Equine Influenza

Abstract

Horses are the third major mammalian species, along with humans and swine, long known to be subject to acute upper respiratory disease from influenza A virus infection. The viruses responsible are subtype H7N7, which is believed extinct, and H3N8, which circulates worldwide. The equine influenza lineages are clearly divergent from avian influenza lineages of the same subtypes. Their genetic evolution and potential for interspecies transmission, as well as clinical features and epidemiology, are discussed. Equine influenza is spread internationally and vaccination is central to control efforts. The current mechanism of international surveillance and virus strain recommendations for vaccines is described.

Figure 1.

Timeline of evolution of equine influenza viruses (EIVs). Events prior to the first isolations of EIVs in 1956 are conjectural, based on extrapolations of phylogenetic data backward in time.

Figure 2.

Phylogenetic analysis of the hemagglutinin (HA)1 nucleotide sequences encoded by H3N8 equine influenza virus (EIV) through 2015, created using PhyML version 3. Key amino acid substitutions are shown with arrows and those in antigenic sites are underlined. Virus lineages are shown by continuous bars on the right and are labeled appropriately. Current World Organisation for Animal Health (OIE)-recommended vaccine strains are in bold and capitalized. (This tree was generated and kindly provided for use in this review by Drs. Adam Rash and Debra Elton, Animal Health Trust, United Kingdom.)