The Modern Equine Vet
December 2023
Vol 13 Issue 12 2023
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Ask the Infectious Disease Expert | Merck Animal Health

“What’s the latest equine herpesvirus information we need to communicate to clients?”

Equine herpesvirus (EHV) is a complex and significant challenge in equine veterinary medicine. Common misconceptions exist that complicate its management. With clear communication, veterinarians can help clients protect their horses.

One important aspect of educating clients is helping them overcome the misunderstanding that all equine herpesviruses are the same. In reality, there are several types: alphaherpesviruses (EHV-1 and EHV-4) and gammaherpesviruses (EHV-2 and EHV-5).

EHV-1 and EHV-4 present the most common risk for respiratory infection, while EHV-1 presents occasional other signs such as abortion and neurologic disease. Emphasize that EHV-1 and 4 are highly contagious and may cause outbreaks in barns.

Clients should know that both EHV-1 and EHV-4 establish a chronic, persistent infection that goes into indefinite hibernation. Make sure clients understand that this means both viruses are capable of reactivating and returning to the respiratory tract to start a new round of infection and potential spread to other horses.

Don’t overlook the gammaherpesviruses in your practice. Even though they rarely cause disease in adult horses, they are ever-present in the equine population. Infections with EHV-2 or EHV-5—as well as EHV-4—are usually detected in foals, yearlings and 2-year-olds, often causing respiratory tract disease and fever. They also may lead to secondary bacterial infections in that group of young horses.

Vaccination is typically targeted at EHV-1 or a combination of EHV-1 and EHV-4, and it is the most effective way to prevent infection. Remember that a highly infectious strain at a high dose can overrun a horse’s existing immunity, so vaccines don’t provide 100% protection, but can decrease severity of clinical signs as well as viremia.

Significant advances have allowed for earlier and more rapid detection of EHV-1 and EHV-4 infection. If you diagnose a patient with a herpesvirus, explain to clients that quarantine is crucial because of high contagion and virus shedding.

Also stress the importance of testing all in-contact horses to help avoid an outbreak. For in-contact horses, consider post-exposure prophylaxis with antivirals such as valacyclovir or ganciclovir, which is used less frequently due to its cost.

There’s no doubt that equine herpesvirus presents a multifaceted challenge. By understanding the various virus types and taking the time to explain prevention and management tactics, veterinarians can help clients avoid the dangers.



Sharing these succinct facts will give clients a strong basis of knowledge about the most significant equine herpesvirus.

1. Watch for signs
Keep an eye out for the appearance of sudden onset of clinical signs such as fever, swelling in limbs (pairs or all four), gait anomalies and recumbency. These could all indicate an ongoing EHV-1 infection. Isolate any horse showing these signs and start testing.

2. Test in-contact horses
Appearance of clinical signs mentioned suggests an EHV-1 infection occurred seven to 10 days ago and may be ongoing. This means the horse may have been shedding the virus the entire time, exposing in-contact horses. Once EHV-1 infection is confirmed, all exposed and in-contact horses should be tested and isolated.

3. Stay vigilant year-round
Cases of EHV-1 are most likely to occur between November and April, and they are least likely in August; however, the period of vigilance has soft margins. Always watch for clinical ‘red flags’ and alert the veterinarian if any are observed. This allows for timely treatment, isolation and testing of exposed horses.


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Provide your clients with more quick facts about EHV-1 and EHV-4 with downloadable infographics from Merck Animal Health.


Lutz Goehring, DVM, MS, PhD, DACVIM (large animal), ECEIM (European College of Equine Internal Medicine), is the Warren Wright, Sr.—Lucille Wright Markey Endowed Chair in Equine Infectious Diseases at the University of Kentucky Gluck Equine Research Center. His research expertise is EHV-1, specifically its effects, transmission and epidemiology.