A Quarterly Newsletter from Your Equine Health Care Provider | Spring 2013
A Better Understanding of Equine Herpes Virus By: Chris Newton, DVM
IN THIS ISSUE EQUINE HEALTH A Better Understanding of Equine Herpes Virus
Rood & Riddle Equine Hospital and Veterinary Orthopedic Education
Worthy Yearling Given a Shot at Life
NEWS LeBlanc Reproduction Center to be dedicated at Rood & Riddle Rood & Riddle, Again, Takes its Hat Off to Kentucky’s Horse Industry Around The Practice
Clients frequently ask me about the neurologic herpes outbreaks which 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 herpes virus has 4 different types of viruses associated with it. The most common are EHV-1 and EHV-4. These 2 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, depression. EHV-1 can also cause abortion in pregnant mares, and cause 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 2 mutations which make it more likely to cause the neurologic form of the disease. However, the neurologic form does occur less frequently from the nonmutated form of the EHV-1 virus. Once a horse is infected with the virus as a baby they may have recrudescent periods, often associated with stress, where the virus leaves its latent form in the body and enters the blood and then the respiratory tract. If a high enough level of virus is shed, then transmission will occur to others. 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 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 they 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 can also identify the mutated forms of the virus which are more likely to cause the neurologic form which we all fear most. However, the test has a poor positive predictive value. 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.