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What is Equine Herpes Virus (EHV)?

EHV can escape immune system recognition in a horse.
Vaccines do not completely protect horses from the Equine Herpes Virus.
The equine herpes virus dies rapidly in the environment but once safely inside a carrier horse, it can escape immune system recognition.
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  • Written By: KD Morgan
  • Edited By: Bronwyn Harris
  • Last Modified Date: 20 October 2014
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Equine Herpes Virus (EHV) is also knows as Rhinopneumonitis, or simply Rhino. EHV is a highly infectious strain of the herpes virus. The initial symptoms are mild fever, coughing and other signs of respiratory distress. A discharge may also be observed coming from the nose. Advanced or mutated stages exhibit neurological symptoms, such as ataxia: weakness, shakiness, wobbliness and “dog sitting”).

EHV follows the characteristic markers of the herpes virus in that it has the potential to go into a dormant stage and remain in the body throughout the life of your horse. Due to the persistence of the virus, infections travel easily between barns, racetracks, training centers and over state lines.

The virus dies rapidly in the environment but once safely inside a carrier horse, it can escape immune system recognition. When the carrier horse becomes stressed, as in the case of heavy exercise, shipping, injury, other infections or even vaccinations, the immune system can become weakened and allow the carrier to shed the virus. Breeding farms have a high risk of contracting this virus. The show horse and racing horse population are most vulnerable to EHV due to the constant trafficking of these athletes.

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EHV exists in five forms of which EHV-1 and EHV-4 are of most concern. Both of these strains can cause significant respiratory tract disease, mainly in young or immune compromised horses. Abortions in mares and sporadic outbreaks of neurological disease have been observed in more advanced cases. Since 2003, a mutating neurological form of EHV-1 appears to be on the upswing and the outbreaks have resulted in a number of deaths. Adult nonbreeding horses are at greatest risk from the neurological strains of EHV-4.

An outbreak can occur when a shedding horse is in close contact with susceptible horses. Often the carrier horse shedding the virus may not exhibit any symptoms of EHV. The virus is spread through the respiratory tract, infective nasal discharge, placental fluids and aborted fetuses.

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

Barn and horse management is extremely important with EHV. For an active barn, quarantine is essential for any incoming horse for the first six weeks. Stalls should be completely disinfected before and after the occupation of each stall.

Horses at highest risk should be kept strictly isolated from horses that travel in and out of your barn. When away from home, never allow your horse to share equipment, buckets and feeders. If it is necessary for your horse to be stalled in an unfamiliar environment, muzzle your horse to protect him from being exposed to anything that may 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 the symptoms. It is best to avoid antibiotics unless complications or secondary infection are present. It is best to quarantine the barn for 30 days after the last signs of EHV have elapsed. EHV is not prone to frequent major mutations, so a peaceful coexistence may develop between your horse’s immune system and the virus.

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