In the past decade, new diseases such as West Nile virus and vesicular stomatitis have caused their share of damage and anxiety in the horse world. But the disease that really strikes fear into the hearts of horse lovers goes by the acronym EHV-1, or its neurological version, EHM.

With no vaccine to protect horses against the neurological version of equine herpesvirus-1, there is good reason to be concerned. In the absence of any failsafe prevention, the burden is on owners, show organizers and veterinarians to make responsible decisions when moving horses around, and when an outbreak takes place in or near their own back yards.

Identifying the Enemy

Equine herpesvirus is not a new disease. Formerly referred to as rhinopneumonitis, it represents the “rhino” part of what is widely referred to as a flu/rhino shot. According to an EHV-1 Consensus Statement prepared by the American College of Veterinary Internal Medicine in 2009, EHV-1 ‘is ubiquitous in most horse populations throughout the world, and causes disease in horses and extensive economic losses through frequent outbreaks of respiratory disease, abortion, neonatal foal death, and myeloencephalopathy.” It is estimated that 95-98% of abortion outbreaks in the US have been caused by the strain of EHV-1 that is also responsible for the majority of EHV-1 cases that become neurological. A horse that becomes sick due to EHV-1 presents with flu-like symptoms, including fever. If the disease progresses to the neurological phase, the symptoms – and prognosis – become much more dire.

The Consensus Statement contains a sobering statistic about how common the virus is in horse populations: “Current estimates of the prevalence of latent EHV-1 infection suggest a rate in excess of 60%.” It goes on to say that due to limitations with current testing technology, the actual infection rate may be even higher. Because EHV-1 is so common, the idea of screening animals for latent infection, such as is done with the Coggins test for equine infectious anaemia, is impractical. “It’s something that a lot of horses carry around with no problem whatsoever,” says Dr. Maureen Anderson of the University of Guelph pathobiology department. “It wouldn’t make sense to cull, because there are so many of them.”

Dr. Anderson explains that one problem with preventing the devastating neurologic version of EHV-1 is that there is not one specific strain of the virus that is wholly responsible for cases that develop neurological symptoms. “There are lots of different strains of EHV-1 capable of causing the neurologic form.’ When a mutated form of the virus was discovered a few years ago, the veterinary community thought they had found the “magic bullet” that would lead them to a vaccine that would prevent the neurologic disease. ‘It turns out some viruses that didn’t have the DNA mutation were also causing the neurological disease, though it’s more likely with the DNA mutation.”

The Rise of EHV-1

The ACVIM Consensus Report admits that there is very limited knowledge of the disease in many respects. Multiple factors contribute to every outbreak, which makes it very difficult to identify a single factor in any situation. The large number of horses moving around to horse shows is indisputably doing its share in causing outbreaks. Stressors that have been identified as risk factors for EHV-1 include comingling and transportation. Horses that are stressed from travel or intense performance schedules are also more susceptible to all diseases. Among the unknowns is whether EHV-1 is becoming more virulent, or whether the growing incidence of outbreaks is purely a factor of the great numbers of horses moving around and coming into contact with one another.

Dr. Scott Weese of the University of Guelph’s Centre for Public Health says that management of EHV-1 outbreaks is difficult to define, because it varies on a case-by-case basis. “There’s unfortunately not a simple or consistent answer,’ he says. ‘Every outbreak is managed differently, based on the disease, the types of horses, where exposure might have occurred and a range of other factors.”

Adding to the problem is the fact that EHV-1 is not a reportable disease in Canada or the US, (although it has been made a reportable disease in some individual states), which means the government cannot prevent the movement of horses that have been exposed or even infected. Anderson says it is unlikely that EHV-1 will be made a reportable disease. “The diseases that become reportable are usually extremely deadly, can be spread among multiple species, or have a significant impact on agriculture. EHV-1 is a sporadic disease for the most part, and there are low numbers of horses that are impacted.”

“Some provinces have more authority and interest,” says Weese. “For example, the Animal Health Act in Ontario gives the province a mandate and powers to intervene. But often investigation is not a priority for regulatory bodies and it’s left to whoever is around and interested. There are some good outbreak management guidelines from different institutions or groups (such as the ACVIM Consensus Statement on EHV), but there is not a standard approach.” Weese says that because the burden of cost for testing lies on horse owners, there is a varied level of response to the threat of infection. “Getting people to test when it’s indicated can be a challenge,’ he says. “Additionally, getting people to follow quarantine recommendations is a challenge because of inability to effectively quarantine on their farm or unwillingness to do so,” (usually more the latter).

