A decade before the deadly COVID-19 pandemic began, a virus from the same family may have been quietly traveling among the U.S. equine populations.

Equine coronavirus (ECoV) causes fever, lethargy and occasionally diarrhea in horses. It typically occurs in outbreaks among horses at boarding barns and racetracks, but a new study from Washington State University (WSU) shows that it can also affect only a few horses at a time.

Initially, ECoV was believed to be primarily a threat to foals. That changed in 2009, with a rash of outbreaks among mature horses in Japan. Two years later, a surge of ECoV cases began occurring at large boarding barns in the United States. Between November 2011 and April 2012, 161 horses tested positive for ECoV in California, Texas, Wisconsin and Massachusetts. Four of those horses died.

For their study, the WSU researchers reviewed the records of five horses diagnosed in non-outbreak situations between 2010 and 2018. Cases were included only if an ECoV diagnosis was confirmed using PCR, a DNA amplification technology that detects the presence of a pathogen. The average age of the study horses—two Quarter Horses, one Arabian, one Tennessee Walking Horse and one Miniature Horse—was 9 years old. Two of the ECoV cases developed in the spring, two in the winter and one in the summer. None of the horses had traveled to shows or other events, nor had any come in contact with any known ECoV cases.

The study cases underscore why ECoV can be difficult to diagnose, says Macarena Sanz, DVM, PhD. “Fever, anorexia and lethargy are the most common clinical signs of this disease,” she says. “Unfortunately, these clinical signs are also seen with multiple other disorders. [Horses] may also have change in manure consistency, but not all horses have these changes.” The study horses followed this pattern, with all five having anorexia and fever, four having some degree of diarrhea, and three being lethargic. Two horses had colic and one had softer than normal manure.

Larger studies of outbreaks have shown that ECoV can cause more severe gastrointestinal problems. “Some horses with ECoV may develop severe colitis (profuse diarrhea), low white-cell counts in the bloodwork and may be systemically very ill,” says Sanz. Ultrasounds conducted on two of the study horses showed abnormalities in the large colon, but none developed significant colitis.

“The clinical signs are not specific, so clinicians should keep this virus on their list of possible diagnosis, as it is contagious,” says Sanz. “A key clue to many veterinarians is a history of multiple horses affected in one location, which usually points to something infectious.”

Biosecurity is paramount in containing the spread of ECoV, says Sanz. “The virus is shed in the manure of horses who are sick. They can shed for a period of 14 to 21 days and they are highly infective to other horses. That’s is why isolation/quarantine of positive horses for a two- to three-week period is very important. In addition, horses in close contact with the sick ones should be tested even if they don’t develop clinical signs, as they could be subclinical shedders.”

Click here to read an in-depth exploration of the threat coronaviruses can pose to horses.

It’s important to note that horses cannot give ECoV to humans or catch it from them: “ECoV only affects horses,” explains Sanz.

Treatment for ECoV is primarily supportive, including anti-inflammatory medications and intravenous hydration if necessary. Four of the horses in the WSU study were given Banamine, and four received an anti-diarrheal supplement. Three were treated with intravenous fluids and three were given antibiotics to prevent opportunistic infections. The WSU researchers noted that two of the study horses developed abnormal heart rhythms and two had abnormal liver function during hospitalization, which lasted from four to 18 days. All of the study horses survived.

Sanz says this high recovery rate is also seen in the larger outbreaks among horses. “It depends on how affected the horse is but, in general, a large number of horses get infected but the overall mortality is low. However, mortality can be higher in horses that develop colitis—it’s been reported to be as high as 27 percent.”

Reference: “Equine Coronavirus-associated colitis in horses: A retrospective study,” Journal of Equine Veterinary Science, April 2020

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