Equine Herpesvirus (EHV)

What Is Equine Herpesvirus?

Equine herpesvirus (EHV) is a family of DNA viruses that affect horses. The most common strains that impact horses in North America are EHV-1 and EHV-4. EHV-1 causes abortion and respiratory and neurologic symptoms. EHV-4 is mainly a respiratory disease, but in rare cases may cause abortion or neurologic symptoms as well.

Equine herpesvirus is endemic in the horse population, meaning that most horses have had the disease or have been exposed to it. Mild cases of EHV may even go unnoticed to the horse’s owner. When the virus causes noticeable disease, it is often seen in either weanlings between 4 and 12 months of age (commonly referred to as “foal snots”) or 2-and-3-year-olds in racing or training circuits.

Once horses have recovered from equine herpesvirus, it stays in their body in an inactive state. EHV in horses can become reactivated, especially during times of stress or if the horse received steroids that suppress the immune system. The neurologic form of the disease is often seen in older horses where the virus was reactivated.

Depending on what state you are in, EHV in horses is a reportable disease, meaning your veterinarian needs to let the United States Department of Agriculture (USDA) know that they have diagnosed a case of EHV. The disease can be a highly contagious and appropriate control and prevention measures must be taken.

Symptoms of Equine Herpesvirus

EHV in horses may be identified by the following symptoms:

1. Respiratory

   • Fever (>102 F)

   • Coughing

   • Depression

   • Inappetence (going off feed)

   • Nasal discharge

2. Neurologic

   • Incoordination

   • Hind limb paralysis

   • Lying down and not being able to get up (down horse)

   • Loss of bladder and tail function

   • Loss of sensation to the skin around the tail and hindlimb areas

3. Reproductive

   • Abortion during the last 4 months of gestation

Causes of Equine Herpesvirus

Infection of equine herpesvirus occurs when horses come into direct contact (nose-to-nose) or indirect contact (water/feed buckets, grooming supplies) with another horse that is actively shedding the virus. People may inadvertently transfer the EHV if they are in contact with a horse shedding it and then interact closely with another horse.

Additionally, a horse that is shedding EHV may spread it by snorting or coughing. Airborne virus particles may travel a short distance (i.e. to a neighboring stall). Foals can be infected through the placenta of an infected mare.

Symptoms of the EHV in horses often develop two to five days after infection. Infected horses can shed the virus for up to 14 days.

How Veterinarians Diagnose Equine Herpesvirus

Equine herpesvirus is diagnosed by isolating the virus from nasal secretions and blood using molecular laboratory DNA tests (real-time quantitative PCR). Your veterinarian will take both a nasal swab and blood sample to submit for analysis.

Treatment of Equine Herpesvirus

Treatment for EHV in horses is based on the form of the disease that it is present and the severity. In uncomplicated cases of respiratory disease, anti-pyretics and anti-inflammatories like phenylbutazone and flunixin meglumine may be all that is required.

The goal of treatment is to keep the horse comfortable, hydrated, and still eating. Antibiotics may be added to treat bacterial infections that tend to occur secondary to equine herpesvirus. If the horse becomes dehydrated, intravenous fluids may be administered.

In severe cases, and especially neurologic cases, hospitalization may be required. Horses that cannot remain standing will require intensive care.

Recovery and Management of Equine Herpesvirus

Recovery from uncomplicated respiratory infections of EHV in horses is common and will occur in a few weeks. The prognosis is variable for horses with the neurologic form of EHV. Even with intensive care, recovery is not always possible. Horses that recover should be rested and gradually return to work.

Prevention of Equine Herpesvirus

Vaccines are available to help the prevention of EHV in horses. They are aimed at limiting the respiratory and abortive forms of the disease. Though not 100% preventative, they may reduce the severity and duration of disease.

No vaccines are labeled to prevent the neurologic form of equine herpesvirus. Vaccination recommendations are based on the age and lifestyle of the horse.

• Vaccines are administered to broodmares at five, seven, and nine months of gestation.

• Foals should receive a three-vaccine series starting at five to six months old.

• Horses that are less than five years old, live in close proximity to broodmares, or travel often for racing or shows should be vaccinated every six months.

In addition to strong vaccination protocols, biosecurity, and hygiene measures should be put into place to decrease the spread of the virus. New horses to a facility should be isolated for three to four weeks. Caretakers going between horses should wash their hands and clean any clothes/shoes. Cleaning horse equipment and horse housing areas with a disinfectant is also recommended. A 1:10 bleach solution will inactivate the virus.

It is always important to be cautious at horse shows and other events. This includes preventing nose-to-nose contact with other horses, not sharing water buckets or any equipment, and avoiding or cleaning areas like wash racks or hitching posts before and after use.