Equine influenza virus: characteristion, epidemiology and vaccination.

The epidemiology, characterisation and vaccination responses to equine influenza (EI) in Ireland were studied between 2007 and 2010. Epidemiological evidence indicated a seasonal pattern to outbreaks with 50% and 39% occurring over the summer and winter months, respectively. Outbreaks were recorded in 13 of the 32 counties and the majority of cases occurred following animal movement on or off premises. Other key influences included housing type and fomites/personnel. Epidemiological evidence revealed poor vaccine compliance on premises where horses were infected with EI, while on premises where vaccination was widely practiced, the duration of vaccination history and time since last vaccination influenced disease expression. In a population of 1149 horses/ponies with mixed levels of antibody, 34%, 11%, 32% and 23% were considered to have index case potential, partial protection, clinical protection and virological protection respectively. Younger animals were identified as most susceptible to EI and the majority of teaser stallions had little serological evidence of vaccination. Phylogenetic analysis of the HA gene of 18 EI viruses identified suggested that predominant strains circulating among Irish horses were from clade 1 and clade 2 of the Florida sublineage. Equine influenza vaccine efficacy was examined in two different equine populations, mature National Hunt (NH) horses and immunologically naïve Thoroughbred (TB) weanlings. There was no significant difference between the levels of antibody response induced following booster vaccination with any of the six vaccines in the NH population. While the mean SRH levels post booster vaccination suggested that this population was clinically protected, analysis of the individual responses suggested that there was potential for vaccine breakdown among some horses. Antibody responses of weanlings vaccinated with the whole virus vaccine Duvaxyn IET Plus were significantly higher than those of the horses vaccinated with any of the other products. A high incidence of poor responders was observed post first vaccination (V1).Vaccination of weanlings which had maternally-derived antibodies effectively primed the animals but in some cases their response to the primary course was lower than that of seronegative weanlings. Overall, the results indicated that EI is endemic in Ireland. Vaccine compliance is good within certain sectors of the industry however, there are vulnerable subgroups where vaccination is poorly executed thus creating potential pockets of disease and facilitating virus spread.