Ever Vigilant Against EHV – Equine Herpes Virus


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Proper biosecurity, vaccination plans, and communication will help lower the risk of equine herpes outbreaks.

The news was alarming – and unfortunately – frequent. In the early 2000s, and even as late as 2011, news would break following a major horse show, at a race track or other equine industry event, where hundreds of horses gathered, that some of the horses participating were diagnosed with equine herpes virus 1 (EHV-1).

But what happened following those events was just as concerning. Horse owners, equine event producers, veterinarians and stakeholders across the industry would scramble to understand the full scope of these multistate outbreaks when the number of horses affected was not immediately known and accurate information was hard to come by.

Lara Maxwell, DVM, Ph.D., Associate Professor of Pharmacology at Oklahoma State University says those headlines got the attention of the equine industry.

“After several large and well-publicized outbreaks occurred at barns, veterinary hospitals, racetracks, and horse shows since around the beginning of the new millennium, many equine events have become much more rigorous about vaccination status and monitoring horses for fever,” she says.

Indeed, improved communication has been critical to getting a handle on herpes outbreaks, says  Wendy Vaala, VMD, DACVIM, Associate Director Life Cycle Management – Equine, Merck Animal Health. “It was this heightened public rumor mill that prompted the development of the Equine Disease Communication Center,” Dr. Vaala says. “The Equine Disease Communication Center now has its own website. It is an incredible resource for all kinds of infectious diseases, but the main reason it was initially created was because there was almost a hysteria that would accompany these herpes outbreaks and the misinformation that would pass from horse owner to horse owner, veterinarians and carriers, and horse show managers. It created this sense of panic. We realized there was so much misinformation going on that this new website is a safe place for veterinarians, government officials, horse owners, even owners of transportation companies to go as a central source for real-time info.”

Preventing or at least limiting outbreaks will require educated equine stakeholders with a firm grasp of the disease’s symptoms, biosecurity protocols, and vaccination plan, experts say.

Understanding the virus

Herpesviruses represent an ancient group of viruses that affect many different groups of animals. Nine equid herpesviruses have been described and are divided into nine corresponding types, with EHV-1, EHV-2, EHV-3, EHV-4, and EHV-5 tied to differing clinical diseases in horses.

Symptoms of EHV-1 infection vary, but fever is a prevalent sign, says Dr. Maxwell. Upper respiratory disease is also associated with nasal and ocular discharge and lymphadenopathy in foals. “Few clinical signs may be noted in mares prior to abortion associated with EHV-1,” she says. “Fever is often noted in horses prior to the development of Equine Herpesvirus Myeloencephalopathy (EHM), although fever might also be missed if the horse is not being monitored for fever. Other signs, such as nasal discharge and lymphadenopathy, are inconsistently observed in horses diagnosed with EHM.”

EHV-1 virus can cause respiratory disease, abortion, and neurologic disease, says Jeff Hall, DVM, Senior Veterinarian, Equine Technical Services, Zoetis. For respiratory disease, common symptoms include depression, fever, nasal discharge, swollen lymph nodes, poor performance and may advance to more severe respiratory diseases such as pneumonia. Abortion usually has no warning signs and typically takes place in the last third of pregnancy, says Dr. Hall. “Clinical signs for the neurologic form are highly variable and depend on the extent and location of the neurologic lesions. Common signs include partial to complete limb paralysis with the hind limbs most severely affected, bladder dysfunction, and loss of tail tone. Most horses with the disease remain alert and have a good appetite.”

Equine Herpesvirus Myeloencephalopathy (EHM), the neurological form of the virus, is of particular concern. Prognosis for horses who test positive for EHV and then develop neurologic signs of EHM is often poor with fatality as high as 30 percent, according to the AAEP. In rare cases, horses with neurologic signs can recover from the infection but may retain neurologic deficits.


For biosecurity reasons, once a horse is diagnosed with the herpes virus, the first step is to quarantine the horse to reduce spreading of the virus to other horses, says Dr. Hall. “The respiratory disease is generally mild and self-limiting and does not require specific treatment. In certain cases, non-steroidal anti-inflammatory medications, antibiotics, and mucolytics may be indicated. For abortion, no treatment is usually necessary, but rigorous biosecurity measures must be taken to protect other horses, especially pregnant mares.”

