Nearly every animal that has ever been examined for them carries at least one species--sometimes five or six. Horses have five. Humans have eight. You're probably harboring at least one right now, and your horse most likely has two or three. It's a good bet that every creature you've ever known, family and friends included, has carried herpesviruses.
But there's no reason to feel like Typhoid Mary. Herpesviruses are among the most successful viral parasites on Earth, and over the evolutionary ages, most have managed to take up permanent residence in their hosts without causing serious harm. Others are such pathogenic lightweights that they cause disease only in individuals whose immune systems are severely compromised, such as people with advanced AIDS. Some herpesviruses do cause significant human illnesses, though, such as chicken pox and the sexually transmitted disease most people have heard of.
In horses, herpesviruses are associated with diseases that range from relatively mild--rhinopneumonitis, roughly equivalent to the common cold in people--to potentially deadly: myeloencephalopathy, inflammation of the blood vessels that supply the spinal cord and brain. This family of viruses is also responsible for a type of equine genital infection, as well as inflammation of the pharynx and eyes. And at least one strain of herpesvirus can cause sudden abortion "storms" that sweep through large breeding farms. Generally, however, these viruses are a silent presence in the horse population.
Yet it's wise to remember that they're out there, says University of Kentucky researcher George Allen, PhD. "The viruses are constantly spreading, and most of the time the infections are mild enough to go unnoticed, but on occasion they will find the right population and the right circumstances to cause serious disease."
Fortunately, by learning a little about how herpesviruses work and how they are spread, you can take precautions to protect your horse from their worst effects.
Forgotten, but not gone
Herpesviruses get their name from the Greek word "herpein"--to creep--a term which was also the root of the word "serpent" (as in "herpetology"), and like snakes, herpesviruses can crawl in unnoticed, under the radar of the immune system, and they may or may not "strike."
When a horse is first infected with a new strain of the respiratory herpesviruses, for example, the organisms penetrate the cells that line his respiratory system, sparking an inflammatory response. It can take anywhere from a day to more than a week for that response to cause outward signs, such as nasal discharge and fever. Just as often, however, the horse never becomes ill. Either way, he is capable of spreading the virus, by nose-to-nose contact as well as by expelling airborne droplets during coughs or snorts.
And once the virus enters a horse's body, it's there to stay. All herpesviruses have a unique characteristic: They can "hide" from the immune system so that it is impossible for the body to clear them entirely away. Instead, the viruses change their outward forms and go "latent" in some hospitable part of the body, such as nearby lymph glands. While latent, the viruses cause no harm, but when the horse is stressed--by weaning, transportation, fatigue--they sometimes revert to a more active state and revive the infection. These later infections tend to produce less severe signs, often so slight that they go entirely unnoticed. Nevertheless, they again cause the horse to shed the active form of the virus, potentially infecting others.
No wonder herpesviruses are so ubiquitous in the horse population. "First exposure to herpesviruses typically happens very early in life," says Allen, "and then horses are constantly reexposed throughout life as they encounter different strains. Some of those strains are more virulent than others and are more likely to cause disease."
So far, researchers have identified nine herpesviruses in equines, but only five are common in domestic horses; the others are found chiefly in other equids, such as zebras, onagers and donkeys. These five viral species fall into two categories, depending on how seriously they affect the horse.