Abstract
Equine herpesvirus type 1 (EHV-1) is a major cause of winter pyrexia in racehorses in two training centers (Ritto and Miho) in Japan. Until the epizootic period of 2008-2009, a vaccination program using a killed EHV-1 vaccine targeted only susceptible 3-year-old horses with low antibody levels to EHV-1 antigens. However, because the protective effect was not satisfactory, in 2009-2010 the vaccination program was altered to target all 3-year-old horses. To evaluate the vaccine's efficacy, we investigated the number of horses with pyrexia due to EHV-1 or equine herpesvirus type 4 (EHV-4) infection or both and examined the vaccination coverage in the 3-year-old population and in the whole population before and after changes in the program. The mean (± standard deviation [SD]) estimated numbers of horses infected with EHV-1 or EHV-4 or both, among pyretic horses from 1999-2000 to 2008-2009 were 105 ± 47 at Ritto and 66 ± 44 at Miho. Although the estimated number of infected horses did not change greatly in the first period of the current program, it decreased from the second period, with means (±SD) of 21 ± 12 at Ritto and 14 ± 15 at Miho from 2010-2011 to 2012-2013. Vaccination coverage in the 3-year-old population was 99.4% at Ritto and 99.8% at Miho in the first period, and similar values were maintained thereafter. Coverage in the whole population increased more gradually than that in the 3-year-old population. The results suggest that EHV-1 epizootics can be suppressed by maintaining high vaccination coverage, not only in the 3-year-old population but also in the whole population.