Equine Herpes Virus-1
There are five distinct herpesviruses that are known to infect horses. Equine herpes virus-1 (EHV-1) is the most frequently discussed within the horse industry as it is the one that induces the neurologic condition known as equine herpes myeloencephalopathy (EHM). You may have also heard EHV-1 referred to as rhinopneumonitis.
How is EHV-1 spread?
EHV-1 is typically spread by nasal secretions. Horses can become infected by direct contact with an infected horse or from contaminated objects such as water buckets, tack, or humans. If an infected horse coughs or sneezes, the virus can become airborne and travel up to 30 feet. Depending on environmental and surface factors, EHV-1 can survive in the environment between 7 and 30 days.
The incubation period, or period of time from when a horse gets exposed to when they start showing clinical signs, can be variable ranging anywhere from 4 days to 3 weeks.
What are the clinical signs of an EHV-1 infection?
Fever and/or mild respiratory disease may be the first signs of infection. Fevers (rectal temperature greater than 101.5F) associated with EHV-1 can be biphasic, meaning a horse develops a fever, it goes away, and then it returns a few days later. With the respiratory disease, some horses may develop mucoid nasal discharge and ocular secretions while others develop only very mild disease that goes unnoticed. Coughing can occur but is usually mild and infrequent. Please be aware that some infected horses will show no clinical signs.
Neurologic disease (EHM) is a relatively uncommon consequence of an EHV-1 infection. When neurologic disease does occur, it usually has a rapid onset. Common clinical signs include incoordination or weakness (most notable in the hind end), dribbling of urine, inability to rise, and/or lethargy. Many horses affected with EHM will continue to appear bright with a good appetite. EHM may warrant humane euthanasia in severe cases, such as horses who are down and unable to rise. Some horses can make a complete recovery from EHM while some show improvement but have persistent residual signs of neurologic disease. In general, any type of recovery can take months. It is oftentimes difficult to predict prognosis and response to treatment and time is frequently used as an indicator of prognosis.
What is the treatment for EHM?
There is no specific treatment for EHM and therapy is aimed at supportive care and reducing inflammation. Horses who are recumbent or at high risk of becoming recumbent may benefit from a sling. Slings can have many complications and are usually only available at universities and large equine hospitals. The anti-viral, valcyclovir, can potentially reduce the development of EHM if given before a horse is exposed to EHV-1. Otherwise, valcyclovir is often used in outbreaks to reduce shedding of the virus.
How is a horse tested for EHV-1?
Testing for EHV-1 involves a nasal swab and a blood sample. EHV-1 is a reportable disease in most states, and any positive cases must be reported to state officials. State officials help to dictate guidelines on quarantine of infected and/or exposed horses/premises.
Is there a way to reduce the spread of EHV-1?
A vaccine for EHV-1 is available and is often called the rhinopneumonitis or “Rhino” vaccine. Vaccination may reduce the clinical signs of infection but is unlikely to prevent viremia which is what leads to EHM and abortions. Viremia means the virus is circulating through the blood stream. Vaccination may reduce shedding of the virus which will help to reduce horses affected in an outbreak situation.
Any horse that has had a known exposure to EHV-1 or is returning from an EHV-1 infected facility/event should ideally be isolated for at least 21 days. For isolation, the horse should be kept in a separate facility from other horses, and all horse-to-horse contact should be avoided. Isolated horses should have their own dedicated equipment for feeding, cleaning, and handling. Horse handlers should wear personal protective equipment and/or change clothing, gloves, and shoes after handling the isolated horse. Temperatures should be monitored twice daily. If a horse develops a fever (greater than 101.5F) during their isolation period, your veterinarian should be contacted, and EHV-1 testing should be performed.
EHV-1 is susceptible to many disinfectants such as bleach and alcohol disinfectants. However, one should be aware that these disinfectants are inactivated by organic material (dirt, manure). So ideally a detergent or soap is first used to remove any organic material before applying the disinfectant. Contact time of the disinfectant is dependent on what product is being used but is typically between 5-10 minutes. At RFEMC, we use a product called Intervention.
Can EHV-1 cause any other types of illness?
EHV-1 can also rarely cause abortions in pregnant mares. Abortions usually occur in the third trimester and the aborted fetus and placenta can be infectious so they should be disposed of properly. Vaccine protocols for pregnant mares typically require vaccination against EHV-1 at 5, 7, and 9 months of gestation. Because the vaccination does not prevent viremia, there is still a risk of abortion in pregnant mares that are exposed to EHV-1. I always recommend that pregnant mares are kept separate from other horses on the property that may travel and/or contract the disease from an outside source.
How does an outbreak begin?
Most horses will become infected with EHV-1 at a young age with 80-90% of horses infected by 2 years of age. After initial infection, the horse does not clear the virus but it becomes latent in area of the upper respiratory tract (it’s true – herpes really is forever). Due to various factors, the virus can be reactivated, and a horse can begin shedding the virus and possibly even develop severe disease. Stress is often a factor in the reactivation of the virus. It is unclear why some outbreaks of EHV-1 result in a higher number of affected horses and more severe disease. There are some genetic factors of the virus that are believed to contribute to virulence.
Please contact us at Roaring Fork Equine Medical Center if you have questions regarding EHV-1 and/or the health of your horse. We will do our best to keep our community updated on any cases that affect our area and/or the state of Colorado.
The above is brief summary and is not a comprehensive review of EHV-1.