Tracking the spread of EHV-1 

Testing stalls rather than the horses in them may be the key to detecting equine herpesvirus at horse shows and other events.

The key to tracking the spread of the neurological form of equine herpesvirus (EHV-1) at large show grounds may be testing stalls rather than the horses in them. Those are the finding from a recent study from the University of California, Davis.

EHV-1 is one of the five types of herpesvirus that most commonly infect horses. This strain most often causes only mild-to-moderate respiratory illness, but EHV-1 infection occasionally leads to the life-threatening neurologic disease known as equine herpesvirus myeloencephalopathy (EHM). Researchers are still working to understand how EHV-1 infection leads to neurologic disease.

Real life setting

It is still unclear how long EHV-1 
can survive on surfaces or whether environmental transmission of the virus is likely.
It is still unclear how long EHV-1 
can survive on surfaces or whether environmental 
transmission of the virus is likely.

In February and March of 2022, there was an outbreak of EHV-1 at Desert International Horse Park in Thermal, California. “This isn’t just any show,” says Nicola Pusterla, DVM, PhD. “This is one of the biggest show grounds in the United States. The circuit runs from late winter to early March. About 700 horses a day are at the site on any given show day.”

Outbreaks of EHV-1 at large events understandably concern owners, but initial reactions may be counterproductive. “Any time there’s even a rumor of an outbreak, everyone grabs their horses and runs away,” says Pusterla. “But they don’t always go home, they just go to other shows. So we start to see outbreaks in other counties.”

Following the spring outbreak, the owner of Desert International Horse Park contacted Pusterla. “The owner of the facility is a problem solver—not one to stick his head in the sand. He wanted us to use the opportunity to study how best to detect and assess outbreaks.” The UC-Davis researchers accepted the invitation and visited the site 28 days after the first reported case. At that point a few horses were still in quarantine and about 400 total horses still on the grounds.

The researchers collected nasal swabs from 231 healthy horses at the show grounds. Then they took surface swabs from 203 stalls in 16 different barns. “We were looking to compare the accuracy and utility of the two types of swabs,” says Pusterla. “A nasal swab from a horse gives you its status at that particular moment. Collecting from the environment, however, gives you the whole picture of what may have accumulated in the environment over time. We swabbed areas where the horses spent the most time, like inside the front door, the water bucket and feeding pan. These were places where there were likely to be nasal secretions.”

The results

Six of the horses tested came up positive for the presence of the EHV-1 virus. “This wasn’t surprising,” says Pusterla. “Even outside of an outbreak, a small percentage of horses will test positive. Shedding of the virus from healthy horses is to be expected—that’s how the virus persists in horse populations.”

What was surprising, he says, is that 21 stalls at the show grounds tested EHV-1 positive. And there was no correlation between positive horses and stalls. “There were horses who tested positive in a negative stall,” he says. “Those horses may have just started shedding and the environment wasn’t contaminated strongly enough to pick it up yet. In other cases, there were negative horses in positive stalls. They had been shedding but were not at the moment we tested them.”

Pusterla says it is still unclear how long the virus can survive on surfaces. Nor is it known whether environmental transmission of EHV-1 is likely. The researchers then used PCR testing to detect even minute amounts of the virus. “To know if what we detected was viable, we’d have to culture and grow it and that was beyond the scope of this study,” says Pusterla.

What the findings mean for horsekeepers

The utility of testing stalls, Pusterla explains, may be to better detect and track outbreaks. “Swabbing the horse gives you information only about that moment,” he says. “Swabbing the stalls can provide a clearer picture of the past and present history of shedding horses. A single positive result from a horse isn’t cause for panic. But if you see three adjacent stalls that all test positive for EHV-1, then a fourth across the same aisle, you are watching a smoldering fire. You can argue that the best way to monitor EHV-1 at shows is to monitor the environment, not the horses. You could set a threshold where too many stalls means the silent spread is getting out of control.”

Pusterla adds that environmental testing would not replace basic biosecurity measures, like disinfecting stalls and isolating positive horses. “Commonsense biosecurity is crucial,” he says. “If you do that, the risk is low. The problem is, not everybody does it or sees the benefit of biosecurity.”  

Reference: “Molecular monitoring of EHV-1 in silently infected performance horses through nasal and environmental sample testing,” Pathogens, June 2022

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