What’s equine herpes virus (EHV)?

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Equine herpes virus (EHV) is a highly infectious strain of the herpes virus that can cause respiratory distress, neurological symptoms, and abortions in mares. EHV-1 and EHV-4 are the most concerning strains. Outbreaks can occur when a carrier horse sheds the virus, and vaccination does not completely protect against infection. Quarantine, disinfection, and isolation are important for managing EHV. Treatment depends on the degree of symptoms, and it is best to avoid antibiotics unless complications or secondary infections are present. EHV does not undergo frequent major mutations, so a peaceful coexistence may develop between the horse’s immune system and the virus.

Equine herpes virus (EHV) is also known as rhinopneumonia, or simply rhinoceros. EHV is a highly infectious strain of the herpes virus. Initial symptoms are low-grade fever, cough, and other signs of respiratory distress. A discharge coming from the nose may also be observed. Advanced or mutated stages show neurological symptoms, such as ataxia: weakness, tremors, staggering and “dog sitting”).

EHV follows the characteristic markers of the herpes virus as it has the potential to enter a dormant stage and remain in the body for your horse’s life. Because of the persistence of the virus, infections travel easily between barns, racetracks, training centers and across state lines.

The virus dies quickly in the environment, but once safely inside a carrier horse, it can escape recognition by the immune system. When the carrier horse becomes stressed, as with strenuous exercise, shipping, injury, other infections, or even vaccinations, the immune system can weaken and allow the carrier to clear the virus. Farms have a high risk of contracting this virus. The show and race horse population is most vulnerable to EHV due to the constant traffic of these athletes.

EHV exists in five forms, of which EHV-1 and EHV-4 are the most concerning. Both of these strains can cause significant respiratory tract disease, primarily in young or immunocompromised horses. In more advanced cases, abortions in mares and sporadic outbreaks of neurological disease have been observed. Since 2003, a mutant neurological form of EHV-1 appears to be on the rise, and outbreaks have resulted in numerous deaths. Adult non-breeding horses are at greatest risk for neurological strains of EHV-4.

An outbreak can occur when a moulted horse is in close contact with susceptible horses. Often the carrier horse that sheds the virus may not show any symptoms of EHV. The virus spreads through the respiratory tract, infectious nasal secretions, placental fluids, and aborted fetuses.

Some career horses are vaccinated every 3-4 months. There is some suspicion that this frequent vaccination schedule is contributing to the mutation of neurological and more virulent strains of EHV. Although vaccines do not completely protect against infection, they do reduce the level of virus shed by a symptomatic horse.

Stable and horse management are extremely important with EHV. For an active stable, quarantine is essential for any incoming horses for the first six weeks. Stables must be thoroughly disinfected before and after each stall is occupied.

Higher risk horses should be kept strictly isolated from horses entering and leaving the stable. When you are away from home, never allow your horse to share tackle, buckets and feeders. If your horse needs to be stalled in an unfamiliar environment, muzzle your horse to protect it from exposure to anything that might be contaminated with EHV. As with all good equine care, minimize stress and maximize solid nutrition.

For simple cases, isolation and stall rest will allow the virus to run its course in 3-4 weeks. Treat according to the degree of symptoms. It is best to avoid antibiotics unless complications or secondary infections are present. It is best to quarantine the stable for 30 days after the last signs of EHV have passed. EHV does not undergo frequent major mutations, so a peaceful coexistence may develop between your horse’s immune system and the virus.

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