Equine herpesvirus-1 (EHV-1), a significant cause of respiratory and neurological disease in horses, can spread stealthily at equestrian events, often going unnoticed. Nicola Pusterla, DVM, PhD, Dipl. ACVIM, AVDC-Equine, professor of equine internal medicine and epidemiology at the University of California, Davis, School of Veterinary Medicine, recently led research on the role that subclinical shedders—those that do not show signs of infection—play in spreading the virus. He presented his findings at the 2024 American Association of Equine Practitioners Convention, held Dec. 7-11, in Orlando, Florida, and raised concerns about current practices for detecting and managing EHV-1.

Silent Shedding and EHV-1 Environmental Spread

Horses that appear clinically healthy might still shed EHV-1 and contribute to outbreaks, complicating efforts to prevent viral spread. Pusterla’s research team tested 162 apparently healthy horses at a California horse show in April 2022. While only three (1.85%) horses tested positive for EHV-1 using quantitative PCR (qPCR), testing environmental samples from stalls revealed more positive results.

Immediately after the event the researchers collected sponges (specifically used to collect samples) from stalls and identified, among others, five EHV-1-positive stalls in one barn. “This clustering pattern confirms that the respiratory virus can easily spread among horses sharing close quarters and suggests that transmission might occur more subtly than we previously understood,” Pusterla said.

Focusing on EHV-1 Environmental Detection

The research team conducted a second study to evaluate the accuracy of environmental sampling in detecting EHV-1. They collected samples daily for 11 days using nasal swabs, stall sponges, air samples, and stall strips in an eight-stall barn housing seven adult horses aged 5 to 16 years old. Four horses had been vaccinated with a modified-live intranasal EHV-1 vaccine to mimic subclinical shedding; the remaining three horses were unvaccinated.

All intranasally vaccinated horses shed EHV-1 in nasal secretions at levels and for a duration similar to what is expected in subclinical shedders, while none of the controls had detectable EHV-1 in nasal secretions (the EHV-1 vaccine strain is harmless and cannot be transmitted to other horses, hence it does not contribute to the spread of EHV-1). The team used sponges to detect EHV-1 in the stalls of all vaccinated and one control horse. With stationary strips they detected EHV-1 in the stalls of three of four vaccinated horses but none of the controls. Air samples showed low detection rates, likely due to low concentrations of aerosolized EHV-1.

“The highest detection rates came from stall sponges, particularly in feeding areas where droplet contamination from nasal secretions is likely,” Pusterla said. The researchers established that the virus did not necessarily transfer directly between horses but did accumulate in shared and high-traffic spaces such as stall walls and water buckets.

Implications for Biosecurity at Equine Events

Pusterla’s research highlights the importance of implementing biosecurity measures to control EHV-1 at horse shows. He reported that traditional diagnostic methods, such as nasal swabs, are invasive and might miss shedding. Environmental sampling, particularly with stall sponges, could offer a more effective and practical approach to identifying viral clusters and taking preventive measures. However, the study’s limitations include a small sample size and no assessment of viral viability in the environment.

Take-Home Message

Pusterla emphasized the need for vigilant monitoring and proactive strategies to mitigate EHV-1 risks. Farm owners and managers should prioritize good biosecurity practices with their hygiene and cleaning efforts, especially in areas prone to droplet contamination, such as feeding zones and high-traffic areas. Equally important, environmental testing could be a vital tool for early detection and prevention of outbreaks, helping limit the silent spread of EHV-1.