, 12, 14, 17, 20-22) were present in animals from both sentinel herds. The use of a blocking enzyme-linked immunosorbent assay made possible the distinction between infections due to BTV and those due to infections with other orbiviruses. Sera from 13 calves at Shambat and from 17 at Um Benein gave agar-gel tests indicative of BTV infections not attributable to any of the 22 serotypes, suggesting the presence of new serotypes of BTV. Sera from 30 calves gave results interpreted as evidence of infection with BTV-related viruses, rather than BTV serotypes. Comparing these results with those reported from Kenya, we suggest that the pattern of the disease and the types involved are similar in both countries and that the movements of the Intertropical Convergence Zone (I.T.C.Z.) may carry infected midges northwards into the Sudan at the start of the rainy season and southward at the end of this period.