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Larry G. Martin, D.V.M.

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Equine Herpes Myeloencephalopathy (EHV-1) Fact Sheet

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Equine Herpes Myeloencephalopathy (EHV-1)

(Following this Fact Sheet are several links for up to date information about the EHV-1 virus outbreak in California and general facts about the disease.)

A recent disease outbreak of Equine Herpes Myeloencephalopathy (EHV-1) has been traced to horses who attended the National Cutting Horse Associations’ Western National Championships in Odgen, Utah on April 30 – May 8, 2011. California horses who participated in this event may have been exposed to this EHV-1 virus.

The California Department of Food and Agriculture encourages owners of horses who participated in the Odgen, Utah event to isolate and monitor their horses for clinical signs of disease. A rectal temperature in excess of 102F commonly precedes other clinical signs. Therefore, we are urging owners to take temperatures on each individual horse(s) twice a day. If a temperature above 102F is detected contact your private practitioner immediately. Laboratory submission of nasal swabs and blood samples collected from the exposed horse can be utilized for virus detection and isolation.

The EHV-1 organism spreads quickly from horse to horse and the neurologic form of the virus can reach high morbidity (illness) and mortality (death) rates. The incubation period of EHV-1 is typically 2-10 days. In horses infected with the neurologic strain of EHV-1, clinical signs may include: fever, nasal discharge, incoordination, hind end weakness, recumbency, lethargy, urine dribbling and diminished tail tone. Prognosis depends on severity of signs and the period of recumbency. There is no specific treatment for EHV-1. Treatment may include intravenous fluids, anti-inflammatory drugs and other appropriate supportive treatment.

Currently, there is no equine vaccine that has a label claim for protection against the neurological strain of the virus.

The strain of EHV-1 causing neurological symptoms is a mutation of the EHV-1 virus most often associated with abortion in pregnant mares. There is NO vaccine for this strain. The commonly used 5-way or Flu/Rhino vaccine has EHV-1 antigens (the sensors that alert the body to produce antibodies to a specific disease) in the vaccine but the “load” of antigens is too low for protection in horses challenged by the virus. However, the Pneumobort-K vaccine and Rhinomune vaccine have a high antigen “load” against EHV-1, but still does not protect against the mutated strain of EHV-1. There is some thought that giving the Pneumobort-K vaccine or Rhinomune vaccine with a high “load” of EHV-1 antigen may be of benefit, but this is purely antidotal, and there is no research to support this protocol. Information suggest that both vaccines create similar immune response to EHV-1, however neither vaccine create direct protection to the mutated strain of EHV-1.

The disease is a virus and requires direct contact from horse to horse, or perhaps more commonly owners are contaminating their hands, and then contaminating others horses. Also, the virus can be airborne and transmitted as an aerosol to other horses. Common sense regarding direct contact with other horses should be used.

Horse-to-horse contact, aerosol transmission, and contaminated hands, equipment, tack, and feed all play a role in disease spread. However, horses with severe clinical signs of neurological EHV-1 illness are thought to have large viral loads in their blood and nasal secretions and therefore, present the greatest danger for spreading the disease. Immediate separation and isolation of identified suspect cases and implementation of appropriate management measures (contact your veterinarian for proper management, if you suspect you have a neurological EHV-1 horse) are key elements for disease control.

CONCLUSION: Monitor your horses for illness and/or neurological symptoms. Avoid contact with horses that you may suspect have had contact with the disease, and wash your hands between horses that you are not familiar with. Common sense and contacting your veterinarian if you have symptoms or concerns is the best approach. We encourage you to contact your veterinarian with specific questions.

California Department of Food and Agriculture Daily Update

UCDavis Center for Equine Health White Paper on EHM

EHM Owner Update


USDA Resources

USDA Equine Herpes Myeloencephalopathy Brochure

American Association of Equine Practitioners Fact Sheet Equine Herpesvirus-1 Myeloencephalitis (EHV-1/EHM) Resources

CDFA Equine Herpes Myeloencephalopathy Fact Sheet

USDA Biosecurity Brochure

Link to United States Veterinarian’s Directory

National Cutting Horse Association Website