LAUSR

BACKGROUND Equine influenza is an important cause of respiratory disease of horses worldwide. The equine influenza virus (EIV) undergoes antigenic drift through the accumulation of amino acid substitutions in the viral proteins, which may lead to vaccine breakdown. OBJECTIVES To describe the epidemiological findings and the molecular characteristics of the EIV detected during the multifocal outbreak that occurred in Argentina between March and July 2018 and evidence a vaccine breakdown. STUDY DESIGN Observational, descriptive study. METHODS Virus was detected in nasopharyngeal swabs by real-time RT-PCR. Nucleotide and deduced amino acid sequences of the haemagglutinin (HA) and neuraminidase (NA) genes were obtained from EIV positive nasopharyngeal swabs, and phylogenetic analysis was undertaken. Amino acid sequences were compared against the current World Organisation for Animal Health (OIE)-recommended Florida clade 1 vaccine strain and strain components of vaccines used in Argentina. Serum samples were tested by haemagglutination inhibition test. RESULTS EIV infection was confirmed by real-time RT-PCR and serological testing. The phylogenetic analysis of the HA and NA genes revealed all the EIV identified during the outbreak belong to the H3N8 subtype, Florida clade 1. Multiple amino acid changes, some of them at antigenic sites, were observed in the currently circulating virus when compared with the strains included in the most commonly used vaccine in Argentina. Seventy-six percent of the affected horses had been vaccinated with this vaccine, suggesting the occurrence of vaccine breakdown. MAIN LIMITATIONS The study does not include antigenic characterisation and full genome sequencing of Argentinian strains, that could provide additional information. CONCLUSIONS The occurrence of this multifocal equine influenza outbreak in regularly vaccinated horses is field evidence of vaccine breakdown, reinforcing the necessity of keeping vaccine strains updated according to OIE recommendations. It too underlines the importance of the implementation of appropriate quarantine measures and restriction of horse movement in the face of disease.