Chances are good that all these horses are carrying the Equine Herpesvirus, which usually remains inactive. Stress can reactivate the virus and it can on occasion recombine into the often fatal neurologic form. The increased severity of outbreaks has led to the designation of EHM as an emerging disease. Photo credit: Suzanne Bush
In an unthinkable Christmas week tragedy at Mile View Farm in Doylestown, Bucks County, PA, three horses were euthanized after being diagnosed with the neurologic form of Equine Herpesvirus, also called Equine Herpesvirus Myeloencephalopathy (EHM). Just before New Year’s Eve, a fourth horse was euthanized.
“They didn’t deserve this,” Dr. Craig Shultz says of the farm’s owners and those who lost horses. “But they’ve handled everything really well.” Shultz is Director of Pennsylvania’s Bureau of Animal Health and Diagnostic Services and he praised the farm’s owners for their response to the outbreak. “The folks at the facility have been wonderful. They’ve done everything they could possibly do; they’ve cooperated 1,000 per cent.”
This disease is one of those which trigger the “all hands on deck” response from regional and state veterinary authorities. The goal is to contain outbreaks, track horses that have been exposed to horses that have developed full-blown symptoms and monitor quarantines. “It’s a story that has played out across the country hundreds of times,” Shultz explains. “This is by far not the worst, but they’re all terrible. It’s a nasty disease.”
He said that there are still some horses at the farm that are “clinical,” meaning they have fevers or have tested positive for the disease and they’re being observed. “The last update I had from our regional veterinarian was there has not been any deterioration in the horses that were febrile. We just have to wait it out.”
The Carrier in Your Pasture
According to the United States Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS), by the time they are two years old, almost every horse has been exposed to—and thus carries—the Equine Herpesvirus (EHV). The mystery is why the virus reactivates with such apparent randomness and ruthlessness. Shultz says the virus circulates throughout equine populations, and “recombines its DNA like all viruses do.” He says they don’t know which factors come together to create the “perfect storm” that generated so much heartache at Mile View Farm.
Equine herpesvirus can cause spontaneous abortion in pregnant mares, and other strains can cause upper respiratory disease in yearlings and weanlings. “That’s part of the syndrome. Why some horses simply develop these other manifestations and go merrily on their way and never get neurological we don’t know,” Shultz said.
Although there have been nine variants of EHV identified throughout the world, the variants that are most troublesome are EHV-1, EHV-3 and EHV-4. “We know that the virology is very complex,” Shultz explains. “Any of them could progress to the neurologic. Any of those viruses can produce EHM.”
Besides its ability to morph from a less lethal variant to the deadly neurologic form of EHV, the virus is very good at hiding its identity. “It’s a very difficult to diagnose disease because early in the stages these horses will not test positive,” Shultz says. “And then later they’ll go on to demonstrate virus on Polymerase Chain Reaction (PCR) testing or in their blood. It’s very unpredictable. That’s why when we see it, we lock everything down.” PCR is the method used by laboratories to reproduce sections of DNA or RNA, from a virus like EHV, for diagnosis.
Virus Spreads by Contact
Shultz says that EHM can spread quickly—even before anyone realizes an individual horse is sick. “It’s spread by contact, horse to horse,” he says. And people and objects can spread it, too. “Tack, water buckets, feed, humans handling infected horses.” People working around an infected horse can unconsciously spread the virus through the most mundane barn chores such as filling water buckets or feeding. “It’s not an easily aerosolized virus,” Shultz explains. “We certainly see, when these outbreaks occur, those horses close to ground zero are at greatest risk.”
He says it’s critical to contain the disease. “Of the two horses that died on December 21, one of those horses had been at a show at Hidden River, NJ on December 12. New Jersey veterinarians have monitored the situation closely. They’ve reached out to all the people at the show.” He says no cases of EHM have been reported in New Jersey, and that the disease appears to be contained now at Mile View Farm. The farm is under mandatory quarantine, which will last for at least 28 days from the last suspected new infection.
An Emerging Disease
According to APHIS, “the virus can be reactivated during times of stress, such as strenuous exercise, long-distance transport or at weaning.” They have recently classified EHM as an “emerging disease.” That is, there has been a significant increase in the occurrence of EHM. APHIS uses three criteria to determine whether to classify a disease as emergent:
- The disease is identified for the first time in a region or a country;
- A disease changes in severity, type of animal that can be infected, or other pathogen behavior, or
- There is a change in geographic range of a disease or in its incidence within a range.
Because recent outbreaks of EHM have been more severe, and the virus behavior seems to be different, APHIS believes that the second criterion has been met. Although it’s possible that reporting of the disease has been more consistent, APHIS believes more study is required. There is no vaccine to protect horses from EHM, and the only treatment currently available is palliative.
In October 2015 a horse at Parx Racetrack in Bensalem was diagnosed with EHM, and that horse survived, but Shultz says that the horse does have some neurological impairments. No other cases of EHM occurred at Parx.