For the neurologic disease, Dr. Hall says supportive care is essential to maintain adequate hydration and nutrition, placement of a urinary catheter to evacuate the bladder, regular manual emptying of the rectum due to paralysis and, if necessary, placement in a sling to assist with standing. “Common therapies include DMSO, NSAIDs, and antibiotics to prevent secondary infections. Antiviral therapy is becoming more commonplace for treatment of the neurologic form of the disease.”

Dr. Maxwell says that therapy for EHV-1 infection in weanling foals is primarily symptomatic, with the administration of NSAIDs as needed for the comfort of the foal. Neonatal foals can be more severely affected, and there are some reports of oral acyclovir administration to these animals.

“Horses at risk for developing EHM have been treated with oral valacyclovir since orally administered acyclovir is poorly absorbed in horses,” she says. “Early treatment with valacyclovir may be important for its success, as one study indicated that treatment with valacyclovir was ineffective once neurological signs had already been observed. In these horses, intravenous ganciclovir administration limited further EHV-1 replication and disease. In any horse that already has neurological disease due to EHM, supportive care is the primary treatment, although antiviral therapy may be indicated if the horse still has a fever and EHV-1 viremia.”

What vaccines can and cannot do

The problem with the equine herpes virus is its ability to go dormant, says Dr. Vaala. “We figure that the majority of horses out there are infected with herpes virus, regardless if their broodmares were or were not vaccinated against EHV-1 during pregnancy. Most horses probably become infected within the first months of life.” Once exposed to EHV-1, horses often remain latently infected, waiting for some stressor to reactivate the virus.

“When it comes to vaccination, there are a lot of diseases out there that when you vaccinate, you are trying to prevent the initial infection and prevent disease,” Dr. Vaala says. “That’s a cleaner, easier kind of target. For instance, we vaccinate against rabies, because we don’t want the horse to contract rabies. It’s a black or white situation. When we vaccinate for herpes virus, we realize that most of the horses vaccinated already harbor the virus. It becomes more complicated.”

There are several vaccines that are approved to reduce upper respiratory disease to prevent abortions due to EHV-1 infection, says Dr. Maxwell. Current vaccines can reduce the severity of disease but are unlikely to completely prevent infection with EHV-1. No vaccine is licensed to prevent EHM, but since vaccination generally decreases nasal shedding of virus, vaccination is still indicated to reduce the likelihood of EHM outbreaks.

Those limitations highlight what herpes vaccines can and cannot do, says Dr. Vaala. “Most vaccines will produce a good antibody response. We know those neutralizing antibodies can help decrease nasal shedding of free virus. But EHV-1 is good at hiding in protected areas within the horse including inside white blood cells, lymph nodes and neurons. Antibodies alone are often unable to eradicate virus hiding in those areas. So, if I have well-vaccinated horses coming into showgrounds, and they’re exposed to a new herpes strain, or because of transport and competition-related stress they reactivate their own internal herpes virus, with a good vaccination program we hope it decreases their ability to shed the virus to other horses. So, it might help contain a potentially bad outbreak.”

Vaccination frequency

Dr. Vaala says that even with vaccinations, horse owners need to be vigilant about basic biosecurity for their horses, such as maintaining good hygiene practices, and monitoring how horses come in contact with one another at all shows. “None of the current vaccines provide a horse a foolproof plate of armor. Even vaccinated horses still have the potential to develop neurologic disease due to EHV-1.”

For horses that are traveling, or showing, in high-risk conditions such as fairs, tracks or at facilities where other horses are constantly coming and going from other events, it’s recommended they be vaccinated every six months with an EHV vaccine. Dr. Vaala says that an EHV-1 vaccine would be ideal over combination vaccines where herpes virus is just one of several antigens in the vaccine.

There’s also been an ongoing discussion among EHV experts that over-vaccination for herpes virus may produce unpredictable immune responses that could be detrimental to a horse’s health, Dr. Vaala says. “More is not always better, particularly when it comes to this virus.” In one study that looked retrospectively at an outbreak of  EHV-1 /EHM among a large population of cutting horses, positive risk factors included the stress on the horse, spending more time in common areas or being entered into more events. “The other positive risk factor was having been vaccinated within five weeks of arriving at the showgrounds.” Another study from researchers at Cornell demonstrated mixed immune responses among horses that received frequent EHV vaccinations over an 8-month period.

