Tracking the spread of EHV-1

Tracking the spread of EHV-1

The key to tracking the spread of the neurological form of equine herpes virus (EHV-1) in large show enclosures may be to test the stables rather than the horses in them. Those are the findings of a recent study from the University of California, Davis.

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

real life scenario

It is not yet clear how long EHV-1 can survive on surfaces or whether environmental transmission of the virus is likely.
It is not yet clear how long EHV-1
can survive on surfaces or if environmental
virus transmission is likely.

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

EHV-1 outbreaks at large events understandably worry owners, but initial reactions can backfire. “Whenever there is 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 started seeing outbreaks in other counties.”

Following the spring outbreak, the owner of Desert International Horse Park contacted Pusterla. “The facility owner is a problem solver, not someone who sticks their head in the sand. He wanted us to take the opportunity to study how best to detect and assess outbreaks.” UC-Davis researchers accepted the invitation and visited the site 28 days after the first reported case. At the time, some horses were still in quarantine and around 400 horses in total were still on the ground.

The researchers collected nasal swabs from 231 healthy horses at the fairgrounds. They then took surface swabs from 203 stalls in 16 different barns. “We were looking to compare the accuracy and usefulness of the two types of swabs,” says Pusterla. “A nasal swab from a horse gives you its status at that particular moment. However, collecting from the environment gives you a complete picture of what may have accumulated in the environment over time. We took samples from the areas where the horses spent the most time, such as inside the front door, the water bucket, and the feed trough. These were places where there was likely to be a runny nose.”

The results

Six of the horses tested were positive for the presence of the EHV-1 virus. “This was not surprising,” says Pusterla. “Even outside of an outbreak, a small percentage of horses will test positive. It is expected that the virus will be eliminated from healthy horses; this is how the virus persists in horse populations.”

What is surprising, he says, is that 21 booths at the fairgrounds tested positive for EHV-1. And there was no correlation between positive horses and stables. “There were horses that tested positive in a negative barn,” he says. “Those horses may have started to shed and the environment wasn’t polluted enough to pick it up yet. In other cases, there were negative horses in positive stalls. They had been moving, but not at the time we tested them.”

Pusterla says it’s not yet clear how long the virus can survive on surfaces. It is also not known whether environmental transmission of EHV-1 is likely. The researchers then used PCR tests to detect trace amounts of the virus. “To know if what we detected was viable, we would have to cultivate it and grow it, and that was beyond the scope of this study,” says Pusterla.

What the findings mean for horse keepers

The utility of the test posts, explains Pusterla, may be to better detect and track outbreaks. “Taking samples from the horse gives you information only about that moment,” she says. “Rubbing the stables can provide a clearer picture of the past and present history of horse moulting. A single positive result from a horse is not a cause for panic. But if he sees three adjacent stalls that test positive for EHV-1, and then a fourth across the same aisle, he’s looking at a smoldering fire. He can argue that the best way to monitor for EHV-1 in exhibits is to monitor the environment, not the horses. It could set a threshold where too many posts mean silent spread is getting out of control.”

Pusterla adds that environmental testing would not replace basic biosecurity measures, such as disinfecting stables and isolating positive horses. “Common sense biosecurity is crucial,” she says. “If you do that, the risk is low. The problem is that not everyone does it or sees the benefit of biosecurity.”

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

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