Equine Herpesvirus


There has been much concern and discussion in the past few years and particularly the past few months surrounding the neurological form of Equine Herpesvirus (EHV) also known as “Rhinopneumonitis”. There have been numerous cases in the past year in Kentucky and on the East Coast and more recently in Wisconsin and South Dakota. This article will hopefully shed some light on this disease and what it means to you.

What is EHV?

Equine herpesvirus is a viral infection specific to horses which manifests in three different ways – a respiratory form, a reproductive form, and a neurological form. There are five different strains of the virus although EHV-1 and EHV-4 are the two most likely to cause clinical disease. Both are worldwide in distribution and constitute a health risk for domestic horses of all ages.

Both EHV-1 and EHV-4 cause respiratory tract disease characterized by fever, cough and nasal discharge. Although infection with EHV-1 is often confined to the respiratory tract, infection may spread beyond the respiratory tract to cause more serious disease, characterized by abortions, newborn foal death, or neurological disease.

Severity of clinical signs varies depending on age and the general health of the infected horse, but tends to be worse in younger animals. Two and three year old racehorses are particularly susceptible due to their age, exposure level and stress levels.

Previously exposed horses older than 3 years of age continue to show evidence of re-infection by EHV-1 throughout their lifetimes with only minimal or no clinical signs of respiratory tract infection. In such animals, however, the risks for the more serious consequences of abortion and/or neurological disease following low grade respiratory infection by EHV-1 continue to exist. The neurological form of EHV-1 is the concern in the recent outbreaks and the focus of this article.


Horses infected with EHV-1 often present with a fever that may spike to 102 –106?F. The fever is generally biphasic meaning that there will be a fever during the first 1-3 days after infection, which resolves and is followed by a second fever spike at 5-8 days after infection. Other symptoms include lethargy, cough, nasal discharge, and ocular discharge. Secondary bacterial infections may occur due to a compromised immune system.

Abortions may occur, usually in late gestation (after 7 months), and affected mares do not always show clinical signs of infection before aborting. Uterine infections may lead to premature deliveries and unhealthy newborns, or infected newborns may appear normal at birth, but within the first week become weak and lethargic. Clinical signs in these foals may include respiratory problems, pneumonia; tachycardia, and diarrhea. Internal organs, including the liver, may be damaged. The prognosis is poor for these foals.

Infection with EHV-1 may also cause neurological disease in infected adult horses, but this is less common. Neurological disease associated with EHV-1 can range from mild incoordination and weakness to severe hind limb paralysis. The most severely affected animals may die or may need to be euthanized. Previous respiratory infection, either in the affected horse or in surrounding horses, may or may not be present. One of the earliest symptoms to appear in the neurological form is loss of tail and anal tone. Early identification of these symptoms along with early treatment may help lessen the severity of the disease.

The neurological symptoms result from viral damage and inflammation to the small blood vessels in the spinal cord and/or brain. The interval between initial EHV-1 infection of the respiratory tract and the onset of neurological signs is 8 to 12 days. The neurological deficits appear suddenly and reach their peak intensity within 48 hours. The neurological form of the disease is resistant to vaccination, the clinical attack rate is high, and the fatality rate is high.


Clinical signs of equine herpes are similar to other respiratory diseases in horses, so it is difficult to make a definitive diagnosis from clinical signs alone. Blood samples and nasal or throat swabs taken during the febrile stage of respiratory tract infections may identify the virus. Aborted fetal tissues may also aid in diagnosis of EHV-1.

A major advance in understanding EHV was the recent discovery that a single gene mutation is the cause of the neurological form. This discovery has led to a genetic test for identifying latent equine carriers of mutant, neuropathogenic strains of EHV-1.


There are no specific treatments for EHV. Treatment of the respiratory forms is generally based on symptomatic treatment (i.e. fever reduction), rest, and stress reduction of affected horses. Stalls should be well-ventilated and as dust-free as possible.

Immunomodulating drugs help to boost the horse’s immune system and may also help reduce inflammation. Antiviral drugs are available, but they are very expensive and non-specific. They are typically used at the outset of neurological symptoms. Potent anti-inflammatory drugs are also used for neurological symptoms. Antibiotics may be indicated in cases with a secondary bacterial infection.


