The disease was first reported in Pakistan in 1959 and in India in 1964 (Sapre, 1964). After that BT outbreaks have been reported from sheep and goat in several states of India. Being India endemic for BT, 21 different serotypes of BTV have been reported (Prasad, 2000). However, recently the 22nd serotype i.e. BTV21 has also been isolated from India (Susmitha et al., 2012). Bluetongue is endemic in India with frequent outbreaks in sheep. The serological survey on BTV in India has indicated the presence of BTV specific antibodies in Indian sheep, goat, cattle, buffalos, camels and several wild ruminants (Mehrotra et al., 1984; Chandel et al., 2003; Prasad et al., 1998a; De et al., 2009).

Several serotypes of BTV have been isolated from the regions where sheep population is more. The north-western region of the country is mostly affected with BTV1. The BTV1 has been isolated from sheep, goat and Culicoides vector in this region (Jain et al., 1986, 1988; Prasad et al., 1994; Biswas et al., 2010). The BTV2, BTV9 and BTV15 have been reported from sheep in the Andhra Pradesh state (Sreenivasulu et al., 1999; Bommineni et al., 2008; Balam et al., 2011). BTV-23 has been reported from Uttarakhand, Madhya Pradesh, Maharashtra, Karnataka, Jammu and Kashmir (Mehrotra et al., 1995, 1996; Tembhurne et al., 2010). In northeast region (Assam) high seroprevalence of bluetongue in sheep, goat and cattle has been reported. However, virus isolation was not done (Joardar et al., 2013). The serotype specific serum neutralization test of BTV in Andhra Pradesh, during 2005–2009 showed 50.0, 44.23, 21.15, 26.92 and 15.38% seroprevalence of BTV serotypes 1, 2, 9, 10 and 23 respectively (Sairaju et al., 2013). The serological study also confirmed the high prevalence (86%) of BTV in Mithun in North east region (Nagaland) of India (Rajkhowa et al., 2008). The serological survey using dot ELISA and competitive ELISA (C-ELISA) techniques confirmed the BTV prevalence in Kerala state (Ravishankar et al., 2005). The overall BT seroprevalence has been increased to 9.3% in domestic ruminants in Kerala (Arun et al., 2014).

The BTV15 and BTV21 have been isolated from sheep in eastern part of India (West Bengal) (Joardar et al., 2009). The BTV serotype 1 (Isolate PTG-13) from goats in the Pithoragarh area of Uttarakahand has been isolated (Mondal et al., 2013).

Globally, all the BT viruses can be divided into eastern and western topotype of viruses based on geographic origins of the virus (Gould, 1987; Maan et al., 2008). Both eastern and western topotypes of BTV have been isolated from different region of India. The trade of livestock and their products along with live attenuated vaccine from western countries have played major role in occurrence of western BTV genome segments in India (Rao et al., 2012a). The eastern topotype BTV1 (Maan et al., 2012a) and western topotype BTV2 and BTV10 (Maan et al., 2012b; Maan et al., 2012f) have been reported recently. The complete genome sequence study of a reassortant strain of BTV16 of goat origin from India has been done recently (Minakshi et al., 2012). Recently, BTV serotype 1 from Culicoides oxystomavector in Gujarat state (Dadawala et al., 2012) and serotype 10 from Andhra Pradesh (Prasad, et al., 2013) has been isolated.

The Indian isolates of BTV shows a great degree of reassortment between different serotypes and also between eastern and western topotype viruses within same serotype. The multiple serotypes of BT viruses are circulating in a geographical area which leads to infection with more than one serotype to individual animals and herds. It facilitates the exchange of genetic material between different viruses and evolution of new reassortant strains of BTV (Reddy et al., 2015). A reassortant strain of BTV21 having segment 6 of BTV16 has also been isolated from Andhra Pradesh state (Shafiq et al., 2013). A reassortant strain of BTV16 having segment 5 (ns1 gene) of western origin and other segments of eastern origin has been reported from Andhra Pradesh state (Kumar et al., 2013). Moreover, antigen, antibody and nucleic acid based methods confirmed the mixed infection of BTV with peste des petits ruminants virus (PPRV) in goat flock (Mondal et al., 2009).

