This year horsemen are concerned about another possible outbreak, with cases reported in horses that participated or were exposed to horses involved in a Tennessee trail ride April 23-30. By late May there were a few more cases reported in Mississippi, Colorado, Indiana, Kentucky and Iowa.
Traveling to equine competitions and other events poses potential risk for spread of this disease. There are many types of herpes virus, but two main strains (EHV-1 and EHV-4) account for the majority of infections in horses. These viruses are readily spread via nasal secretions and can survive in the environment 7 to 10 days—which means they can be spread by towels, tack, grooming tools, shared feed and water, trailers, clothing.
A horse may become exposed in a barn stall previously occupied by a horse that was shedding the virus. People who handle multiple horses may carry the virus from horse to horse unless good hygiene is practiced. The best protection is to limit movement of horses and avoid mixing horses from various locations.
Commercial vaccines for EHV are inadequate protection. They may reduce severity of respiratory disease and may decrease the period of viral shedding. Appropriate vaccination of pregnant mares can decrease incidence of abortion. However, neither the inactivated nor the modified live-virus vaccines will prevent the neurologic form of this disease.
Dr. Frank Cook (Maxwell Gluck Equine Research Center, University of Kentucky) says herpes viruses are probably transmitted at a very early age. “EHV-1 and EHV-4 tend to be transmitted from mare to foal or from animal to animal within the same group of horses,” he says. By two years of age, almost all horses have antibodies to EHV-1 and EHV-4, demonstrating that they have been exposed to these viruses.
“As a result, these viruses are difficult to control by good management practices. Once a horse has been infected, that horse tends to remain infected because herpes viruses use stealth tactics including a dormant state to avoid being eliminated by immune responses,” says Cook.
“Equine herpes viruses such as EHV-1 can enter white blood cells or nerve cells and hide in those cells by becoming almost completely inactive,” he says. In this latent stage they stop multiplying, and the immune system cannot detect them.
These viruses can then be reactivated at some future time, such as when the horse undergoes stress. Transport or even vigorous exercise may create enough stress to enable the virus to come out of hiding within the body.
“An analogy would be Herpes Simplex Virus-1 (HSV-1) that causes cold sores in humans. Another example is the virus that causes chicken pox (varicella) and later in life might reactivate to induce bouts of shingles (herpes zoster),” he says.
“It’s the same with horses; disease can crop up again later. The two herpes viruses that have the most impact on performance in horses are EHV-1 and EHV-4. Although both infect the respiratory tract, EHV-1 is probably the most dangerous because it can cause a high incidence of abortion—a phenomenon that rarely occurs with EHV-4. This is why most horse breeders vaccinate mares against EHV-1,” says Cook.
Some variants of EHV-1 can also cause severe neurological paralysis. “The paralysis can be anything from mild ataxia in the hind legs to total recumbency—with the horse unable to get up—depending on severity of the lesion and where it is in the central nervous system,” says Cook.
“We think EHV-1 is mainly a respiratory infection. In some cases, however, it can get from the lungs into the blood stream. It then infects the cells lining the blood vessels, including capillaries in the spinal cord. It closes them down because of all the dead cells and inflammation. This starves the neural tissue for oxygen, and causes damage to the spinal cord, or sometimes the brain,” explains Cook.