In most cases, following infection with EHV-1 or EHV-4, the virus becomes latent, surviving in the horse forever, but not causing signs of disease until the animal is stressed. In response to stress (weaning, transport, social disruption, etc.), the virus may be reactivated and transmitted to other susceptible horses.

Direct contact with virus particles is the most common means of transmitting the disease between horses. Transmission through indirect contact is possible, but less likely. Therefore, horses that are in close nose-to-nose contact with infected horses, infective placentas, or infective aborted materials are more likely to become infected themselves than horses that are kept isolated. Recovered horses may continue to shed virus for up to 3 weeks before it becomes latent, and the virus may survive in the environment for up to 2 weeks in certain conditions.


Prevention is based on isolation procedures and implementation of a regular vaccination program. New horses, horses that have been traveling, and sick horses, should be isolated from resident horses, particularly pregnant mares, for up to six weeks. Pregnant mares should be kept in small groups, and new arrivals should not be added to these established groups. These mares should be isolated from all other age groups, and mares carrying their first foals should not be mixed with older mares.

New horses should have their temperature taken and recorded twice daily for 3 weeks. Any horse found with a temperature >101.5oF should be isolated and examined. If a horse presents clinical evidence suggesting a herpes infection, diagnostic samples (nasal swabs and whole blood) are collected and submitted for testing.

If a herpes virus is detected or it is believed the diagnosis is highly probable, the animal is isolated from other horses to minimize risk. The remaining horses housed in the same barn are quarantined and tested by PCR. Results of the initial test are used to determine the current disease status of the barn. After the initial assessment, testing of the barn’s population is conducted at 7 to 12 day intervals to monitor the disease progression.

Steps for isolating the infected horse should be as follows:

1) Limit people in the barn to only necessary individuals. Anyone entering the barn is required to wear a protective outer covering, disposable gloves, rubber boots (covers), etc. When exiting a restricted zone, foot baths are placed at the door for disinfecting the outer foot wear. All disposable gloves, boot covers, etc are left in the barn in sealed containers.

2) Minimize the use of shared equipment. Water buckets, lead ropes, etc should not be shared by horses. Other equipment such as twitches, shovels, forks, etc are disinfected daily and between each use.

3) Care should be taken when filling water buckets and feed troughs. Neither the hose nor the feed scoop should have contact with the bucket or trough.

There is great controversy over vaccination. No current vaccine has been demonstrated to protect against the neurological manifestation of EHV-1 infection. When used as an adjunct to sound management practices, vaccination may be helpful in reducing severity of EHV-1 and EHV-4 respiratory disease in young stock. Vaccinated animals may still become infected, and are able to shed virus to other horses in nasal secretions, but the severity and length of illness are usually shortened by vaccination and by reducing the amount of circulating virus, it may help reduce the spread.

It is uniformly agreed that pregnant mares should be vaccinated at 5, 7, and 9 months of gestation with a killed EHV-1 vaccine to limit the occurrence and severity of abortion storms in broodmares. Foals should be vaccinated at 3-4 months of age, then 4-8 weeks later with EHV-1 and EHV-4.

Horses that travel or are in boarding facilities with horses that travel may benefit from EHV-1 vaccination. It has not been recommended routinely in our practice due to the rarity of EHV infection in this area and the relative ineffectiveness of the vaccine. However, in light of the outbreak as close as Madison, Wisconsin, we do have vaccine available for those horses that may be at higher risk of exposure.

There are multiple vaccines available against EHV-1 and EHV-4, usually in combination with influenza vaccine. Most are killed virus intramuscular vaccines although there is one killed virus vaccine which is intranasal (Calvenza®). There is one modified live virus vaccine (Rhinomune® by Pfizer) which was shown in one limited study to be more effective at reducing the neurological symptoms though that is controversial.

The protocols for timing of vaccination have also been controversial. The EHV respiratory vaccines produce a very short-lived immune response, so previously horses have been vaccinated 2-6 times per year. There is some thought that the more frequently vaccinated horses are more susceptible to the neurological form.

Our current recommendation for traveling horses or those exposed to traveling horses is to vaccinate with EHV-1/4 vaccine, After the initial 2-shot series, it should be given twice a year.

Remember that EHV has been around forever and will probably be around forever. There has been panic recently due to the proximity of the disease outbreaks to us; however, the common sense and the recommendations above should help to prevent widespread outbreaks.

Dr. Lisa Borzynski prepared March 2006