The equine herpesviruses are found worldwide and they have been around for a long, long time. The role of these herpesviruses in causing disease in the equine family is of a very complex nature and not fully understood by researchers, let alone the lay community.

With these facts in mind I will try and explain to you, as simply as I can, the effect these viruses can have on the health of your horses and mules.

Equine herpesvirus is commonly abbreviated (EHV) and there are five distinct types. They are therefore listed: EHV-1, EHV-2, EHV-3, EHV-4 and EHV-5. These abbreviations save much writing or printing and identify each type of the virus.

EHV-1 is associated with respiratory problems in the horse but primarily will cause abortion in mares and will rarely cause neurologic disease in the equine. EHV-2 can be isolated from most horses but has not been found to cause any particular disease in them. The same can be said for EHV-5. EHV-3 is the cause of a venereal disease known as coital exanthema and is sexually transmitted. EHV-4 is known to be a primary cause of respiratory disease in the horse as well as being responsible for abortion in pregnant mares.

As is the case with all herpesviruses, the EHV have the ability and can establish a latent infection in its host and reappear during the lifetime of the animal. A horse experiencing this reactivation phase may not produce antibodies and may shed the virus thus serving to spread the disease and become a source of infection for others. Causes of this reactivation process are stress associated factors such as long distance transportation, pain, debilitating conditions and even vaccination against other viruses and diseases. Therapeutic doses of corticosteroids may also cause reactivation of the virus in the host animal.

The horse owner and trainer has to contend with just two of the five types of EHV, namely EHV-1 and EHV-4. Let’s examine what these viruses can do to your horses.

EHV-1 and EHV-4 both can cause respiratory disease in horses. EHV-1 is a less common cause but is the more virulent of the two and spreads more rapidly through the herd. EHV respiratory disease complex often starts with a mild rise in temperature, loss of appetite, lethargy, a clear nasal discharge, an occasional cough and possible edema of the hind limbs. There may or may not be enlargement of lymph nodes in the mandible and throat regions. This latter swelling of the lymph nodes may confuse the issue with that of strangles, especially since the clinical respiratory form of the disease often occurs in young animals; as its also a fact concerning strangles. When the infection occurs in older horses it is often sub-clinical and very mild. This respiratory form of herpesvirus infection will often last several months in a large stable of horses and pass slowly from animal to animal especially where EHV-4 is involved. The duration of the respiratory form is often prolonged by secondary infections of the upper respiratory system. An occasional abortion may occur due to EHV-1 or EHV-4; however multiple abortions that occur on premises are usually due to EHV-1, especially where racehorses and young stock are the primary source of infection. Most mares will abort during the last trimester of the pregnancy. A few mares may abort as early as four months in their gestation period. These aborted fetuses are loaded with virus and become a great source of infection for other mares on the same premises. Occasionally one of these infected mares will give birth to a live foal which will be weak, jaundiced and have an acute respiratory problem. The foal will usually die in a few days. These foals are a source of infection for others as they are usually heavily infected with the virus.

EHV-1 myeloencephalitis was first described in the literature 60 years ago. Since then, outbreaks of this disease have occurred with ever greater frequency world-wide and in the United States. The disease not only affects horses, mules and donkeys but has been found in llamas and alpacas. In this form of the disease the EHV-1 virus affects the brain and spinal cord of the victim. The disease almost always occurs with or following the respiratory form of infection, having an incubation period of four to seven days.

Any animal can be infected. However, pregnant mares, mares that have just foaled and younger horses seem to be the most susceptible. The literature reports that affected pregnant mares rarely do not abort but other exposed mares will abort while showing no symptoms of this “nervous form” of the disease.

The source of the infecting virus may be a newly acquired animal or the reactivation of EHV-1 in the patient’s body due to stress from foaling, castrating, training, working, transportation or the administration of corticosteroids. Again, the carrier animal that shows no symptoms of the disease, but sheds the virus, is probably the most important source of the infection.

So what are some of the clinical signs or symptoms which are seen in a horse with the neurologic form of EHV-1 infection? The very first sign is the rapid onset of symmetric ataxia with the rear limbs often exhibiting the greatest amount of ataxia and incoordination. The head may be tilted and the tongue may protrude from the mouth and appear to be weak. The horse will exhibit a very staggering gait and show evidence of urine dribbling. Edema of the limbs is common especially in the rear legs. Scrotal edema may be seen in stallions. Geldings and stallions may also exhibit penile prolapse. Other symptoms such as tail weakness and the loss of perineal sensory factors are seen. Other signs may include bladder distention, with accompanying urine and fecal retention.

As a rule these symptoms will develop very quickly for up to 48 hours before leveling off. In this period of time, if the animal goes down, the mortality rate is very high. If the patient can stay on its feet for two or three days its chance for recovery is good. Some horses will return to normal in three to five days. Others may take several months to undergo full recovery.

