Non-Outbreak Equine Coronavirus Tests Rarely Positive, but Reliable

Veterinarians detect coronavirus in horses—which isn’t COVID-19 and isn’t transmissible to and from humans—by testing feces in the laboratory. If those tests are positive and the horse is showing classic clinical signs of fever, anorexia, and lethargy, you can safely assume he has coronavirus disease without further testing, according to new study results.

Current diagnostic procedure calls for multiple additional tests to rule out other sources of disease that might be causing the clinical signs, said Macarena Sanz, DVM, MS, Dipl. ACVIM, PhD, assistant professor at Washington State University, in Pullman.

Even so, rapid and accurate diagnosis of equine coronavirus (ECoV) is necessary to maintain control over outbreaks of the “highly contagious” disease, she said. Horses can shed the virus for 14 days in their feces, and if other horses consume the virus in soiled bedding or pastures, for example, they could become infected. “It is important to isolate positive horses for a minimum of two weeks, with three weeks being preferred,” said Sanz.

ECoV frequently affects the gastrointestinal (GI) tract, leading to colitis (infection of the colon) and causing fever, anorexia (refusing to eat), and lethargy (fatigue) in horses. However, these are common clinical signs that could result from many infectious and noninfectious diseases, Sanz explained. As such, practitioners generally include ECoV testing in their work-ups, but they don’t rely on a positive ECoV test as being the explanation for the horse’s illness until they’ve ruled out other sources. In other words, the general rule of thumb has been only to confirm ECoV as the “culprit” as long as the ECoV test is positive and all other tests are negative.

“The clinician has had to rule out multiple other infectious diseases before they could call a horse positive,” Sanz said. This adds to the wait time for results and, hence, to a conclusive diagnosis, when time is of essence for ensuring good biosecurity measures. It’s also a financial burden for horse owners, she added. Her group’s research, however, indicated that a positive fecal test combined with clinical signs gives a strong indication that “you are most likely to be in front of a positive horse,” she explained. Nonetheless, she cautioned against generalizing her results, which were focused on a single geographical area (Washington).

In their study, Sanz and her fellow researchers tested 130 hospitalized horses—unrelated to any ECoV epidemic—for the presence of ECoV virus in their feces. They aimed to determine just how common viral presence is, because few studies have examined the prevalence in horses outside of the context of an outbreak. They ran fecal testing with both polymerase chain reaction (PCR, a DNA-based test) assays and scanning electron microscope (SEM) analyses on two fecal samples from each horse, spaced about 48 hours apart (sometimes less if the horses weren’t hospitalized for at least two days). Half the horses in the study had been admitted to the hospital for gastrointestinal issues, whereas the other half were admitted for anesthesia for imaging procedures involving the limbs (as a control group).

Only one—an 8-year-old Miniature Horse gelding—had a positive PCR test for ECoV, Sanz said. The horse had been admitted to the hospital after six days of fever and anorexia and was later confirmed to have colitis. At the time of admission, the horse’s PCR test for ECoV was negative, she explained. It was the second test—at 48 hours—that showed a positive result. He showed improvements within a week of testing and was discharged after a 20-day hospital stay.

The fact that the general presence of ECoV in horses’ feces is so infrequent suggests that when it is present, veterinarians should take it seriously and consider it a likely cause of the horse’s fever, anorexia, and lethargy, Sanz said.

Interestingly, the scientists noted nine positive results for coronavirus when testing with scanning electron microscope. However, the microscope doesn’t detect the type of coronavirus—only the characteristic “crown” shape that give coronaviruses their name (“coronam” is Latin for “crown”; see sidebar). Hence, it’s possible these horses had acquired some kind of coronavirus other than ECoV, which would have shown positive on the PCR. Five of those positive results came from horses in the control group, with no signs of gastric disease, said Sanz.

The appearance of viral shedding after 48 hours of hospitalization (for ECoV as well as other possible coronaviruses, in addition to other viruses they tested for, such as parvovirus and rotavirus) suggests that stress might cause increased viral shedding, Sanz said.

“The results of our study are important for clinicians because ECoV is currently a ‘rule-out disease,’ as the clinical signs of this condition (fever, anorexia, and lethargy) are very common to many other diseases,” Sanz said.

“Because this virus is very contagious, it is important to have ECoV in the list of differential diagnosis when dealing with horses with unspecific clinical signs, especially if multiple horses are affected.”