Colorado State University

Overview

How it Starts

How it Spreads

Reducing Your Risk

Equine herpesvirus is a highly contagious virus that causes upper respiratory infection in young horses, abortion in pregnant mares, and other damaging problems. Researchers believe that only one or two strains of EHV-1 (equine herpesvirus) have the potential to cause neurological problems, but when one of these strains does strike a herd, the results can be devastating. A severe case can cause a neurological disease that affects the horse's brain and spinal cord resulting in paralysis and death.

There is currently no known method to reliably prevent the neurologic form of EHV-1 infection. Neurological symptoms include incoordination that can progress to the inability to stand, lower leg swelling, the inability to urinate or pass manure, urine dribbling, and reduced tail tone. Some of these symptoms also occur in other neurological diseases such as rabies, EPM, and West Nile Virus infections, so it is important that the animals be examined by a veterinarian as soon as possible.

First exposure to herpesviruses typically happens very early in life, and then horses are constantly re-exposed throughout life as they encounter different strains. Young horses may become sick and then recover uneventfully. Some of those strains are more harmful than others and are more likely to cause disease. So far, researchers have identified nine herpesviruses in equines, but only five are common in domestic horses; the others are found chiefly in other equids, such as zebras, onagers (wild asses) and donkeys. These five viral species fall into two categories, depending on how seriously they affect the horse. The viruses are constantly spreading, and most of the time the infections are mild enough to go unnoticed, but on occasion they will find the right population and the right circumstances to cause serious disease.

When a horse is first infected with a new strain of the herpesviruses, the organisms penetrate the cells that line his respiratory system, sparking an inflammatory response. It can take anywhere from a day to more than a week for that response to cause outward signs, such as nasal discharge and fever. Just as often, however, the horse never becomes ill. Either way, he is capable of spreading the virus, by nose-to-nose contact as well as by expelling airborne droplets during coughs or snorts.

Once the virus enters a horse's body, it's there to stay. All herpesviruses have a unique characteristic; they can "hide" from the immune system so that it is impossible for the body to clear them entirely away. Instead, the viruses change their outward forms and go "latent" in some part of the body, such as lymph glands. While latent, the viruses cause no harm. However when the horse is stressed by such things as weaning, transportation, or fatigue, these viruses sometimes revert to a more active state and revive the infection. The biggest problem with EHV-1 (herpesvirus) is that once horses have been infected, they can become latent carriers of the virus for the rest of their life. Although infected horses are no longer sick or shedding the virus (nasal discharge and fever), they carry the virus and can spontaneously begin shedding it during periods of stress.

EHV-1 is spread primarily through coughing or sneezing, but can also be carried in fetal tissues, the placenta and uterine fluids from mares that have aborted. Studies have shown that the virus doesn't live long in the environment, but transmission via coughing or sneezing can occur over a distance of up to 35 feet. Direct contact with infected horses as well as contaminated feed, equipment, clothing, and tack can also spread the disease. The intermittent shedding of the virus by carrier horses is thought to be the source of sporadic outbreaks of the disease, including closed herds where no new horses have been introduced.

EHV-1 is a virus and does not respond to antibiotics. Therefore, supportive treatment is the only option and is tailored to the individual horse depending on the severity and range of symptoms. It usually includes anti-inflammatory drugs, fluids to maintain hydration, and slinging of horses that are unable to stand. In most cases, horses that remain standing have a good prognosis, although recovery may take weeks or months. Horses that go down and are unable to stand have a poor prognosis.

We strongly encourage horse owners to seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy. After all, it is your horse's life.