The eastern-western reassortant strains of BTV such as BTV2 (Maan et al., 2012c), BTV3 (Maan et al., 2012d), BTV23 (Maan et al., 2012e), BTV9, BTV1 (Rao et al., 2013; Rao et al., 2012b; Rao et al., 2012c) and BTV12 (Rao et al., 2013b) have also been reported from India.

The analysis of individual genome segments of Indian isolates of BTV reveals its genetic relationship with global isolates. The partial vp2 gene sequences analysis of BTV1 revealed 79-89% identity between Indian and Australian isolates and only 73-75% with South African isolates (Dahiya et al., 2004). The vp2 gene based phylogenetic comparisons of some of the Indian BTV1 isolates (Isolate IND1992/01from Rajasthan State) shows eastern topotype origin (Maan et al., 2012). Vp2 gene based phylogenetic analysis of some of the Indian BTV16 isolates from goat origin showed the close resemblance with Japan and Greece isolates than Australia isolate of BTV16 (Dadawala et al., 2013).

The sequence based phylogenetic and restriction enzyme analysis of vp2 gene of an Indian BTV2 isolate showed close relationship to the Taiwan isolate (Balumahendiran et al., 2009).

Similarly, vp2 gene of Indian strains of BTV10 (isolates IND2004/01 and IND2005/04 and IND2005/05) showed maximum identity with BTV 10 from the USA. It suggests that some of the Indian BTV serotype could have been imported to India from USA. Although it is not much clear that how and when this happened. However, it is possible that animals imported from USA for breeding programme were infected with BTV. It also explains the higher incidence and severity of BT observed in local Indian sheep breeds (Prasad et al., 2009).

The vp5 gene based analysis of Indian BTV isolates also showed the eastern and western topotype origin. Vp5 gene of some of the Indian BTV2 (BT2, Accession number JF899598) and BTV9 (Ong5/06/Ind, Accession number JF969307) showed maximum sequence identity with western viruses showing their western origin (Ranjan et al., 2014a; Ranjan et al., 2013). However, some of the viruses such as BTV16 (BT16/07, Accession number JF980714) and BTV1 showed eastern origin (Ranjan et al., 2014b; Manjunatha et al., 2010).

Segment7 sequence analysis shows that Indian BTV isolates are comparatively closer to some Australian isolates than American, Chinese or French isolates (Kovi et al., 2006). The Segment7 gene sequence of Indian BTV18 and BTV23 forms a single monophyletic group with Australian isolates, while BTV1 form a separate group (Kovi et al., 2006). It indicates the separation of segment7 nucleotide sequences of Indian BTVs into distinct eastern and western clads (Bonneau et. al., 2000).

The vp7 gene sequence analysis of Indian isolate of BTV23 shows its close phylogenetic relationship to Australian BTV1 and Chinese BTV1, 2 and 4 serotypes, indicating its close evolutionary relationship with Australian and Chinese BTV serotypes (Kataria et al., 2006).

The phylogenetic analysis of segment7 revealed that Indian isolate of BTVl (isolate Avikanagar) cluster with eastern viruses from India (BTV18 and BTV23), South Africa, Australia and China (BTV1, BTV2 and BTV4). However, Indian BTV15 (isolate Hyderabad) was found closer to western viruses from USA (BTV10 and BTV17) (Dalal et al., 2008). The phylogenetic analysis suggests that grouping of vp7 gene based Indian BTV isolates are at random and it does not assort according to serotype, even over distant geographical location.

The segment 10 (S10) based phylogenetic analysis are important in understanding the BTV evolution. The BTV serotype 1, 18 and 23 from India shows closer relationship with viruses from Mediterranean Basin origin. Indian isolates of BTV 18 and 23 are closer to each other and form a distinct cluster from BTV 1. However, S10 of Indian isolate of BTV 18 and 23 showed close relationship with BTV from China, Indonesia and Australia (Desai et al., 2009).