Equine herpes virus

Farm Forum

Equine herpesvirus (EHV-1, EHV-4) is also known as rhinopneumonitis and has received a lot of media attention in the last two years due to the rare neurologic form it has exhibited at some major equestrian events.

Clinical Signs

Equine herpesvirus has three clinical forms that include respiratory disease, abortion and neurological disease. EHV-1 causes respiratory disease, abortions and neurological disease. EHV-4 causes mainly respiratory disease.

Respiratory Disease

Clinical respiratory herpesvirus infections most commonly occur in young horses, usually weanlings and yearlings. Widespread outbreaks can occur, especially in highly populated and stressful environments. Older horses are important in the outbreak since they can become subclinically infected, but still shed virus. Clinical signs of respiratory disease include fever, coughing and nasal discharge. The discharge is clear, but progression to a yellow thick exudates is common. Incubation period is 2-10 days and outcome is improved by minimization of stress and rest. Testing is needed to discern a herpes infection from influenza.


There are usually no warning signs of abortions which can occur as early as 90 days, with most cases occurring between seven and nine months of pregnancy. (Red bag abortions are common in horses infected with herpesvirus).

Neurological Disease

The neurological form of this disease can be fatal and is the greatest cause of concern at boarding facilities, racetracks, and horse shows. Most horses experience respiratory signs of distress for one to two weeks prior to the development of neurologic signs. Stress may trigger the onset of neurologic signs in which the virus attacks the spinal cord and brainstem. The clinical signs commonly seen are: hind-end weakness, in-coordination, toe-dragging, dog-sitting, as well as urinary/fecal incontinence.

How is Herpesvirus Transmitted?

Transmission occurs via respiratory route with infective droplets obtained from coughing and snorting horses. Shedding of the virus in nasal secretions can occur for 14 days. Contaminated hands and equipment can also spread the virus. With-out disinfection, the virus can actually survive several weeks. All horses have the potential to be carriers of the virus whether or not they demonstrate clinical signs.

What are the methods to confirm a Herpesvirus infection?

Detection of the virus can be done from either a nasal swab or blood by virus isolation. Polymerase Chain Reactions (PCR) tests can look for the virus in nasal secretions, blood or placental and fetal tissue. Demonstration of a relative drop in lymphocyte and neutrophils on a complete blood count and/or a rising titer in serum collected 2-4 weeks apart.


Because EHV-1 is a virus, it does not respond to antibiotics. Supportive treatment is the only option and is tailored to the individual patient, guided by the severity and range of clinical signs. Supportive treatment usually includes anti-inflammatory drugs, anti-viral drugs, and fluids to maintain hydration. In most cases, horses that remain standing have a good prognosis, although recovery may take weeks to months. Horses that go down and are unable to stand have a poor prognosis.


There is NO vaccination currently available that is labeled to protect against the neurologic form of herpesvirus. Vaccination for the respiratory form may not always prevent the disease, but it will decrease the frequency and severity of clinical signs and more importantly, decrease shedding of the virus to other horses. Vaccinate all broodmares at the beginning of the 5th , 7th and 9th month of pregnancy. The American Association of Equine Practitioners recommends the following vaccination schedule for EHV-1 and EHV-4 in the prevention of respiratory disease. The vaccine for rhinopneumonitis (Herpes) is commonly combined with influenza.

Foals/Weanlings: First dose 4-6 months of age, second dose 5-7 months of age, third dose 6-8 months of age, then at 3 month intervals.

Yearlings: Every 3-4 months.

Horses in Training: Every 3-4 months.

Dr. Darin Peterson, DVM, was born and raised on a horse and cattle ranch in Rosholt, S.D., and received his B.S. in Animal Science from SDSU. He concentrates most of his work time with large animals. He can be reached at 701-347-5496 or