Several outbreaks of equine herpesvirus have been reported at large training barns and racetracks around the U.S. in the past year, North Dakota State University Extension Service equine specialist Carrie Hammer warns.

Confirmed cases include horses in North Dakota and surrounding states.

Equine herpesvirus (EHV), also known as Rhinopneumonitis, is common throughout the U.S. It can occur in multiple types, with types 1 and 4 being the most common. EHV-1 and EHV-4 both cause infections in the respiratory tract; however, EHV-1 also can cause infections in the reproductive tract, leading to abortion, and in the central nervous system (brain and spinal cord). EHV-4 infections almost always are limited to the respiratory tract and are a common cause of “colds” in young horses.

Researchers do not know why EHV-1 causes disease in the reproductive tract or central nervous system periodically, Hammer says.

Once a horse becomes infected with EHV, it is infected for life. An estimated 80 percent of the U.S. horse population has been exposed to the virus. After the initial clinical signs, the virus lies dormant, resurfacing during times of stress, such as weaning, trailering, training or competition.

Clinical signs for EHV respiratory disease are a cough, thick nasal discharge, fever and depression. The respiratory form of the disease is by far the most common. Signs of EHV reproductive infection usually occur in late pregnancy, with the loss of the developing fetus at seven to 11 months of gestation.

Infection of the central nervous system results in difficulty walking and moving. The disease affects the hind limbs most often, with the horse adopting a dog-sitting posture or lying down in severe cases. Failure to defecate and dribbling of urine also are signs of EHV central nervous system infection.

Hammer recommends that if horse owners suspect their animal may have EHV, they should isolate the horse from others and call their veterinarian immediately.

Infected horses can shed the virus through the air in nasal secretions and by coughing, so they should be quarantined for a minimum of two weeks, Hammer says. Aborted fetus and fetal membranes are the main source of the virus from aborting mares, so these items should be removed immediately to prevent contact with other pregnant mares. Mares that aborted also should be quarantined for at least two weeks.

Although the respiratory form of EHV generally is uncomplicated, the central nervous system and reproductive forms can be devastating to horse owners. Because the disease is so common in the horse population, complete prevention often is difficult. Therefore, minimizing risk factors becomes very important.

The main risk factors for EHV include increased stress, exposure to other stressed animals and exposure to animals infected with the virus. Because stress can trigger the dormant virus to become active again, minimizing stress is very important. Also, the virus is carried through the air, so confinement in close quarters, such as in a training barn, or in barns with poor ventilation can lead to exposure of multiple animals very quickly. Horse owners should prevent their animals from sharing water buckets or having horse-to-horse contact with unknown horses at shows, rodeos, trail rides or other competitions.

“One of the biggest guns in our arsenal against EHV is vaccination,” Hammer says.

A variety of vaccines are available for protection against EHV-1 and 4. However, while vaccines reduce the risk of infection and severity of clinical signs of respiratory infection, they do very little to prevent the occurrence of the central nervous system form.

The duration of protection from the vaccine is very short-lived; therefore, horses at high risk (those traveling and exposed to multiple horses frequently) should be vaccinated every three to four months. Pregnant mares should be vaccinated with an approved (killed virus) vaccine at the fifth, seventh and ninth month of gestation.

“The bottom line is that EHV is very common and horse owners should do their part to minimize the risk of infection,” Hammer says.

NDSU Agriculture Communication