Reprinted from The Horse Report (April 2007) with permission from the Center for Equine Health, School of Veterinary Medicine, University of California, Davis.
Equine herpesvirus-1 is one of a large group of viruses that causes potentially serious disease in horses. EHV-1 has two forms. One causes abortion in mares, while the other causes respiratory infection and neurological problems or myeloencephalopathy—damage to the brain and/or spinal cord. This latter form is of particular concern because it results in a high death rate, it is resistant to prevention by vaccination, and it affects horses of all breeds, ages and vaccination status. It has the potential to cause catastrophic losses to both the health of horses and the economy of the horse industry.
EHV-1 occurs throughout the world and indeed almost all horses older than 2 years of age have been exposed to it, similar to the herpes simplex type 1 virus in humans, which affects about 85% of the world population sometime during childhood. Following initial exposure, EHV-1 has the ability to develop into an inapparent, latent infection—that is, it remains in a dormant state and does not produce any clinical signs. This ability to reside as a silent and persistent infection in horses provides a reservoir of virus that may play a role in transmission.
According to the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS), the clustering of outbreaks in certain regions of the country—primarily the eastern United States—could be related to where high-level performance horses are located or where they tend to travel. However, since this disease is not well understood, other factors could also play a role in these outbreaks.
The neurological form of EHV-1 is not a new disease, but the evidence currently supports the observation that it is emerging as a more virulent strain than in the past. It is possible that a strain of EHV-1 has a mutation that allows the virus to reproduce rapidly to create very high levels of EHV-1 in the blood. Since it has a preference for nervous tissue it can produce a fulminating disease (one that occurs suddenly and with great intensity) that rapidly leads to death. However, in other individuals—even in the face of outbreaks—the virus appears to be restricted to latency, much like the herpes simplex type 1 virus in humans.
How Horses Become Sick
EHV-1-induced neurological injury occurs when large numbers of the virus damage small blood vessels in the brainand spinal cord. This leads to inflammation of the blood vessels and the formation of clots that obstruct the flow of blood through the circulatory system or hemorrhages into the nervous tissue. Ultimately, this results in tissue infarct (tissue that is dead or dying because of a lack of blood supply) or a restriction in blood supply to regions of the brain and spinal cord, with resultant damage or dysfunction to tissues. This is not unlike what happens when cardiac blood supply is impaired in human heart attack.
One of the typical neurological effects of EHV-1 infection is the “dog-sitting” posture reflective of pronounced weakness particularly in the hindlimbs.
EHV-1 is contagious and is spread by direct horse-to-horse contact, by contaminated hands, equipment and tack, and for a short time, through aerosolization of the virus within the environment of the stall and stable. Therefore, to prevent the spread of infection, it is essential to institute isolation and quarantine measures immediately.
It is also possible that stress factors may reactivate the virus and elicit the onset of clinical signs. These stress factors may include stress from transport, strenuous physical exercise, suppressed immune system, and excessive fatigue.
Clinical Signs of EHV-1
The initial clinical signs of EHV-1 infection may be nonspecific and include fever of 102°F or greater. Fever may be the only abnormality observed. Other signs may be combinations of fever and respiratory symptoms of nasal discharge and cough. Some horses may have reddish mucous membranes, puffy and red eyes, and swollen legs.
Horses with the neurological form of EHV-1 can soon become uncoordinated and weak and have difficulty standing. They may also experience difficulty in urinating and defecating. Often the hindlimbs are more severely affected than the forelimbs. Hence, “dog-sitting” is not uncommon in sick horses (see photo on previous page). Signs of brain dysfunction may occur as well, including extreme lethargy and a coma-like state.
The incubation period for infection is from 2 to 8 days. Once a fever occurs, clinical signs can progress to nervous system involvement over the next 1 to 7 days.
While several vaccines are available for protection against the respiratory and abortogenic forms of EHV-1, at this time there is no equine vaccine that has a label claim for protection against the neurological strain of the virus.
Until such time as a vaccine is developed to protect horses from the neurological form of EHV-1, the best way to prevent the spread of disease is through isolation, quarantine and the practice of biosecurity—a series of management steps taken to prevent the introduction of infectious agents into a herd.