ABSTRACT
High sheep population density, congenial climatic conditions for Culicoides propagation, and susceptible sheep breeds may be contributing to the higher incidence of Bluetongue (BT) in Southern states of India. Sheep farming in this part of the country is nomadic in nature and BT is one of the major infectious diseases inflicting huge losses. Andhra Pradesh is one of the Southern states with high sheep population in India. Although isolation studies in this region were started in 1993, concerted efforts only began in 2002. More than 50 isolates were obtained in the last decade, and 7 Bluetongue virus (BTV) serotypes (1, 2, 9, 10, 12, 16 and 21) were isolated. Among them, BTV-10, BTV-12, and BTV-21 were reported for the first time from India and the genome analysis of these viruses revealed that BTV-10 and BTV-12 have high sequence identity with the modified live virus (MLV) vaccines used in USA and South Africa, respectively. At the same time, BTV-21 has probably originated from Southeast Asia. Furthermore, some of the BTV isolated from Europe have high sequence identity with viruses isolated from Andhra Pradesh indicating common ancestry. The analysis of different isolates involved in outbreaks revealed that more than 1 BTV serotype is involved and that mixed infections with different serotypes is not uncommon. In a limited study conducted during 2005-2009, it was observed that most of the sheep seroconverted to more than 1 serotype, which further supports circulation of multiple serotypes and mixed infections in Andhra Pradesh. Based on the virus isolation data, in this study it was observed that a few serotypes dominate for 3-4 years followed by domination of others. Continuous monitoring of circulating serotypes is essential to understand the distribution and spread BTV in endemic areas and for devising suitable control measures.
