Equine Herpesvirus Cliffs Notes

Equine Herpesvirus-1 has been reported in several states causing alarm in equine communities. EHV-1 is highly contagious among horses and can cause a variety of ailments in equines, including respiratory disease, abortion in broodmares, and myeloencephalopathy (EHM, the neurologic form).Here are the basic facts you should know about Equine Herpesvirus:

Overview of Herpesvirus

Herpesviruses are a species-specific family of DNA viruses. They are characteristic because of their ability to cause life-long infection by becoming latent. The virus is reactivated in times of stress, such as shipping, competition, etc. This results in re-infection and transmission of the virus to susceptible hosts. Herpesviruses cause disease by damaging a host’s blood vessels.

Herpesvirus in the Horse

Herpesvirus is common worldwide in the horse. A majority of the time the virus is dormant and does not cause illness. The most common sign of Herpesvirus is respiratory disease. While there are 5 subtypes of Equine Herpesvirus (EHV), 3 are considered health risks for horses (EHV-1, EHV-3, and EHV-4)

• EHV-1: respiratory disease, abortion, neonatal death, neurologic disease

• EHV-2: pharyngitis and lymphoid hyperplasia (respiratory disease affecting the upper respiratory tract)

• EHV-3: causes equine coital xanthema, a benign venereal disease affecting the external genitalia

• EHV-4: respiratory disease (uncommonly causes abortion, neonatal death, neurologic disease)

• EHV-5: respiratory disease

Equine Herpesvirus Myeloencephalopathy (EHM)

Infection with a neuropathogenic strain of EHV-1 causes neurologic disease by damaging the blood vessels of the brain and spinal cord. Altered blood flow injures the tissues, impairing their ability to function normally.

Typical signs of EHM:

• incoordination
• weakness
• recumbency or inability to rise
• dribbling urine or inability to urinate
• decreased tail tone
• fever (occurs before other signs)

Transmission

The virus is shed in respiratory secretions, typically via coughing and snorting, and infected aborted fetuses and fetal membranes. Horse to horse contact is the most common way the disease is transmitted, but indirect transmission via contaminated lead ropes, tack, buckets, trailers, and people (hands and clothing) also occurs.

Diagnosis

Polymerase Chain Reaction (PCR) is a test that detects the viral DNA in the blood or on a nasal swab. It can also be performed on neural tissues to confirm the diagnosis during a post-mortem examination. An ELISA test measures the antibodies to the virus in the blood. The levels should be measured 2-3 weeks apart.

Treatment

Treatment is primarily directed to relieve symptoms, including anti-inflammatories. In some cases antiviral drugs may be used.

Prevention

Biosecurity is essential in order to prevent horses’ exposure to the virus:

• isolate horses with clinical signs and seek veterinary advice
• isolate horses in contact with EHV-1 positive cases and take temperature twice daily if possible
• proper hand washing
• disinfect equipment between horses
• minimize contact between horses if traveling off-farm

Vaccinations are recommended. Commercial vaccines for EHV-1/4 are available, but are not labeled to prevent the neurologic form of the disease

Take Home Message

Herpesvirus is common world-wide and causes respiratory, abortion and occasionally neurologic signs. The key to prevention is appropriate vaccination and management. Horse owners should consult with their local veterinarian for recommendations on an appropriate vaccination schedule and the risk in their area. For additional information, we recommend these resources:

Biosecurity for the Traveling Show Horse – Rood & Riddle Equine Hospital

Equine Herpes (EHV) Myeloencephalopathy – A Guide to Understanding the Neurologic Form of EHV Infection – USDA