Equine Herpesvirus and Equine Herpesvirus Myeloencephalopathy

Protect your horse from the risks of EHV-1.

From Pfizer

With the recent outbreaks of the neurological form of equine herpesvirus-1, horse owners should be aware of the risks posed by this disease and methods to help protect their horses from exposure and infection. Equine herpesvirus is extremely contagious, and many horses can become latently infected – infecting other horses while not displaying symptoms themselves. Owners should contact their veterinarian to determine their horse’s risk level for EHV and best practices to help reduce the risk of infection and transmission of this disease.

Equine herpesvirus-1 and EHV-4 are the most likely to create clinical disease, such as respiratory distress, fever, nasal discharge, cough, anorexia and abortions. Recent outbreaks of EHV-1 have also involved an uncommon but devastating secondary disease known as equine herpesvirus myeloencephalopathy, which causes neurologic symptoms such as hind limb ataxia or weakness, decreased tail tone, urinary incontinence and death. The prognosis for recovery for non-recumbent horses is favorable, but poor for horses that remain recumbent for more than two days. EHM occurs rarely with EHV-4 respiratory infections and can also occur with no previous signs of respiratory disease.

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EHV-1 can also cause abortions and stillbirths in pregnant mares exposed to the virus, as well as foals that are born alive but die soon after birth. Abortions usually occur from mid-gestation until term. The virus may be found in the placenta and/or fetus following abortion.

Equine herpesvirus is transmitted through both direct and indirect contact with infected horses and can be spread via water buckets, feed tubs, tack, grooming equipment and even on the hands and feet of people caring for affected animals. In the event of an outbreak, all affected horses should be isolated or removed from the premises to an isolation area. Owners should disinfect all areas where the virus may have spread with a disinfectant that is not neutralized by organic material. Hand washing and foot baths are recommended for anyone with direct contact to infected horses. A quarantine of at least 21 days after the last clinical sign of disease is recommended.

EHV is particularly hard to manage because a horse may become latently infected and not display symptoms for many years after infection. The disease may then be activated by periods of stress caused by training, competition, transportation or other environmental situations.

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While there are no vaccines labeled for the prevention of EHM, horse owners should discuss EHV vaccination protocols with their veterinarian.

Viremia, or the level of the virus circulating in the blood, has a direct correlation with the severity of the disease and clinical symptoms.¹ A vaccine, such as PNEUMABORT K®, has been shown to decrease levels of viremia (the virus circulating in infected horses).² Vaccination should not be performed on horses with a fever or showing signs of clinical disease.

When administered prior to exposure to stressful situations such as horse shows, transportation or exposure to disease, immunomodulators can help stimulate and prepare a horse’s immune system to function more efficiently. ZYLEXIS® from Pfizer Animal Health is a demonstrated safe and effective immunomodulator that stimulates the immune system and helps to reduce upper respiratory disease caused by EHV-1 and EHV-4. As with all vaccinations, owners should discuss their options with a veterinarian. Immunomodulators should not be used in horses with a fever or showing signs of clinical disease.

While vaccination programs and hygiene management practices may not completely prevent the spread of EHV, informed owners can help provide their horses with the best level of protection. Ongoing herd management practices, such as isolating new horses from the main herd, not sharing feed pans or water buckets between horses and thoroughly disinfecting tack and grooming equipment, should be part of a facility’s daily routine, especially during proximity to an outbreak.

1. Lunn, D.P., Davis-Poynter, N., Flaminio, M.J.B.F., Horohov, D.W., Osterrieder, K., Pusterla, N. and Townsend, H.G.G. Equine Herpesvirus-1 Consensus Statement (2009). Journal of Veterinary Internal Medicine, 23 (3) 450-461. Available at: Accessed May 25, 2011.
2. Goehring, L.S., Wagner, B., Bigbie, R., Hussey, S.B., Rao, S., Morley, P.S. and Lunn, D.P. Control of
EHV-1 viremia and nasal shedding by commercial vaccines (2010). Vaccine, 28 (32) 5203-5211.

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