Clients frequently ask me about the neurologic herpesvirus outbreaks that occur from time to time at horse shows. There has been a tremendous amount of misinformation circulating about the disease, and the "outbreaks," so I hope to quickly clarify the disease and how we can best deal with it as an active sport horse community.

Equine herpesvirus has four main types of viruses associated with it. The most common are EHV-1 and EHV-4. These two viruses commonly (over 90% of the population) infect horses in the first few months of life. This infection occurs in spite of vaccinating mares prior to foaling, so the foals have strong immunity from mom. They generally cause respiratory disease with fever, nasal/ocular discharge, mild cough, vasculitis and depression. EHV-1 can also cause abortion in pregnant mares and equine myeloencephalopathy, which is the neurologic form of the disease. This form has a fever followed by weakness to paralysis, often worst in the hind end, and potential death.

The EHV-1 virus has been shown to have two mutations that make it more likely to cause the neurologic form of the disease. However, the neurologic form occurs less frequently from the non-mutated form of the EHV-1 virus. Once a horse is infected with the virus as a baby, it may have recrudescent periods, often associated with stress, where the virus leaves its latent form in the body and enters the blood and the respiratory tract. If a high enough level of virus is shed, then transmission can occur to other horses. This transmission is generally from nasal secretions in the air or on carrier objects. The virus does not live long out of the body, and it is easily killed with a 10% bleach in water solution.

The general incubation period from infection to clinical signs is 3-8 days, with 14 days being the outer limit. If a horse is to develop neurologic herpes it will generally show signs 8-12 days after infection.

It is possible to test for EHV-1 in the blood and nasal secretions. A PCR test will identify DNA of the virus, and it can also identify the mutated forms of the virus that are more likely to cause the neurologic form, which we all fear most.

However, the test has a poor positive predictive value (they can be positive without showing signs of disease). This is because most horses are carrying the virus in its latent state, and some become intermittently viremic, but never show illness or affect those around them. Therefore, a positive test is only useful in horses showing clinical signs of the disease, and it would likely result in the elimination of healthy horses if used as a screening tool.

We therefore have to look at protecting our equine athletes, as it appears that eliminating the potential of exposure with a screening test is not currently possible. There are three types of vaccines currently available to prevent equine herpesvirus. Unfortunately these vaccines make no claim to prevent the neurologic form, only the respiratory and abortive forms. The types of vaccines are killed single component, killed multiple component and modified live.

Initial vaccination should involve a series of three shots 3-4 weeks apart. Non-pregnant show horses should receive boosters every six months, and if they are entering an outbreak type situation, they should be boostered 7-10 days prior to exposure if it has been longer than 60 days since the last booster.

These vaccines have not been shown to prevent the disease consistently, but studies have shown they diminish the amount of virus shed in the nasal tract, and one study showed a diminished incidence of the neurologic form of the disease when exposed after vaccination. Vaccines are not perfect, but they provide one component of prevention.

It is is easy to see why everyone is confused about the disease. We can't reliably screen for it, we can't reliably vaccinate against it, and our best tool for early identification is a fever.

How many of you take your horses temperature daily at competitions? Based on my experience, that number is rather low. However, practicing good biosecurity is likely the best way of preventing infection. Unfortunately the chain of biosecurity is only as strong as its weakest link, and that is the owner of the horses who do not take temperatures daily. We all have to show each other respect by monitoring temperatures, having horses with fevers evaluated by a veterinarian, and keeping those horses away from others. Vaccinating will help diminish spread, but not necessarily disease. Keeping our horses happy and healthy takes an entire community.

Hopefully this article will improve the understanding of equine herpesvirus, and the feared neurologic form. This disease is endemic in the U.S. horses, and in spite of significant research and effort, we continue to have intermittent cases at racetracks, horse shows and areas of commingling. We can help by vaccinating appropriately, but biosecurity monitoring and practice can also help significantly. This is not easy, and with the irregularity of the disease, we easily return to old habits.

Edited and reprinted with permission of Rood and Riddle Equine Hospital.

Chris Newton, DVM