HORSE MANAGEMENT ARTICLES
THE FACTS: EQUINE
HERPES VIRUS TYPE 1 DIAGNOSED IN VIRGINIA
Joseph P. Garvin, D.V.M.
Director/Diagnostician Warrenton Laboratory
Virginia Department of Agriculture and Consumer Services
Warrenton, Virginia
Amid erroneous reports and rumors of a "mysterious illness" caused by a "mutant" strain of virus, an outbreak of an uncommon form of disease caused by equine herpesvirus type 1 (EHV-1) at a boarding stable in Middleburg last month highlights the difficulties in dealing with this common equine disease agent.
During the week of April 19 through 26, 2002, three horses out of the 50 residents at a boarding stable in Middleburg, Virginia, developed sudden profound incoordination of the rear limbs. In addition, all three had large bladders and a lack of tail tone, and one horse was incoordinated in the front limbs and had a head tilt. Each of the three horses eventually went down and was
euthanized. Early the next week, two ponies on the farm demonstrated mild neurologic signs, but each made an uneventful recovery.
Based on the clinical signs he observed, Dr. Norris Adams of the Middleburg Equine Clinic made a preliminary diagnosis of equine
herpe svirus yeloencephalopathy. The farm established a strict quarantine regimen in the barn, and all the other horses were monitored for fever or other signs of disease. Two of the three euthanized horses were sent to the Virginia Department of Agriculture and Consumer Services. Warrenton Animal Health Laboratory for necropsy and further testing. Tests for other agents causing neurologic disease in horses, such as equine protozoal
myeloencephalitis, Eastern equine encephalitis, West Nile virus and rabies, were all negative.
EHV-1 was isolated from a blood sample taken by Dr. Adams and sent to the Gluck Equine Research Center at the University of Kentucky. Likewise, portions of spinal cord from the two necropsied horses were positive for EHV by polymerase chain reaction
(PCR) at Michigan State University. PCR testing detects the deoxyribonucleic acid (DNA) of a particular organism, in this case
EHV, in the tissues of suspect animals.
Equine herpes viruses are in the same family of viruses that include human chicken pox and feline viral
rhinotracheitis. Both EHV-1 and equine
herpes virus type 4 (EHV-4) cause respiratory disease in horses, a condition known as rhinopneumonitis, and abortion in pregnant mares. In addition, EHV-1 can also sometimes cause disease of the nervous system as seen in the horses housed in the Middleburg stable.
One characteristic of all herpes viruses is their ability to establish a latent, or
unapparent, infection and re-occur in times of stress. EHV-1 infects a large proportion of the equine population, and almost all young horses are infected with EHV-1 early on in life. Many of these young horses will develop rhinopneumonitis, and many of them remain latently infected and can shed virus during times of stress. Vaccinating horses against EHV does help reduce the severity of illness and the shedding of virus, but it does not prevent a
horse from becoming infected. For this reason, most horses are continually at risk for EHV exposure, which usually occurs via aborted fetuses, infected placentas and the nasal secretions of horses with rhinopneumonitis.
The neurologic form of EHV-1 infection fortunately is not common, and the exact reasons why outbreaks such as this occur are not clear. The damage caused to the nervous system of affected horses is thought to be, at least in part, a reaction of that individual
horse's immune system to EHV-1. Although the strain of virus isolated in this case was found to be similar to other strains associated with neurologic disease, the occurrence of disease is much more complicated than a simple infection with the virus. Research continues in
an effort to determine the exact mechanism, but one theory is that a horse may need prior exposure to a particular strain of EHV earlier in life, and then a subsequent re-infection to cause the
neurological disease.
For these reasons, specific prevention against equine herpes virus myeloencephalopathy is difficult. The EHV-1 vaccines available for prevention against rhinopneumonitis make no claims for prevention against the
neurological disease. Nevertheless, vaccination is only one of several strategies used to prevent all forms of disease associated with equine
herpes virus infection. These strategies should include:
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Reducing stress whenever possible
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Segregating groups of horses, especially pregnant mares from young horses
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Quarantining new arrivals on the farm for three weeks
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Isolating any horse exhibiting respiratory disease signs or any mare that aborts
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Following the standard vaccination protocol recommended by your veterinarian
--
Ben Chase
North Carolina State University
College of Agriculture and Life Sciences
North Carolina Cooperative Extension,
Rockingham County Center
Physical address: 525 NC 65, Suite 200, Reidsville, NC 27320
Mailing address: PO Box 200, Wentworth, NC 27375-0200
Phone : (336) 342-8235 Fax: 336-342-8242
Email : ben_chase@ncsu.edu
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