Can Vaccination Protect Horses from Neurologic EHV-1?

In the equine industry three simple letters, when said in order, can silent a room of horsemen, turn a showground into a ghost town, and send shockwaves through barns. They're E, H, and V, and they stand for equine herpesvirus-1, a contagious equine virus that can cause serious neurologic problems in affected horses. Fortunately for owners, veterinarians and researchers are constantly working to learn more about the virus and how to keep horses safe.

At the 2013 American Association of Equine Practitioners' Convention, held Dec. 7-11 in Nashville, Tenn., Lara Maxwell, DVM, PhD, Dipl. ACVCP, presented the results of some recent research that horse owners might find encouraging: One EHV-1 vaccine could have at least some protective properties against the neurologic form of the virus.

EHV-1 is highly contagious among horses and can cause a variety of ailments in equids, including rhinopneumonitis (a respiratory disease usually found in young horses), abortion in broodmares, and myeloencephalopathy (EHM, the neurologic form). Myeloencephalopathy is characterized by fever, ataxia (incoordination), weakness or paralysis of the fore- and hind limbs, and incontinence.

Current vaccines are not labeled for use in preventing the neurologic form of EHV-1—only rhinopneumonitis and abortion—and clinical signs of EHM have been observed in well-vaccinated horses. Therefore, it's remained unclear whether vaccination provides any protection against neurologic disease.

Maxwell, an associate professor of pharmacology at the Oklahoma State University (OSU) Center for Veterinary Health Sciences; Lyndi Gilliam, DVM, an associate professor of internal medicine at OSU; and colleagues vaccinated one group of aged mares with the EHV-1 vaccine Pneumabort K three times at one-month intervals and one control group with saline at the same time intervals before inoculating six control and six vaccinated mares with a neuropathogenic EHV-1 strain. She and colleagues monitored horses for disease and to see if vaccination had any impact on clinical signs.

Key findings included:

  • Control mares exhibited more severe neurologic signs compared to vaccinated mares—five of the six control mares had at least a two-grade change in ataxia scores (a rating of how ataxic, or uncoordinated, a neurologic horse is while walking) compared to one of six test mares;
  • Viral load in neurologic tissues was lower in vaccinated mares than in control mares; and
  • Vaccinated mares' viremia (virus circulating in the bloodstream) levels were lower than those found in control mares.

Other indicators of disease (body temperature, clinical score, and nasal virus shedding) were lower in the vaccinated mares, but as a pilot project this study lacked sufficient horse numbers to confidently state that there was a difference in these other indicators, Maxwell cautioned. She noted that while vaccination appeared to decrease clinical signs of EHM, "the differences were not statistically significant." Nonetheless, "the consistent positive trend for all parameters measured did suggest that vaccination was protective," she said.

So should owners consider adding the EHV vaccine to their horses' annual schedules if it's not already a part of the protocol?

"There are presently epidemiological data suggesting that frequent vaccination might actually increase the risk of neurological disease in some outbreaks, so veterinarians are understandably cautious about recommending frequent vaccination to protect horses from EHM," Maxwell told The Horse. "However, these retrospective studies can be difficult to assess, and it may be that some vaccines are more protective than others. Our positive results should encourage more study of the protective effects of vaccination that would provide more definitive recommendations to guide owners and practitioners."

Of course, she said, it's still best to proceed with caution: "Any drug or vaccine has the potential to cause harm, so I view it as a risk versus benefit question," she explained. "Right now, there is evidence to suggest harm (possible increase in neurologic disease as well as vaccine reactions) and also contrasting evidence to suggest benefit (our study and a study by Goehring et al. 2010 showed a reduction in viremia with vaccination) from vaccination," she continued. "It may be that some vaccines are more effective than others or that we are missing which factors are really important for an individual horse to get sick. Right now, owners should discuss with their veterinarians the best vaccination plan for their individual horse. Vaccination plans should take into account the current evidence, the likely level of viral exposure for that horse, and vaccination requirements of particular equine events."

Maxwell said her group does not currently have additional studies planned on this topic, "but hope that we can continue helping owners and the equine industry to better deal with outbreaks of EHV-1."

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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