Epidemic Strains of Equine Influenza Virus: Cross-Reactivity with Vaccine

Equine influenza virus (EIV) is one of the two most common viral causes of upper respiratory disease in horses. This past year (autumn 2018 through spring 2019) saw an unusually high level of EI activity in both the USA and Europe and importantly, there were multiple reports of vaccinated horses contracting EI disease. This kind of spread is suggestive of the emergence of a novel antigenic variant of EIV, which would call for updating of the EIV strains used in vaccines. However, analyses of the responsible virus strains, done independently in laboratories in England, France, and the USA (my own) all came to the same conclusion—that there was no antigenic difference between the new strains and the virus strains already used in EI vaccines. There was thus no rational basis for any change in the EI vaccine strains: the existing vaccine strains ought to have been effective. This leaves unresolved why some horses that had been vaccinated in accordance with recommended best practice, had nonetheless been poorly protected. The implication is that our antigenic testing is missing some feature of these latest circulating EIV strains that is relevant to clinical protection. The standard antigenic analysis, for EIV as well as for human influenza, relies on hemagglutination-inhibition (HI) testing against large panels of reference ferret sera. To examine the comparative antigenic character of these new EI strains from a different point of view, I propose (Specific Aim 1) to use the Serum Neutralization Test to learn whether horse antibody responses to vaccine virus strains are efficiently neutralizing the new circulating strains. The viral hemagglutinin (HA), the target antigen of the HI test, is the most important viral antigen for immunity but not the only one: the viral neuraminidase (NA) also stimulates immune responses that are believed to contribute to protection. In EIV we know that NA changes over time just as HA does, but the HI test does not measure the antigenic character of NA, and we do not understand the importance for vaccine strains of the changes that we see in NA. To increase our understanding, I propose (Specific Aim 2) two approaches to compare the antigenicity of NA of a recent strain with the most relevant vaccine strain: (1) standard ELISA to directly measure total NA-specific serum antibody binding; and (2) NA-inhibition (ELLA) assay to measure antibody-mediated inhibition of NA functional activity. To facilitate these approaches, I include a small horse experimental infection with one of the recent strains (Arizona/2019) to obtain strain-specific convalescent serum antibodies to use for comparison with post-vaccination horse sera. The results of both specific aims will shed new light on the similarity and differences between the new EIV epizootic strains and the current vaccine strains, and provide new evidence from two new directions to consider with regard to the question of updating the current vaccine strain recommendations. This would be a health benefit for all horses