Your friend’s voice is panicked on the phone. He returned home from a horse show last week and has just been informed that one of the horses there was euthanized yesterday due to severe neurologic disease from equine herpesvirus-1 (EHV-1) infection.
“But your horse is vaccinated, right?” you ask.
“Yes, but they are saying the vaccine may not do any good,” he says.
“Well, what now?”
“We just have to wait and see.”
This is becoming an all-too-common scenario for horse owners. I took a quick look at the Equine Disease Communication Center this morning, and the first five disease outbreak alerts were regarding equine herpesvirus myeloencephalopathy (EHM), or neurologic disease associated with EHV-1 infection, in five states. Equine herpesvirus-1 was first discovered in association with equine abortions in the United States in the 1930s, and it’s a very important cause of equine reproductive loss and foal death. Neurologic signs have also long been identified as an uncommon consequence of EHV-1 infection; however, in the past 20 years EHM has been reported with increasing frequency, especially in the United States.
These cases can be sporadic but can also manifest as large outbreaks that spread from sources at exhibitions or competitions. Horses that develop EHM can be mildly or severely affected. They typically have a fever and neurologic problems primarily affecting the back half of the body, with signs ranging from wobbliness and weakness to complete paralysis. Decreased tail tone and bladder dysfunction are hallmark signs. Veterinarians treat horses with supportive care, based on the clinical picture, because there’s no evidence from controlled studies at this point to guide treatment. Similar to other neurologic diseases such as West Nile virus, horses that go down and are unable to rise on their own have a poor prognosis and commonly must be euthanized.
One of the most frightening things about EHV-1 is that it seems to “come out of nowhere.” The principal reservoir of infection for all equine herpesviruses is latently infected horses—those that carry the virus without showing signs of infection. Most horses have been infected with EHV-1 at some point and do not show clinical signs, or they might only have a fever and nasal discharge. However, in certain situations, the virus can reactivate and spread to other horses, usually by the respiratory route. Susceptible horses can develop serious problems such as abortion or EHM. While horses that travel and have contact with a diverse population of horses are at greater risk of exposure to diseases like EHV-1, the fact that the virus can reactivate means even horses in closed herds can potentially be affected.
So what is a horse owner to do? It would be fantastic if we could reliably prevent infection with vaccination against EHV-1. Unfortunately, while there are several EHV-1 vaccines labeled to protect against abortion and respiratory signs, this virus presents many challenges and no available vaccine provides complete protection. The relative rarity of EHM and problems researchers have reproducing the disease experimentally make it difficult for them to assess vaccine performance. Causing further confusion, recent and/or frequent vaccination against EHV-1 has been identified as a risk factor for EHM development in epidemiological studies. Note that increased age and event attendance have also been shown to be risk factors, both of which could be associated with frequent vaccination. The bottom line is vaccination is not a standalone prevention; rather, your best bet for averting EHV-1 is to employ management and hygiene measures that might include vaccination as recommended by your veterinarian.
Infection control recommendations for EHV-1 are similar to those for other contagious pathogens. The disease is transmissible directly through contact with infected horses and indirectly through contact with contaminated hands or equipment. Practice good biosecurity when you are traveling with your horse. Don’t let him have nose-to-nose contact with horses from other farms, don’t share tack and equipment, and make sure the stall and transport vehicle you place your horse in are clean. Take your horse’s temperature daily when you are at an event, so you know as soon as possible that something is amiss. Wash your hands or use hand sanitizer frequently. If you have a group of horses on your property that travel often and a group that doesn’t, keep the globetrotting horses separate from the homebodies. In doing so, you’ll minimize disease spread if EHV-1 is introduced back into your home population despite your best efforts.
Consult your veterinarian for more infection control tips. While it isn’t possible to protect your horse from everything, you can reduce disease risk by using common-sense precautions.