The diagnosis of EHV-1 and EHV-4 outbreaks that are mild and respiratory in nature is somewhat difficult. Laboratory examination of blood samples drawn from the patient at the onset of the disease and 10 to 14 days later are very helpful. If the animal has a history of no recent vaccinations, the blood testing can indicate a virus infection. Nasopharyngeal swabs collected during the acute respiratory stage of an infection are very helpful in making a diagnosis as the virus can be isolated and cultured from the samples.

EHV abortion cannot be diagnosed except by postmortem examination of the aborted fetus or foal. To really diagnose the cause of an abortion, stillbirth or death of a foal, the remains should be properly submitted to a diagnostic lab within 10 days of its birth where the proper procedures can be implemented to recover the virus or test for its presence. Virus isolation is very important since the virus can survive for several weeks in a damp or moist environment. There are many, many causes of abortion in the mare and it behooves the owner or manager of the animals to determine the cause, should an abortion occur.

If a horse exhibits an acute onset of neurologic symptoms, especially those which show hind leg involvement, the possibility of EHV-1 infection should always be considered. These symptoms usually do not appear until 10 days after exposure to the virus so blood testing for high antibody titers would be the thing to do. It is always recommended to process a second blood sample taken 10 days later to check the titer. If the patient dies or is euthanized, a diagnosis can be made in a laboratory by isolating the virus from brain or spinal cord tissue which has been sent in. These neurologic cases of EHV-1 must be differentiated from E.P.M. (so called "Possum Disease") and encephalitis caused by the West Nile virus and the Eastern, Western and Venezuelan encephalitis viruses. In many of these neurologic virus infections the afflicted horses will often exhibit symptoms which are common to all of these diseases.

The treatment of EHV-1 and EHV-4 infections is best left in the hands of able professionals. The veterinarian should be able to make an accurate diagnosis of the condition and prepare a correct treatment for its resolution.

Today there are a number of vaccines on the market containing EHV-1 and EHV-4. No vaccine on the market today will protect against the myeloencephalitis caused by EHV-1. In fact, “because it has been suggested that this condition may result from an inflammatory response to virus antigen, vaccination in the face of the neurologic form of the disease is not recommended by the vaccine’s manufacturers.”

Immunity will not last over three months in the horse who has recovered from the respiratory form of the disease caused by EHV-1 and EHV-4. The best program for prevention of this respiratory form of the disease is to vaccinate every two to three months, those horses who will be exposed to other horses, with a vaccine containing only EHV-1 and EHV-4 as it is not necessary to vaccinate so often against influenza.

Prevention of abortion in mares from EHV-1 and EHV-4 depends on vaccination and very good management of the mare herd. Vaccines containing only EHV-1 and EHV-4, the so called “rhino shot,” should be used and the manufacturer's directions should be followed exactly. Good management will dictate that all pregnant mares be kept separate from other horses. If this cannot be done the other horses should be vaccinated. If a mare should have a herpesvirus abortion she should be kept from others and the rest of the pregnant mares should be kept on the premises until they have foaled. To keep the disease localized no horse should leave the premises for one month after the abortion. Of course, on the average farm, this is nearly impossible to accomplish, hence the disease can be spread.

In many states it is mandatory that a case or cases of EHV-1, involving the nervous system, should be reported. The stables are then quarantined and no horses are allowed on or off the property. Often these quarantined horses are placed in small groups to facilitate their treatment and observation. Handlers must change their clothing and disinfect themselves when leaving the premises or when going from one isolated group of animals to another. Horses are not allowed to leave these areas until it has been determined that they are free of serologic or virologic evidence of the EHV-1 infection. Horses that have recovered from the disease can spread the virus for several weeks and must be kept in quarantine until it is determined that they will no longer shed the virus and perpetuate the disease.

In recent years there have been some severe outbreaks of EHV-1 myeloencephalitis. State and Federal governments have reacted strongly to curtail the spread of the virus by quarantine methods and strictly enforcing the movement of these animals. In some states this is a reportable disease, in others it is not. As of January, 2007, there have been outbreaks of this disease in Florida, New Jersey and California. Several years ago there was a severe outbreak of EHV-1 myeloencephalitis in Ohio.

Four years ago I experienced a bout with EHV-1 neurological infection in one mare out of a herd of animals. When I first saw her she was ataxic but could eat and drink well. She periodically would become unmanageable, smashing through fences, tearing herself up and appearing absolutely crazy. One could not control her but she would eventually become calm, or have to be sedated. On the third day she was euthanized and it was the postmortem laboratory findings that diagnosed the disease. No other animals in the herd were involved.

The best advice I can give you is to watch your stock closely. Be very wary of long term upper respiratory problems which may arise. If you should have a mare abort, clean and disinfect the area, isolate the mare and have the aborted fetus sent to a laboratory for examination and a possible diagnosis. Last but not least, if a horse, mule or donkey exhibits any neurological symptoms, don’t just assume it is EPM. The single neurological case can appear suddenly out of nowhere. Make sure a proper diagnosis is made, even though it is by post-mortem examination.