Equine herpesvirus type 1 (EHV-1) can cause three different forms of disease that include: a respiratory disease (rhinopneumonitis, or sometimes called just rhino), which affects mostly young horses; abortions in pregnant mares; and neurologic disease (equine herpesvirus myeloencephalopathy).
Like herpesviruses in other species, equine herpesvirus has the ability to lay dormant as a latent infection. This allows the virus to continually reside within the horse, and at any time it can become an active viral infection, especially if the horse is stressed. Equine herpesvirus affects only equids and does not pose a health risk to people or other animals.
Most commonly, this virus manifests itself as a respiratory disease in young horses. Abortion storms also occur in unvaccinated pregnant mares.
The originally identified strain of EHV-1 appears to have mutated into a strain that replicates more rapidly and has a predilection for neurologic tissue. The mutated virus circulates at higher levels in the body earlier in the course of the disease and maintains the high level of virus within the body for a longer period of time. However, both the mutated and original virus can cause neurological disease.
The neurological form of EHV-1 traditionally has been less common. However, in the 2003 Findlay, Ohio, outbreak, the respiratory outbreak of EHV-1 involved the neurological form and has been the most devastating outbreak to date.
Neurological EHV-1 has been responsible for quarantines and horse mortality in most areas of the country during the past decade. Most recently, a case was diagnosed in Ontario the end of February and a case was diagnosed at Michigan State University this the past week. Also, Ohio has had a couple of cases in the past week. The Michigan Department of Agriculture has quarantined the farm of origin for the Michigan case and is currently identifying all contacts the Michigan horse had and testing any horses that may demonstrate clinical signs suggestive of EHV-1. The virus that caused the neurological disease in the Michigan horse was the original strain of the virus.
The incubation period for EHV-1 may be as short as one day or up to 10 days. Typically signs are seen within the first week. The virus is shed for seven to 10 days, but shedding has been documented for up to 28 days after clinical signs have been recognized.
Initially, horses may present with fevers up to 105 degrees Fahrenheit. Some horses will develop nasal discharge, depression and loss of appetite.
Horses may also develop neurological signs, including toe-dragging, weakness and incoordination of the hind end. The nerves of the head (cranial nerves) are often not affected. Other neurological signs that may occur include a weak, floppy tail, inability to defecate and urinary incontinence.
Once innervation to the urinary bladder is affected, the horses cannot urinate freely and the bladder becomes greatly distended. After reaching the point of overfilling the bladder, they start to “dribble” or “squirt” urine. If these horses are not immediately treated by bladder catherization, permanent urinary incontinence may develop.
Some horses will progress to the point of recumbency. Once recumbent, a horse is extremely difficult to manage and is often euthanized due to treatment difficulties or for humane reasons.
The neurological form of EHV-1 must be differentiated from other diseases (such as rabies, EPM, EEE, WEE, VEE, and WNV).
In the past, EHV-1 was diagnosed by paired serum-titers and/or culturing the virus from the blood or from nasal swabs. It would take several weeks for the results of these tests to become available.
Currently, with PCR (polymerase chain reaction) technology, EHV-1 can be diagnosed within days from a nasal swab. (The test can be done within the day once the sample is at the lab when a suspect horse is identified.)
Additional testing is required to determine if the neurologic or original strain of EHV-1 is involved. About 75% of the neurological cases are caused by the neurologic strain while the remaining causes are caused by the original strain.
No specific treatment is available for EHV-1. Supportive therapy (such as IV fluids, bladder catheterization, etc.) and nursing care (keep horses quiet, minimize stress, etc.) are extremely important for an animal’s survival.
Anti-inflammatory agents should be used in an effort to minimize damage to the spinal cord.
Antiviral drugs acyclovoir, valacyclovir, famiciclovir and penciclovir have been used, but efficacy of these drugs has yet to be determined in equids. In a recent study, valacyclovir may be the most promising therapeutic antiviral.
In horses that survive, recovery can take several months. In those horses that don’t progress to recumbency, a complete recovery can be expected, while horses that become recumbent and survive may continue with some degree of neurological deficit.
The best form of prevention is to keep your horses at home, thus limiting horse-to-horse contact. This is often not possible in the horse industry but Michigan State University Extension recommends limiting horse travel and horse-to-horse contact is advisable. Under strict biosecurity protocol, a horse should be isolated when moving to a new facility or returning to the home facility.
Although this is often not possible by the nature of the horse industry, steps can be taken to minimize the risk of spreading disease. For example, keep horses that travel housed separately from horses that do not travel.
Also, precautions should be taken to prevent fomite-spread of disease. Fomites include people, tack, feeding supplies, barn cleaning equipment, etc.
An isolation period of 30 days is advised as the herpesvirus may be shed in various secretions (tears, respiratory secretions, and abortive fluids, etc.) for at least 14 days and has been documented for up to 28 days
In addition, stress may cause a horse (as in people) that has recovered to again begin shedding herpesvirus. Anytime a horse travels the risk of viral replication, shedding and disease is possible.
Vaccines are available for the prevention of respiratory disease and abortions related to EHV-1; however, no vaccine is currently available to prevent the neurological form of EHV-1
A theory is that vaccination may decrease the amount of virus shed by an individual, thus decreasing the amount of virus in the environment. It is hoped that if less virus is shed, a disease outbreak may be averted or reduced.
Biosecurity remains the best means of minimizing the risk of an EHV-1 outbreak. Additional information regarding EHV-1, can be obtained at: http://www.aphis.usda.gov/vs/nahss/equine/ehv/. Another source of information can be obtained from the American Association of Equine Practitioners (AAEP) website.
This article was written by Dr. Judy Marteniuk from Michigan State University Extension.