Outbreaks of Neurologic Equine Herpes Virus (EHV-1)
Continue- Florida’s Show Horses Most Recent Victims
Equine Herpes Virus (EHV) most commonly causes a respiratory illness
characterized by fever, nasal discharge, and cough. Clinical disease is most commonly seen in horses that travel, show or are otherwise exposed to a changing population of horses, or under stress. In pregnant mares, exposure to EHV (also known as rhinopneumonitis) may cause abortion. An estimated 80% of horses over the age of 2 years are carriers of EHV, which lies latent until activated by stress. When the virus becomes active, the horse sheds virus in his nasal secretions that is contagious to other horses, even if he does not become ill. Because carriers can both become ill repeatedly, and spread the disease to others, future outbreaks are inevitable in the horse population.
Along with abortions and respiratory disease, EHV can also cause a neurologic
syndrome with weakness, ataxia (loss of balance), and hind end paralysis. A recent mutation in the virus has occurred, causing an increase in the number and severity of cases of neurologic EHV. The new strain of neurogenic EHV appears to be much more contagious, and causes its devastating symptoms due to attacking endothelial cells throughout the body. In 2003, many horses in Ohio at a Findlay riding school were affected with the neurologic form, and in 2004-2005 there were clusters of cases in Canada, Pennsylvania, and KY. Early in 2006, there were multiple premises under quarantine for the neurologic form of EHV in Maryland, West Virginia, Pennsylvania, Michigan, and Kentucky, and restrictions on travel to many states were in effect. In the
winter of 2006-2007, there were 10 stables under quarantine in Florida following the illness of 12 horses, of which 6 died or were euthanized. This recent outbreak
originated with a horse imported from Germany that came through the Newburgh, NY USDA quarantine station prior to shipping to Florida. Several other horses originating in the same shipment also became ill, one in California and one in New Hampshire. The emotional and economic cost of this disease outbreak is extreme, as competitions are cancelled and loved horses succumb. Additional information on the Florida EHV outbreak is available at www.faep.net.
Control of EHV will require rigorous attention. Excellent and detailed guidelines are available on the AAEP website at
Horses that are at risk due to travel, showing or racing should have daily temperatures taken on a routine basis. If a fever develops, nasal swabs and whole blood should be submitted for PCR testing for EHV of the neurogenic strain. In addition, these horses should be isolated from all others, and cared for by people that do not contact the other horses. Some horses develop fevers and shed virus, but do not become terribly sick or develop neurologic problems. However, they do shed high numbers of virus particles in highly contagious nasal secretions. Although the majority of clinical cases recover, 30-50% of these horses are ultimately euthanized. Disinfecting surfaces with a 10% bleach solution or a quaternary ammonium disinfectant is effective in killing the virus. Hand washing and laundering clothes is essential.
Vaccination with killed vaccines does NOT provide protection against the neurologic form of the disease. Recently studies by Dr. Klaus Osterrieder at Cornell
have shown convincing evidence that use of the modified live vaccine, Rhinommune, may provide protection against the neurological form of EHV. Dr. Osterreider’s study
exposed 15 horses to the strain of EHV isolated from the Findlay outbreak. Five of the horses were not vaccinated, five had been vaccinated with a killed vaccine, and five had been vaccinated with the modified live product. Of the groups, only the horses in the modified live group did not show neurologic signs, and the amount of virus shed in nasal secretions was reduced. In addition, the duration of shedding was reduced to 2 days from the 5-7 days observed with the other groups.
We currently recommend vaccination of all horses likely to be exposed to EHV or stressed by their lifestyle, with Rhinommune, a modified live vaccine. Use of
the vaccine every 90 days provides maximum protection, but timing of the vaccine to coincide with likely exposure is another good approach. In pregnant mares, vaccine boosters for "Rhino" need to be given at 3, 5, 7, and 9 months of gestation. The use of the modified live product has been shown to be safe and effective in pregnant mares.
Please do not hesitate to contact the office with your questions and concerns. In addition, the following websites are a helpful source of information: