Equine influenza outbreak in India (2008-09): virus isolation, sero-epidemiology and phylogenetic analysis of HA gene.

An outbreak of equine influenza (EI) was reported in India in June, 2008 after a gap of two decades. The outbreak started from Jammu and Kashmir (Katra), northern state of India and spread to the other parts of the country affecting equines in 11 states. The virus (H3N8) was isolated from nasal swabs obtained from clinical cases in various locations in the country including Katra (Jammu and Kashmir), Mysore (Karnataka) and Ahmedabad (Gujarat) using embryonated chicken eggs. The virus isolates were identified as H3N8 by haemagglutination inhibition (HI) test titration with standard serum and by sequencing of full-length haemagglutinin (HA) gene and partial sequence of neuraminidase (NA) gene. Paired serum samples (n=271) showing more than fourfold rise in antibody titres tested from 11 states confirmed equine influenza. Serum samples (n=2517) of equines from 13 states of the country screened by HI test revealed 687 (26.85%) samples positive for antibodies to EI (H3N8). Phylogenetic analysis of the haemagglutinin (HA) gene confirmed the virus to be closely related to Clade 2 of the Florida sublineage in American lineage. Comparison of deduced amino acid sequence of HA gene with EIV isolates from various lineages showed substitutions in the antigenic regions C and D. HA1 gene sequence had highest amino acid identity to A/eq/Gansu/7/08 and A/eq/Hubei/6/08 isolates from China and Inner-Mongolia isolate, while the complete HA gene sequence was closest to A/eq/A/eq/Newmarket/5/03, A/eq/Bari/05 and A/eq/Kentucky/05/02 isolates. Recent outbreaks of Mongolia, China and India by clade 2 EI viruses imply their predominance in Asia in addition to Europe. (c) 2009 Elsevier B.V. All rights reserved.