“We’re still learning how to best stimulate the immune system, but I think moderation when it comes to how frequently we vaccinate against equine herpes virus is probably the way to go,” Dr. Vaala says. “I think the AAEP vaccination guidelines and USEF guidelines – which recommend to vaccinating horses in high-risk environments every six months with an EHV vaccine – is a good place to start unless there’s a sound medical reason to vaccinate more frequently.”

Good biosecurity practices

Dr. Vaala says biosecurity is critical when it comes to this disease. “While EHV vaccines help prevent the spread of disease, they are only part of the control strategy. When dealing with highly contagious diseases like EHV we should not become overly reliant on vaccines alone to provide complete protection. Good husbandry and common sense are critical.”

Dr. Maxwell says biosecurity has been successful at controlling the impact of EHV-1 outbreaks. “For example, racetracks may refuse to admit horses that are arriving from an area currently under quarantine for EHV-1 and require a recent health certificate showing that the horse was free from signs of EHV-1 infection,” she says. “Successful quarantine measures, including monitoring of EHV-1 spread by PCR and body temperature, separation of infected horses, and use of appropriate disinfectants, have helped to limit the impact of outbreaks when they do occur. Many venues require proof of prior vaccination of horses for EHV-1 and may require that vaccination be administered every few months.”

Since the movement of horses is critical to so many aspects of the equine industry, the development of a vaccine that can effectively prevent EHM would help the industry enormously, Dr. Maxwell says. “In the meantime, the use of current vaccines and biosecurity measures are key to preventing the dissemination of EHV-1, although outbreaks are still expected to occur. Twice daily monitoring of body temperature is an inexpensive and useful tool for screening horses at risk for EHM, and valacyclovir therapy can be instituted early in these patients. Unfortunately, antiherpetic drugs are not presently approved for use in horses, so the use of the human drug can be quite expensive. Therefore, the development of more effective drugs, like ganciclovir, for use in horses at risk for EHM could also be an important management tool.”

Good biosecurity measures include limiting horse to horse contact, washing your hands between handling horses, and not sharing common use items like a tube of paste, such as oral medications, among horses. Sharing bits and being careful about having multiple horses using the same feed or water buckets, are other things to consider, says Dr. Vaala. “Even trailers that transport a lot of horses back and forth between shows need to be cleaned between shipments.  We know the virus can linger outside the horse long enough that it could be transmitted via respiratory secretions. The virus is not difficult to kill with good soap and water and routine disinfectants.”

Dr. Hall says that both the AAEP and the ACVIM have taken leadership in providing guidelines and documentation on how best to deal with an EHV-1 outbreak and how to help limit its impact. “The keys are to detect sick horses early with identification of fevers through twice-daily rectal temperatures and recognizing clinical signs early,” he says. “If there are any signs of disease, it is best to quarantine the horse immediately for treatment and perform the proper diagnostic tests to identify the infectious agent. Horses with neurological disease should be kept isolated for a minimum of 14 days, although longer periods of up to 28 days have been recommended. Use of good hygiene measures can go a long way.”

Additional steps to limit EHV-1

Jeff Hall, DVM, Senior Veterinarian, Equine Technical Services, Zoetis, offered the following steps for horse owners to consider:

  • Minimize your horses exposure to other horse’s
  • Quarantine new horses that are brought onto the farm for 21 days before introducing them to the herd
  • Keep tack separate for each horse
  • Never share water sources
  • Reduce stress
  • Stay current on the respiratory and abortion EHV-1 vaccines
  • Call your veterinarian EARLY if you detect any illness in your horse

EHM & EHV-1 Frequently Asked Questions

Source: American Association of Equine Practitioners Guidelines website

  1. How do we handle horses returning from events where they may have been exposed to EHV-1?

For horses that may have been exposed to the risk of infection, there are some steps to take to mitigate the risk at their home facility. Even if these horses are returning home from events at which no disease was reported, and even if these horses appear healthy, precautions are needed as these horses could bring a contagious pathogen home and spread it at their home farm – this is the classic way EHV-1 spreads:

