EFFICACY OF EQUINE INFLUENZA VACCINES FOR PROTECTION AGAINST A/EQUINE/JILIN/89 (H3N8) - A NEW EQUINE INFLUENZA-VIRUS

A new H3N8 equine influenza virus [A/Equine/Jilin/1/89 (Eq/Jilin)] appeared in Northeastern China in 1989 and caused high mortality in horses; the available evidence indicates that it has not yet spread outside this region of the world. Serological analysis with postinfection ferret sera in haemagglutination inhibition (HI) tests confirmed that Eq/Jilin is antigenically distinct from H3N8 equine influenza viruses isolated between 1963 and 1991 and also showed that a current equine influenza virus [A/Equine/Alaska/1/91 (H3N8)] had undergone antigenic drift. In the present study we determine if vaccine against a recent H3N8 influenza virus [A/Equine/Kentucky/1277/90 (Eq/Kentucky)] that was standardized for haemagglutinin content will protect mice against lethal challenge with the new H3N8 influenza virus from China. Complete protection is defined as prevention of virus replication in the lungs of mice 3 days after challenge. High doses of Eq/Kentucky vaccine in aqueous suspension (0.5-5.0 mug HA per dose) provided minimal protection against Eq/Jilin challenge as judged by virus titres in the lungs of vaccinated animals. Eq/Kentucky vaccine in adjuvant (1.0-5.0 mug HA per dose) did provide complete protection against challenge with Eq/Jilin in mice. Eq/Jilin vaccine in aqueous suspension induced complete protection of mice against challenge with Eq/Kentucky at doses from 0.5 to 5 mug HA and in adjuvant doses of Eq/Jilin from 0.1-5.0 mug HA were efficacious. Homologous protection against Eq/Jilin or Eq/Kentucky was induced by doses of vaccine from 0.5-5.0 mug HA per dose in aqueous suspension and from 0.01-5.0 mug HA per dose in adjuvant. Lower doses of vaccine (0.01-0.1 mug HA) failed to induce detectable HI antibodies yet the mice were protected; ELISA and neutralizing antibodies correlated with protection. Studies showed that commercially available equine influenza vaccines provided minimal cross-protection with Eq/Jilin virus but did prevent mortality. The present studies indicate that cross-protection can be induced against the new H3N8 equine influenza virus from China with vaccine to the currently circulating equine H3N8 viruses but high doses of vaccine are needed; commercially available vaccines do not contain sufficient antigen to induce complete protection in mice.