Equine herpesvirus-1 (EHV-1) is endemic to the United States. There are 4 manifestations of the disease: respiratory tract disease, abortion, neonatal mortality and a paralytic neurological disease. Horses that recover from the disease may become inapparent carriers and be biological reservoirs to transmit the disease to other horses.

Clinical signs may be nonspecific and a fever of a 102° F or higher may be the only thing seen. The incubation period is 2 to 8 days. Respiratory signs of a cough and nasal discharge may be seen along with the fever. Abortion is almost always seen in the last 4 months of gestation and there are no impending signs. The placenta is expelled with the fetus, which contains a high level of virus. Foals infected are born at term but are obviously sick at birth or become ill with in one or two days of birth. The foals fail to nurse, become lethargic and exhibit severe respiratory distress. Despite intensive supportive veterinary care foals die of viral pneumonia in a few days. Equine herpesvirus myeloencephalopathy (EHM), a neurological disease of horses caused by EHV-1, causes a horse to be weak, uncoordinated and have trouble standing. The rear limbs are more severely affected than the front creating the inability to rise from sitting position. There may be difficulty in urinating and defecating. Non-recumbent horses can have a favorable prognosis but those down longer than 24 hours have a poor prognosis.

The virus can spread through the air, contaminated equipment, clothing and hands. The virus does not last long in the environment and can be easily killed by disinfectant but a cough or sneeze can cause transmission up to 35 feet. Disinfection of premises, stalls and trailers is indicated. Not washing hands after handling a horse with EHV-1 can spread the disease. A solution of 1 part chlorine to 10 parts water is effective for decontaminating equipment and environment after removing gross dirt, debris and manure.

Once EHV-1 confirmation is made, quarantine or isolation measures to restrict the movement of all potentially exposed horses will be necessary to prevent the spread of the disease to other locations. Horses in the immediate contact area of the clinically affected horse should be tested as well as any showing clinical signs.  Isolation of sick horses is important along with early determination of the cause. Isolation of horses with a fever is a good practice in any case. If a horse develops fever, respiratory signs, or neurological signs, a veterinarian should be notified immediately. The horse and horses in the immediate area should not be moved except for the movement of the affected horse to an isolation stall or enclosure located away from high traffic areas.

Vaccination is recommended for all horses. There are several commercially manufactured vaccines for controlling abortion and respiratory disease caused by EHV-1. No current vaccine will protect against the neurological manifestation or EHM. Annual boosters are required. Vaccination, along with good management practices, will decrease the severity of EHV-1 respiratory disease of young horses and limit the occurrence and severity of abortion storms.

NDDA 2014 Equine herpesvirus myeloencephalopathy (EHM) Cases Map


Equine herpesvirus-1 Contacts

North Dakota State Board of Animal Health

600 East Boulevard Ave. Dept. 602 Bismarck, ND 58505-0020
701-328-2655 1-800-242-7535
doa-bah@nd.gov https://www.nd.gov/ndda/animal-health/north-dakota-state-board-animal-health