The New Normal

The absence of a standard protocol for what to do if an outbreak should take place means that those in the horse industry are fully responsible for educating themselves about EHV-1 – and for managing outbreaks. Communication is critically important, but there is some reluctance out there, both from those who would do the educating and those who need to learn. Horse show organizers are leery of the subject, because an outbreak would almost certainly devastate their bottom lines if people became afraid to bring their horses to competitions. One major show facility that was contacted for this article did not respond to several emailed inquiries. Another, Craig Collins at Equestrian Management Group and the Caledon Equestrian Park, did confirm that his team is taking proactive steps to make sure they will be ready if an outbreak were to take place during a tournament. “As a venue, we constantly monitor the situation worldwide to ensure that we are cognizant of the status of outbreaks,” he says. “We communicate constantly with the official veterinarians who are on site as well as the entire veterinary community in our area. We continue to review and enhance our procedures to insure that we can quickly react to conditions in the field. We proactively communicate with other organizers throughout North America to gain experience from them.”

There are some members of the veterinary community who are reticent to talk to clients and the public about EHV-1 because of a perceived expectation that it could cause widespread panic. Unfortunately, that attitude reinforces ignorance, which is more likely to lead to panic in an actual outbreak. Veterinarians are also bound by client confidentiality, which prevents them from informing others whose horses may have come into contact with an infected animal. “We need to get the message across to owners that they should give the vets permission to disclose,” says Anderson. “The head-in-the-sand technique isn’t going to work. The biggest thing when there is an outbreak is communication. If people don’t know what’s going on, they are more likely to panic. People can be reassured when someone tells them what the plan is for keeping the disease under control. It’s important to define a plan for people, and communication is really key.” Veterinarians have a significant role in both educating their clients and providing guidance and information when an outbreak takes place. Trying to “keep the lid on” an outbreak by not telling the public about it is not only counterproductive, it’s dangerous.

EHV-1 isn’t going away any time soon. Testing and quarantine are expensive, but the horse industry needs to accept the fact that it’s up to everyone to play an active part in preventing and containing outbreaks of this potentially fatal disease. horse%20ender.tif

In next month’s issue, read about how the outbreak in Ocala, Florida last winter was effectively contained, as well as what Equine Canada is doing to create a strategy for addressing potential EHV-1 outbreaks in Canada.

How to Manage an Outbreak

Dr. Weese provided a general set of procedures for management of any infectious disease outbreak. For EHV-1, there are specific requirements.

  1. Identification and diagnosis – the challenges of EHV-1 include problems with testing. A horse that is quite sick can test negative, while a healthy horse with no clinical signs may actually be “shedding” the virus.
  2. Communication – let people know that there is a potential outbreak and keep them informed as more becomes known.
  3. Identify potentially exposed and infected horses – testing exposed horses for EHV-1 is part of this process, keeping in mind the potential challenges identified in the first point. Because of the potential for inaccurate test results, the quarantine policy (see below) is to preventively quarantine any horses that were potentially exposed to the virus, regardless of test results.
  4. Quarantine – it was previously believed that a quarantine period of 21 days was adequate, but the ACVIM Consensus Report recommends a quarantine period of 28 days, given recent discoveries that some horses shed the virus for a longer period than was previously observed. Quarantine must take place in a separate building due to the ability of the virus to travel by airborne means. Biosecurity (disinfectants, gloves, foot baths, dedicated equipment such as wheelbarrows and buckets) must be rigorously practiced in a quarantined stable. A person can infect a horse merely by carrying the virus on hands or clothing.
  5. Develop and communicate a plan to maximize compliance with quarantine and testing; create a means to centralize data collection and communications; and keep people in the loop as the investigation progresses to maximize compliance and decrease loss of compliance because of boredom or fatigue with the recommendations.
  6. Finally, for those who would consider not vaccinating for EHV-1 because it doesn’t cover the neurological disease, vaccination continues to be recommended.