In a recent study, horses with nasal swab testing positive for equine herpesvirus type 1 (EHV-1) also had positive swabs from their muzzles/outer nostril regions more than 75% of the time. The finding could offer alternatives for EHV-1 testing when sampling deeply into the nostrils is not possible or feasible, said Nicola Pusterla, DVM, Dipl. ACVIM, professor of medicine and epidemiology at the University of California, Davis.

“We’ve all known these horses that are very difficult to catch and even more difficult to get a nasal swab from,” Pusterla said, speaking at the 2022 American Association of Equine Practitioners (AAEP) Convention, held Nov. 18-22 in San Antonio, Texas.

“Therefore, it is sometimes of advantage to find alternative ways (of testing for EHV-1), especially when we’re involved with outbreaks and large numbers of horses that have to be swabbed or when dealing with horses that need to be swabbed and are highly refractory to the procedure.”

A Real Need—and Opportunity—for Less Invasive EHV-1 Testing

Deep nasal swabbing can be uncomfortable for some horses—and even the treating veterinarian due to the standing position to get the swabs, Pusterla said. And when dozens of horses need testing, it can be prohibitive in terms of time and physical effort.

Because of how EHV-1 propagates on and around horses, simpler swab opportunities could hint at infection, he said.

“We … know that EHV-1 is shed in high numbers, and the way I reflect that is it paints the environment and the horse,” Pusterla explained. “Anything that is in that room will be painted with EHV-1. You can find this virus on the wall, on the floor, feeder, water trough, on the neck, on the back, on the sling (in cases of neurologic disease associated with EHV-1), on any piece of equipment that enters that stall.

“For those who like to comply with biosecurity, that involves the phone that you pick up; that involves a stethoscope, your ultrasound machine, grooming equipment,” he continued. “And it’s on the person that is in that room. It’s on the gloves; it’s on the lab coat; it’s on the boots. It even goes as far as to the (humans’) nostrils.”

Alternative Testing During an Outbreak

Pusterla and his fellow researchers took advantage of such widespread shedding to try to find reliable alternatives to deep nasal swabbing—the gold standard of EHV-1 testing. In addition to standard deep nasal swabs, they took swabs from the muzzle/outer nostril area, front limbs, and rectum of 17 horses known to be clinically infected during a recent equine herpesvirus myeloencephalopathy (EHM, involving neurologic signs) outbreak, as well as from 19 healthy herdmates. The researchers also swabbed the horses’ feed bins and water troughs.

The team took these samples two to four times, two to six days apart, from and around each of the horses—which ranged in age from 4 to 27 years old and included about 60% geldings and 40% mares. They ran quantitative polymerase chain reaction (qPCR) testing on the samples, specifically targeting genes gB and ORF 30.

The Closer to the Inner Nostrils, the More Reliable the Swab

Of the 94 swabs, the researchers detected EHV-1 in 30 from the muzzles and outer nostril area, seven from the front limbs, six from the rectum, seven from feeders, and six from water troughs, Pusterla said.

This translated into an agreement between nasal swabs and the muzzle/outer nostrils in 77% of positive cases, he explained. For the other testing areas, that rate was 23% for front limbs and feeders and 19% for rectal regions and water troughs.

“What we found is that the nares/muzzle swabs … showed actually overall good agreement with the gold standard of nasal swab compared to the other swabs,” he said. “So it seems that the swabs could be used—not necessarily as the primary swab if you have a compliant horse but definitely in horses that have to be reswabbed—or if you have the chance to be involved with a large outbreak and have to collect samples or repeated samples in large numbers of horses.”

Such swabs could be more easily collected by clients and other nonprofessionals with brief training, he said. “And it’s speedy,” he added. “(It takes a) very short time to collect each of these swabs with really little or good compliance from the horse.”

While rectal samples were not reliable for diagnoses, the fairly high number of positive results confirms EHV-1 could be passed to other horses through exposure to feces, he said.

“From a biocontainment standpoint, it’s interesting,” he said. “If you’ve ever seen how barns are cleaned, where the wheelbarrow filled with manure is just moved from one stall over to the next, that potentially could represent a risk of transmission.”

Fecal sampling could also point to EHV-1 infections in herds of wild or feral equids, he said.

Finally, environmental sampling could aid in general outbreak management in a noninvasive way, Pusterla said.