EHV-1: Separating Fact, Fiction, And Confusion

by | 02.02.2016 | 1:42pm

As Sunland Park and Turf Paradise remain under quarantine due to an outbreak of EHV-1 (equine herpesvirus), we turned to the experts for answers on this serious and highly contagious illness. According to the American Association of Equine Practitioners, equine herpesvirus is found in most horses, though many of them do not become ill as a result. There are several types of viruses in the EHV family, including EHV-1, which can cause respiratory disease, abortion, neonatal death, or neurological disease. Officials announced that a strain of EHV-1 associated with neurological disease was detected at Sunland Park Jan. 21 and at Turf Paradise Jan. 27. Horses exposed to EHV-1 may or may not become feverish, and of the horses that develop a fever, not all develop neurological signs. However, there is no cure for the neurological symptoms associated with EHV-1 if they do occur.

We spoke with Dr. Nathaniel White, professor of large animal clinical sciences at the Virginia-Maryland Regional College of Veterinary Medicine at Virginia Tech and Dr. Josie Traub-Dargatz, professor of equine internal medicine at the James L. Voss Veterinary Teaching Hospital at Colorado State University, about the disease, its spread, and good biosecurity practices.

We've been hearing there are actually two kinds of EHV-1—a “wild” strain and a second strain. What's the difference, and how do health officials handle these differently?
Dr. Traub-Dargatz: If we have a case that's suspected or confirmed EHV-1, neurologic form, the actions that would be indicated to control the spread of disease would be the same whether it's the “wild type” or the neuropathogenic strain. Our initial actions aren't going to vary based on which subtype of the virus it is.

The wild and neuropathogenic strains are different only by one amino acid in their DNA. They do seem to vary in a couple of ways—in challenge studies, the amount of virus that's in the blood of the horse seems to be higher with the neuropathogenic strain than the wild type. Also, some people believe the scope of disease and the severity among an exposed population would be greater for this neuropathogenic strain, but there have been a couple of outbreaks associated with the wild type and multiple horses were affected.

Some laboratories only report that a sample is EHV-1 and don't do subtyping.

How reliable are nasal swabs or whole blood testing for this?
Dr. Traub-Dargatz : The recommendation from the U.S. Animal Health Association (USAHA)  is that both types of samples be tested, not just one. Depending where the horse is in the course of the disease and how much virus it might be shedding in the nasal secretions, one [test] may be positive and the other not. When we collect whole blood for testing, we're going to get a representative sample of the horse's blood in a tube; with the nasal swab we're sticking a swab up the horse's nose and some horses are not very tractable for that procedure so we may not get an optimal sample of the nasal secretions on the swab. So it is important that we recognize it doesn't just come down to the lab test methods, it's how good a sample we get on the swab for the lab to then test.

There have been one or two outbreaks where at the onset of fever, the horse might not be positive for EHV-1 on the swab and by the time they develop the neurologic disease, they are positive. If we have highly suspect horses, that might mean doing more than one test on that horse a few days later in order to make the diagnosis.

So diagnosis is a matter of watching both the symptoms and the test results?
Dr. Traub-Dargatz: Yes. I think the key in the USAHA guidelines is the recommendation to focus testing on horses who have signs of the disease. We know that a small percentage of horses that are clinically normal can shed EHV-1 in their nasal secretions and look perfectly normal. Those horses may not pose much of a risk that we're aware of. There was one study that sampled horses at shows and sales and around 3.8 percent had EHV-1 detected in nasal swabs by PCR testing when there was no outbreak going on. Thus testing on clinical horses is the recommendation versus testing the general population.

Is that three percent of the population the reason for an outbreak?
Dr. Traub-Dargatz: I think that is a possibility, but it's impractical to test all horses every day to see if they're shedding. The majority of horses have been exposed to EHV-1. Some of them will develop a latent infection, which means the virus lives in places in the body (mostly in lymph nodes and areas around the head) and the body doesn't recognize it's there. That horse could then shed the virus out their nose and look perfectly normal.

