Full Genome Sequence of a Western Reference Strain of Bluetongue Virus Serotype 16 from Nigeria

Co-infection of bluetongue virus serotypes 12 and 16 in sheep from Haryana, India

World Organization for Animal Health has listed bluetongue (BT) under notifiable diseases. The BT is an arboviral infectious disease of domestic and wild ruminants caused by the bluetongue virus (BTV). Southern states of India had remained the point of attention for BT since first presence in 1964 in Maharashtra. Recently, northern states of India have also been reported positive for BTV in small ruminants. The present study reported the dual infection of BTV serotypes, BTV-12 and -16 in sheep population from Sirsa district of Haryana in the year 2016. After detection and serotyping with Seg-2 specific real time polymerase chain reaction (PCR), the Seg-2 and Seg-6 of BTV were PCR amplified and sequenced. On phylogenetic analysis it was detected to be clustered in nucleotype G and nucleotype B specific for BTV-12 and BTV-16, respectively. This was the first report of BTV-16 from Haryana. The results signified the co-infection of two different serotypes in an animal from a single outbreak.

Epidemiology of Bluetongue in India

Bluetongue (BT) is an insectborne endemic disease in India. Although infections are observed in domestic and wild ruminants, the clinical disease and mortality are observed only in sheep, especially in the southern states of the country. The difference in disease patterns in different parts of the country could be due to varied climatic conditions, sheep population density and susceptibility of the sheep breeds to BT. Over the five decades after the first report of BT in 1964, most of the known serotypes of bluetongue virus (BTV) have been reported from India either by virus isolation or by detection of serotype-specific antibodies. There have been no structured longitudinal studies to identify the circulating serotypes throughout the country. At least ten serotypes were isolated between 1967 and 2000 (BTV-1-4, 6, 9, 16-18, 23). Since 2001, the All-India Network Programme on Bluetongue and other laboratories have isolated eight different serotypes (BTV-1-3, 9, 10, 12, 16, 21). Genetic analysis of these viruses has revealed that some of them vary substantially from reference viruses, and some show high sequence identity with modified live virus vaccines used in different parts of the world. These observations have highlighted the need to develop diagnostic capabilities, especially as BT outbreaks are still declared based on clinical signs. Although virus isolation and serotyping are the gold standards, rapid methods based on the detection of viral nucleic acid may be more suitable for India. The epidemiological investigations also have implications for vaccine design. Although only a handful serotypes may be involved in causing outbreaks every year, the combination of serotypes may change from year to year. For effective control of BT in India, it may be pertinent to introduce sentinel and vector traps systems for identification of the circulating serotypes and to evaluate herd immunity against different serotypes, so that relevant strains can be included in vaccine formulations.