2025 Volume 36 Issue 2 Pages 67-74
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 (P<0.05). The reduction in these parameters was likely associated with the efficacy of the updated vaccination program.