We know EHV-1 is very contagious, but just how contagious is it—could using the same track surface as an exposed horse pose a risk?
Dr. Traub-Dargatz: I  think sharing the same track surface would be minimal risk but having nose contact with contaminated surfaces like starting gates would be more concerning. The virus has to get up the horse's nose in a great enough amount to cause infection. How much virus it takes to cause an infection depends on the horse's immune status but it's really the droplets that come out of the nose that are a risk. Contaminated hands, contaminated equipment including buckets and wipe rags are much more worrisome. In the open air it's probably less of a concern than in a confined airspace. We really don't know how long the virus lives on a pitchfork or how far it can spread in the air. Some of the literature says 30 feet away, but that really hasn't been investigated. Horses with EHV don't typically cough as much as those with flu to move those droplets a long way.

What do we know about incubation times?
Dr. Traub-Dargatz: It depends how much virus they're exposed to. If we get a huge exposure and they have very little immunity it could be as short as one or two days. Also, onset of clinical signs can be different. They may have a fever one to three days after they're exposed to this virus, and then some horses' temperatures return to normal, and then they get a second fever. The neurologic signs usually develop six to 10 days after exposure if they're going to become neurologic.

There are vaccines for EHV, so why can't they prevent an outbreak like this?
Dr. Traub-Dargatz: There are around 25 different brand names of vaccines that contain EHV. None of them carry a claim for prevention of the EHV-1 neurologic disease. However, some of the vaccines could reduce the amount of virus being shed within a population, as some of the vaccines carry a claim to reduce virus shedding. There's still a lot we're learning.

When a facility has an outbreak, who decides what the quarantine protocols are going to be?

Dr. White: In a state where EHV is a reportable disease, (that means that if it occurs, the veterinarian would call the state vet), the state vet would make the decision about the quarantine. The practicing veterinarian that recognizes it initially would probably recommend at least immediate quarantine. And the state vet would try to assess where that horse has been and what other contacts may have happened. If you cross state lines, it goes to the next state veterinarian.

We really would like all states to make it a reportable disease. If it's not, it would be the treating veterinarian making a recommendation but there's no way to enforce it. Most people would want to do that but if it's not reportable there's no regulation that there has to be a quarantine.

Does the U.S. Department of Agriculture (USDA) or the Animal and Plant Health Protection Service (APHIS) get involved at a certain point?
Dr. White: They don't do that normally because each state has its own regulations and USDA is more involved with foreign disease and imports. Back in 2011 when the Ogden, Utah outbreak occurred, state vets involved called USDA and asked them to help coordinate recording of the outbreaks. We've asked them to do more with that to work with the epidemiology of any reportable disease. Sometimes, state vets aren't required to notify each other when there's an outbreak going on, even if it's in an adjacent state (though many times they will do so anyway).

Is there a standard recommendation for how long a quarantine should last?
Dr. White: It's 21 days from the last fever. That wouldn't necessarily make it 21 days from the last neurologic symptoms, because some horses develop neurologic symptoms that last for years. It's really the fever indicating an active infection. They should then be negative on testing by that point. If it's 21 days and they're still positive, you'd still want to keep them quarantined. There are times when others have recommended 14 days, but each state can set their own policy.

Any other tips for racetracks and competition venues for dealing with an outbreak like this?
Dr. Traub-Dargatz: It's not easy. This is not a disease that's simple. Consider having precautions in place to keep sick horses from coming in, and have a plan for what to do if horses become sick while at an event like a race meet to minimize the risk of further exposure. Owners need to take precautions themselves to reduce the potential for exposure—monitoring temperatures before, during, and after shipping in and out for an event. Fever is an early sign of several different causes of infection.  Getting a veterinarian to examine a horse with a fever promptly to determine the cause and getting recommendations on how to reduce risk of disease spread can be best for the sick horse and for other horses at the event.

To get updates on this and other communicable disease outbreaks, follow updates online from Dr. White's Equine Disease Communication Center.

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