  • These horses should be isolated from any other horses when they return to their home facility.
    Isolation requires housing them away from other horses, using different equipment to feed, clean and work with them than is used with any other horses, and rigorous hygiene procedures for horse handlers (hand hygiene, wearing separate clothes when coming in contact with isolated horses, etc.). Please discuss this with your veterinarian.
  • We strongly advise owners to call their veterinarians to discuss how long to keep the horses isolated at home, but even if they don’t develop fevers this should be at least 21 days.
  • These horses should have their temperature taken twice a day, as elevated temperature is typically the first and most common sign of infection – horses with elevated rectal temperatures (101.5 F or greater) should be swabbed by your veterinarian to find out whether they are shedding EHV-1.
  • If a horse develops a fever and is found to be shedding EHV-1, then the level of risk to other horses on the premises increases significantly. Affected farms should work closely with their veterinarian to manage that situation, if it develops.
  • The American Association of Equine Practitioners (AAEP) has an extensive set of Equine Herpesvirus (EHV) Control Guidelines that serve well as a resource for practitioners.
  1. What do we do if we already have a potentially exposed horse on a farm?
  • It still makes sense to isolate the exposed horse from other horses; even though it may have already been in contact with other horses, start isolation procedures to stop further exposure. It is very important to not mix horses from different groups to accomplish this. Try and isolate the suspect horse without moving other horses from one group to another – segregation of horse groups is the key, because this will help you reduce spread if an outbreak starts.
  • Check temperatures of all horses on the farm twice daily (fever spikes can be missed if you check once daily). If fevers are detected, then isolate the horse and test for EHV-1.

The value of starting healthy horses on anti-viral treatment when there is no evidence of disease on the farm is questionable. The treatment is expensive, the drug (Valtrex™ – valacyclovir) may have limited availability, and prophylactic therapy against EHM will only work while a drug is being administered. Therefore, it is more likely to be effective if administered when fever is first detected (see below).

  1. What anti-viral treatments can I use against EHM on a farm?
  • If EHM is present on a farm, then the risk of developing EHM to other horses at that farm is greatly increased. Stringent quarantine and biosecurity procedures must be implemented immediately.
  • Treatment of horses with clinical neurological disease (EHM) is largely supportive – the use of anti-viral drugs is not known to be of value at this stage. The use of anti-inflammatory drugs is recommended: flunixin meglumine (0.5 to 1 mg/kg, IV, q 24 hours).
  • For horses on the farm that develop fever, test EHV-1 positive, or have a high risk of exposure, anti-viral drugs may decrease the chance of developing EHM.
  • Currently, the treatment of choice in a febrile EHV-1 infected horse to prevent the development of EHM is Valacyclovir (Valtrex™), given orally. The use of oral acyclovir is unlikely to be of any value, as it is poorly absorbed from the GI tract.
  • We currently recommend Valacyclovir (Valtrex™) for prophylactic therapy at a dose of 30 mg/kg q 8 hr for two days, then 20 mg/kg q 12 hr for 1-2 weeks. Maintain on higher dose rate if the horse is still febrile.
  • The use of Valacyclovir in horses that have already developed signs of EHM is questionable at this time. In that circumstance, the use of intravenous Ganciclovir is preferable as it may have greater potency against the disease. The dose of Ganciclovir is 2.5 mg/kg q 8 hr IV for one day then 2.5 mg/kg q 12 hr IV for one week.
  • Administration of a zinc-containing supplement may be beneficial based on a recent epidemiologic study that found a decreased risk of EHM associated with owner-reported dietary zinc supplementation.
  1. Is there any value to using booster vaccination against EHV-1 at this time?
  • Unfortunately, there is not a licensed EHV-1 vaccination product with a label claim for prevention or control of EHM.
  • EHV-1 vaccines have been shown to reduce nasal shedding and, in some cases, reduce viremia. These products may, therefore, have some theoretical value against EHM by reducing viremia, and certainly against spread of the virus by reducing viral shedding in the environment.
  • If horses on the farm are previously vaccinated against EHV-1 then booster vaccination should quickly increase immunity, and perhaps reduce the spread of EHV-1, if it is present.
  • Vaccination in these circumstances is controversial, as some authorities speculate that immunity to EHV-1 may play a role in the development of EHM. While this is unproven, it remains a possibility. The use of vaccination is, therefore, a risk-based decision.
  • Vaccination has no value as a treatment in affected horses.

Protect your Practice & Facility

During an outbreak, caution is recommended at all times to reduce spread of infection. Movement of horses on and off farms should be limited whenever possible. Reducing stressful activity of the at-risk horse can assist control methods as well.

Protect your own practice and hospital from becoming part of the problem. Heighten your biosecurity, as mentioned in the AAEP EHV Control Guidelines.