Reduction in endemic equine herpesvirus type-1 and type-4 infection among Thoroughbred yearlings through an updated vaccination program

The endemic situation of respiratory disease caused by equine herpesvirus type-1 (EHV-1) and type-4 (EHV-4) was investigated in a training facility for Thoroughbred yearlings in Japan. Vaccination typically starts in mid-September or early October-only after all yearlings have arrived-leaving those introduced earlier unprotected. To bridge this immunity gap, a revised vaccination program that started earlier was implemented. In 2021-2022 and 2022-2023, yearlings were allocated to three groups according to their introduction dates. Each group received a live EHV-1 vaccine (Equi N Tect ERP, Nisseiken, Tokyo, Japan) as early as possible after arrival, with a second dose administered two months later. Virus-neutralizing titers to EHV-1 rose significantly after the first vaccination in each group as anticipated. Virus type-specific enzyme-linked immunosorbent assays showed that, in 2018-2019 to 2020-2021, pyretic horses (≥38.5°C) infected with EHV-1/4 occurred more frequently between August and September than in other months: 15 cases with EHV-1, 9 cases with EHV-4, and 5 cases with both viruses across the three years. In 2021-2022 and in 2022-2023, only three pyretic horses with EHV-1 infection were confirmed in the corresponding period. The infection rates for EHV-1/4 in the entire population peaked between August and September in 2018-2019 to 2020-2021 (6.8‰-10.9‰). Under the new program, infection rates were much lower, at 2.6‰-3.8‰ in 2021-2022 and 1.2‰-1.7‰ in 2022-2023 (<i>P</i><0.05). The reduction in these parameters was likely associated with the efficacy of the updated vaccination program.