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

Overview

• This study evaluated the impact of an updated vaccination program on reducing infections of equine herpesvirus type-1 (EHV-1) and type-4 (EHV-4) in Thoroughbred yearlings at a Japanese training facility.
• The updated schedule, which involved earlier vaccination upon arrival, led to significantly lower rates of EHV-1/4 infections compared to previous years.

Background and Problem

• Equine herpesvirus types 1 and 4 (EHV-1/4) commonly cause respiratory disease in horses, leading to fever and other symptoms.
• At the studied facility, vaccination traditionally began mid-September or early October, after all yearlings had arrived. This created a vulnerability gap for horses introduced earlier in the season who remained unvaccinated during peak infection months (August–September).
• The presence of this immunity gap resulted in repeated cases of pyretic (feverish, ≥38.5°C) horses infected with either or both virus types during late summer.

Updated Vaccination Program

• To address the vulnerability period, the revised vaccination protocol started vaccination earlier—immediately upon arrival of yearlings rather than waiting until all had arrived.
• Yearlings were divided into three groups according to their entrance dates in the facility to enable timely vaccination administration.
• Each group received two doses of a live EHV-1 vaccine (Equi N Tect ERP, Nisseiken, Tokyo): the first dose as soon as possible after arrival and a second dose two months later.

Immune Response Assessment

• Virus-neutralizing antibody titers against EHV-1 were measured in the yearling groups.
• A significant rise in neutralizing titers was observed after the first vaccination in all groups, demonstrating that the vaccine effectively stimulated immunity.
• Enzyme-linked immunosorbent assays (ELISAs) specific for EHV-1 and EHV-4 were used to detect active infections among pyretic horses over several years.

Findings on Infection Rates and Seasonality

• In the pre-updated vaccination years (2018–2021), the peak incidence of EHV-1/4 infections corresponded with August and September, the months before the traditional vaccination start, comprising 15 EHV-1 cases, 9 EHV-4 cases, and 5 co-infections in pyretic horses over three years.
• During the two years of the updated vaccination program (2021-2023), only three pyretic horses infected with EHV-1 were detected in this high-risk period.
• Infection rates in the overall population during peak months previously ranged between 6.8 to 10.9 per thousand (‰) but dropped significantly to 2.6 to 3.8‰ in 2021-2022 and further to 1.2 to 1.7‰ in 2022-2023.
• The reduction in infection prevalence during the critical summer months was statistically significant (p < 0.05), indicating the program’s success.

Conclusions

• Starting vaccination earlier upon yearling arrival effectively closed the immunity gap during the vulnerable pre-vaccination period.
• The updated vaccination schedule significantly decreased the incidence of EHV-1/4 infections and related fevers during the previously high-risk late summer months.
• These findings suggest that vaccination timing is a critical factor for controlling endemic respiratory equine herpesvirus infections in Thoroughbred yearlings.
• The use of a live EHV-1 vaccine induced robust early immune responses, which likely contributed to the decreased viral transmission in the facility.
• Overall, the study supports early immunization strategies as a means to reduce endemic viral respiratory disease burden in horse populations.