Recent equine herpesvirus (EHV) outbreaks around the country have led to great interest in the disease and equally great anxiety among veterinarians and horse owners. In the last five years, EHV has appeared at a variety of race tracks, show grounds, and veterinary hospitals. Such incidents of are tremendous concern as a result of the impact on individual horses, but also due to the need to close facilities in order to prevent the spread of infection. The financial costs to the equine industry caused by infectious disease outbreaks can be huge, affecting both individual horse owners as well as equine-related businesses.

Equine herpesvirus is a common virus that occurs in equine populations worldwide. It has been estimated that up to 70% of adult horses may have the virus in their system, yet not show clinical illness. While there are several strains, the two most common are EHV-1, which causes abortions, respiratory disease and nervous system disease, and EHV-4 which primarily causes respiratory disease. The most common signs of infection with either form are fever, depression and loss of appetite. Cough and nasal discharge are frequently seen as well.

In horses affected with EHV-1, these initial signs may be followed by nervous system disease which is characterized by stumbling and lack of a coordinated gait in the rear limbs. Urine dribbling and an inability to defecate can also be seen. In more severe cases, the horse may be unable to stand. The severity of the nervous system disease is quite variable, and depends upon the animal's general health and immune status, and perhaps factors related to the virus itself. Most patients recover with supportive care, but fatalities do occur. Late term abortions are sometimes seen with equine herpesvirus infection, or foals that are born weak may die within a few days of birth.

EHV is highly contagious and can be readily transmitted from one horse to another. The most common methods of transmission are by aerosol or direct contact. Indirect transmission can occur when the virus contaminates hands, feed and water buckets, etc., which then pass the virus to other horses by contact. Aborted fetuses and associated placental membranes are also significant sources of infection. This rapid transmission requires that once an affected animal has been identified, nearby animals and properties must be quarantined to prevent spread to other horses and farms.

In patients suspected of infection with EHV, there are a number of diagnostic tests that can be performed to confirm or rule-out the infection. Culture of the virus, detection of virus DNA, or testing for an antibody response to the virus have all been employed. The most common method of diagnostic testing and screening at this time is the PCR test, which detects the presence of the DNA of the virus. This test is rapid, and requires only a nasal swab or blood sample. In managing a disease outbreak, it can also be used to determine when horses are no longer shedding the virus, and are safe to return to normal housing and activity. The typical infected horse will usually not shed the virus for longer than 14 days after the fever subsides. Routine cleaning with soap and hot water is typically adequate to kill EHV which lives for only a few days in most environments.

There are methods of minimizing a horse's risk of infection with EHV. Vaccination is commonly employed, and is highly effective against the respiratory and abortion forms of the disease. It is important to recognize, however, that viruses can mutate or change their form, and there is no effective vaccine at this time for controlling the neurologic form of the virus. Control of the neurologic form is therefore dependent upon proactive infection control measures on the farm.

The most effective method of on-farm infection control is to limit horse movement onto the farm, and to minimize co-mingling of new arrivals until a two to three week isolation period has been completed. The isolation facility need not be elaborate but should serve to keep the new arrival physically separate (by 20 feet at least) from other horses on the farm. New arrivals should be attended to after the other horses, and the caretakers should be sure to wash and change clothes after touching the isolated animals. These horses should also have separate feed buckets and water tanks that are cleaned with soap and water daily.

All horses coming to a property should be recently vaccinated against the common equine infectious diseases (including EHV). Requiring a current veterinary health certificate ensures that the horse has been examined prior to transport. In addition to isolation of new arrivals, rectal temperatures should be taken twice per day — fevers should prompt an examination by a veterinarian.

These simple procedures have value in limiting infection from many viruses and bacteria, not just EHV. It must be recognized that they are not foolproof, however. If a horse becomes ill and EHV is suspected, prompt attention from a veterinarian is absolutely necessary. Delaying examination or testing out of fear of the results will only maximize the risks while prompt and forthright action will likely minimize the potential damage caused by the disease.

At the Marion duPont Scott Equine Medical Center, we follow stringent biosafety procedures in order to protect the health of our patients. By practicing similar biosafety practices on the farm, owners can minimize the likelihood that infectious diseases like EHV will affect their horses.

To schedule an internal medicine appointment or consultation at the Marion duPont Scott Equine Medical Center, call (703) 771-6800.

Dr. Furr is the Professor and Adelaide C. Riggs Chair in Equine Medicine at Virginia Tech's Marion duPont Scott Equine Medical Center. He can be reached at mfurr@vt.edu. To view his online profile, visit www.equinemedicalcenter.net. Virginia Tech's Marion duPont Scott Equine Medical Center is a premier full-service equine hospital located in Leesburg, Va., that offers advanced specialty care, 24-hour emergency treatment and diagnostic services for all ages and breeds of horses. One of three campuses that comprise the Virginia-Maryland Regional College of Veterinary Medicine, the center and its team of equine specialists are committed to providing exceptional treatment to patients, superior service to clients and cutting-edge research to the equine industry.