What does a cold sore on your lip have in common with herpes virus infection in horses? The blister of a cold sore is caused by a herpes virus; the infection lies dormant in a person until triggered by fatigue, anxiety or stress. Each animal species is subject to herpes infections, although most are host-specific, meaning the virus cannot pass from one species to another. While it’s not transmissible from human to horse or vice versa, the herpes virus readily circulates between individuals within a species, i.e. from horse to horse.
The horse can be affected by many different strains of equine herpes virus (EHV), also known as rhinopneumonitis, but five strains are most prevalent: EHV-1, EHV-2, EHV-3, EHV-4, and EHV-5. Of these, EHV-1 and EHV-4 are associated with viral respiratory disease, with EHV-4 mostly affecting young horses younger than 3 years of age. EHV-1 is the most prevalent concern in horse populations of all ages, not only because its respiratory disease is more virulent than that of EHV-4, but also because it can cause viral abortion in pregnant mares or neurologic disease (myeloencephalopathy).
EHV-2 and EHV-5 typically infect the upper respiratory tract of young foals and may predispose the lower tract to bacterial pneumonia by Streptococcus equi or Rhodococcus equi. Conjunctivitis and eye inflammation may occur from EHV-2 infection as well.
EHV-3 causes a self-limiting equine venereal disease known as equine coital exanthema–active lesions on a stallion’s penis or prepuce or on a mare’s vulva may cause discomfort during breeding. However, it’s not likely to cause infertility or life-threatening problems.
Exposure and infection
By the time a horse is 2 years old, it is likely that he will have been exposed to herpes virus, particularly EHV-1 and EHV-4. Like a person with a cold sore, the horse remains a carrier for life once infected. The virus hides out within the trigeminal nerve of the face or in specialized white blood cells of the lymphatic system. While resting in a dormant, the herpes virus doesn’t replicate and is “silent” to the immune system, effectively escaping detection and destruction. During stressful periods such as training, competition, transport, management changes or illness, high levels of circulating corticosteroids suppress a host’s normal defense mechanisms. This reactivates the virus to elicit shedding in nasal secretions. A horse may appear clinically normal yet still serve as a reservoir of disease as a “silent shedder,” thereby increasing the potential to cause an epidemic within a herd.
Incubation of EHV-1 is rapid, with clinical signs appearing within one to 10 days following exposure. Initially, an infected horse develops fever and respiratory signs, such as nasal discharge or coughing; however, these initial signs may be so subtle as to pass unnoticed. While the virus is circulating in the bloodstream, infected cells may spread to other organs, such as a pregnant uterus, potentially causing abortion; the central nervous system, potentially eliciting myeloencephalitis; or the eye where it can cause ocular disease like chorioretinitis.
The neurologic form of herpes virus, referred to as equine herpes myeloencephalopathy (EHM), manifests through a variety of neurologic signs related to blood vessel damage in the central nervous system: ataxia (incoordination); hind limb weakness; difficulty defecating or urinating; urinary incontinence; diminished tail tone; extreme lethargy; and/or inability to stand. A horse with a fever higher than 103.5o Fahrenheit is more likely to develop EHM. Neurologic herpes virus is also associated with a high rate of mortality.
Other than taking care of your horse’s symptoms, there is no specific treatment for most viral infections; rather, the virus must run its course. For respiratory illness from herpes virus, a horse may be medicated with non-steroidal anti-inflammatory medications to control fever and to make him feel well enough to continue eating and drinking.
If the herpes virus circulates to the uterus or central nervous system, the disease will take hold and nothing can avert the consequences: The mare will lose the foal or the EHM-affected horse will demonstrate varying degrees of neurologic signs. Anti-viral medications may reduce the extent of the disease, but the most important step is to isolate infected individuals and implement active biosecurity measures to control viral spread.
Prevention and Containment
Horses with respiratory signs are very contagious and should be isolated immediately. Direct horse-to-horse contact facilitates exposure, but the virus can also be transmitted on people’s hands, clothing and shoes, as well as grooming equipment, tack, buckets, rakes, hoses and water tanks. Aerosolized particles from coughs or sneezes can become airborne and can infect horses a distance away.
A mare that has aborted her fetus should not be allowed to have contact with other horses, and the foaling area must be disinfected extremely well. The fetus and fetal tissues that contain high levels of the virus must be incinerated or disposed of away from other horse or human contact.
Commercial vaccines are available against EHV-1 and EHV-4 for respiratory disease and abortion. Because the virus’s genes mutate in neurologic manifestations, currently available herpes vaccines won’t stimulate protective antibodies against neurologic disease. However, immunization may reduce nasal shedding that would otherwise facilitate viral spread.
Prevention and containment rely on systematic biosecurity strategies, particularly in the face of an outbreak:
- Don’t move horses off the property until an outbreak has run its course.
- Don’t bring any new horses on to the property.
- Refrain from mixing groups of horse –keep the horses in their usual herds and stalls.
- Check rectal temperatures twice daily. At the first sign of fever, take proactive steps to isolate a suspect horse and contact your veterinarian immediately.
- Designate specific personnel to handle only suspect or sick horses.
- Don’t share any tack or equipment between horses since the virus spreads through contaminated objects.
- Wash your hands between handling other horses and/or wear disposable gloves.
- Change contaminated clothing and shoes before entering stabling or handling other horses, and use disinfectant baths for footwear.
- Minimize horse stress as much as possible through good management practices.
Prevention also relies on effective immunization of all horses on a farm according to the vaccine manufacturer’s recommendations. While no vaccine is yet available for EHM, many herpes virus vaccines target both EHV-1 and the less worrisome strain of EHV-4. An adult horse should receive boosters twice annually, although more frequent intervals may be dictated by an increased risk of exposure, stress-related travel and competition, or during pregnancy.