Overview of bluetongue disease, viruses, vectors, surveillance and unique features: the Indian sub-continent and adjacent regions.

The occurrence of bluetongue (BT) disease in India was initially confined to exotic breeds of sheep and subsequently became endemic in native breeds. BT virus (BTV) antibodies are common in cattle, buffaloes and goats although clinical disease has not been reported. Exotic breeds of sheep and their cross-breeds are more susceptible to disease than native breeds. Overall, morbidity, mortality and case fatality rates of 9.3%, 2.7% and 28.8%, respectively, have been reported in rural flocks; these rates are higher than in organised farms. The disease is mostly cyclical in occurrence. Outbreaks usually occur between June and December during the monsoon period when livestock biting midges greatly increase. BTVs have been isolated from native sheep, and sentinel herds have been used to demonstrate virus activity. A total of 21 serotypes of BTV have now been reported in the country. Major impediments to control the disease include the presence of multiple virus serotypes, the broad vertebrate host range of the virus and a lack of detailed knowledge of vectors. Inactivated vaccines prepared from local isolates are currently under field trials. BTV occurs in regions adjacent to India. An antibody prevalence of 48.4% has been reported in Pakistan with serotypes 3, 9, 15, 16 and 18 identified. BTV antibody, but not disease, has been reported in Bangladesh and Sri Lanka.