Seroprevalence of Equine Rhinitis Virus in Louisiana Horses

Equine influenza caused byequine influenza virus (H3N8) is considered themost important respiratory disease of horses due to its rapid transmission. Inactivated vaccines are widely used for limiting the spread of disease and reducing the clinical severity in individual level. However, many outbreaks have been reported among vaccinated horses primarily because of the antigenic differences between the vaccine strains and epidemic strains. It is therefore necessary to periodically review the strain composition of vaccines. Thehemagglutination inhibition (HI) test has been widely used for assessing the cross-reactive antibody responses induced by vaccines to epidemic strains. Though there is general agreement between results of HI tests and vaccine efficacies, it remains unclear whether cross-antibody titer measured by the HI test correlates with crossneutralizing reactivity. Here, we assessed the cross-reactivity of the antibodies elicited by the Japanese local vaccine strains to the current epidemic strains, by virus neutralization (VN) test.Wemade the horse antisera by inhalations of the Japanese vaccine strains [A/equine/La Plata/1993 (American) and A/equine/Ibaraki/1/2007 (Fc1)] [108.3 50% egg infectious dose (EID50)/horse each] to horses. Two weeks after the inhalations, each serum was taken from each horse. All the antisera were treated with trypsin-heatpotassiummetaperiodate to remove non specific inhibitors before VN tests. Then the required final dilution of treated antiserum (1:8) was prepared and absorbed with packed chicken red blood cells. Two-fold serial dilutions of the antiserumwere prepared and added to the equal volume of each virus (approx. 104.0 EID50/200 ml). After incubation for 60 min at 34 C, 200 ml of the mixture was injected into an embryonated hen’s egg (5 eggs per each serum dilution). After three overnights incubation at 34 C, the allantoic fluids were harvested and examined hemagglutination activities. VN titers were expressed as the log (2) of the reciprocals of the dilution of antisera which reduced infectivity to1 EID50/200ml.Whereas thehorse antiserumraised to A/equine/Ibaraki/1/2007 (Fc1) showed significantly lower VN titer (6.2) to A/equine/Yokohama/aq13/2010 (Fc2) than that to thehomologous virus (9.0), thehorse antiserum raised to A/equine/La Plata/93 neutralized A/equine/Yokohama/aq13/2010 well at the similar VN titer (9.3) to the homologous VN titer (9.2). The World Organization for Animal Health (OIE) annually reports the antigenic traits of circulating viruses in the world. It is recommended that vaccines for the international market should contain both Fc1 and Fc2 viruses. Our results however showed that the inoculation of A/equine/La Plata/93, which is one of the Japanese local vaccine strains and not genetically classified into Fc2, can elicit the antibody cross-neutralizing the Fc2 viruses well in horse serum.