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 micrograms 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 micrograms 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 micrograms HA and in adjuvant doses of Eq/Jilin from 0.1-5.0 micrograms HA were efficacious. Homologous protection against Eq/Jilin or Eq/Kentucky was induced by doses of vaccine from 0.5-5.0 micrograms HA per dose in aqueous suspension and from 0.01-5.0 micrograms HA per dose in adjuvant.(ABSTRACT TRUNCATED AT 250 WORDS)