Learn about this recently recognized, highly contagious virus and how its discovery impacts your horse

On a balmy Nebraska morning in June 2017, the owner of a small boarding facility found one of her horses showing signs of mild colic. While the gelding’s herdmates grazed happily in the morning sun, this particular horse was alone, alternating between lying down and standing, kicking at his abdomen, nostrils flared, glaring at his flank. As she walked toward him he passed a small amount of very moist feces … definitely not normal for this horse. With every step the facility owner’s heart rate and blood pressure rose, as did her sense of panic. She immediately called her veterinarian and isolated the colicky animal in his stall until the vet arrived.

This scenario might feel all too real for anyone who has gone through it like this owner did. In this situation veterinarians tested the horse, diagnosing him with equine coronavirus (ECoV). They ultimately diagnosed six horses on the farm with this colitis (inflammation of the colon lining) causing infection, one of which succumbed to disease—a relatively rare occurrence with ECoV. 

Here we’ll share essential information about ECoV, how it spreads between horses, why it’s important to diagnose, ways to minimize chances of infection, and how to diagnose and treat the disease.

Getting to Know ECoV

Coronaviruses are essentially microscopic strands of RNA (genetic material similar to DNA) wrapped in an envelope of proteins. So named because they look like they’re wearing crowns when viewed under a microscope, coronaviruses can be subdivided into different groups and genera. The ECoV is a species-­specific member of the coronavirus family, classified within the Betacoronavirus 1 genus. Unlike bacteria, which are capable of multiplying on their own, viruses like ECoV must invade a host cell (e.g., one lining the inside of a horse’s gastrointestinal tract), taking over its resources to divide and make more virus particles. Other examples of coronaviruses include severe acute respiratory syndrome (SARS-CoV), a sometimes-fatal human disease that might sound familiar from news reports in the early 2000s, and bovine coronavirus (BCoV), known for causing winter dysentery and respiratory disease in cattle.

“Horses become infected with ECoV by ingesting the virus that was shed in the feces of another horse,” says Scott Weese, DVM, MSc, Dipl. ACVIM, a professor in the Department of Pathobiology at the University of Guelph’s Ontario Veterinary College, in Canada.

He explains that a recent in-depth review on coronaviruses and how they cause disease reveals that once a horse ingests ECoV, the virus appears to travel to the small intestine, where it attaches to specific receptors on intestinal cells. It does so using proteins protruding from its outer envelopes—akin to the jewels on a crown. From there, the virus particle fuses with the host’s cell, and the RNA enters the host cell and integrates itself into that cell’s DNA. The virus then uses the host cell’s resources to replicate, making thousands more disease-causing virus particles. 

Nicola Pusterla, DVM, PhD, Dipl. ACVIM, has dedicated years to studying this pathogen (disease-causing organism). A professor in equine internal medicine in the University of California, Davis, Department of Veterinary Medicine and Epidemiology, he says that “ECoV is a relatively newly recognized disease affecting the enteric system of adult horses. Since 2010, when the ECoV test was developed, outbreaks of ECoV have been diagnosed in Japan, Europe, and throughout most of the United States.”

Pusterla recently co-authored a study in which he and colleagues summarized their findings from examining 472 horses involved in 20 ECoV outbreaks from November 2011 to March 2017. Based on that data, he says these are the most common signs of illness associated with ECoV infection in adult horses:

He notes that only 130 of the 472 study horses that tested positive for ECoV displayed clinical signs of disease. In fact, an estimated 4-83% of affected horses remained healthy, meaning they showed no clinical signs of disease despite tests identifying ECoV in their feces.

“We estimate that only about 20% of horses that become infected with ECoV and test positive will show any of the above-described clinical signs of disease,” says Pusterla.

Horses that do develop clinical signs most often respond to basic supportive care, usually involving fluids and non-­steroidal anti-inflammatory drugs (NSAIDs) such as phenylbutazone or flunixin meglumine. It’s uncommon for horses to die from the disease, but when they do it’s typically due to disease complications rather than the primary infection, says Linda D. Mittel, MSPH, DVM, senior extension associate at Cornell University’s Animal Health Diagnostic Center (AHDC), in Ithaca, New York.

“One potentially serious fatal complication is called hyperammonemia,” she says. “This causes mild to severe neurologic conditions due to excess ammonia production in the bowel. This occurs due to a change in the bowel bacteria, called the intestinal microbiome, that produce high levels of ammonia that get absorbed into the bloodstream and circulate throughout the body, including the brain.”

Mittel adds that some owners call coronavirus infection the lying-down disease. “Even though the horses are not usually severely colicky, they often are found down and just not quite right,” she says.

Risk Factors for ECoV Infection

Pusterla’s current data show that approximately 2-6% of horses suspected of infection ultimately test positive for ECoV. This makes ECoV a seemingly insignificant condition; however, the number of affected horses identified annually has increased over the past eight years. Pusterla and colleagues suggest this reflects increased recognition of ECoV and owners’ willingness to test for it.

Moreover, after testing 5,250 apparently healthy horses from 18 states, Pusterla reports the horses most at-risk for ECoV:

• Reside in the Midwestern U.S.;
• Are draft horses; and
• Participate in ranch work/farming or are used in breeding programs.

“These horses appear to have the highest chances of testing positive for ECoV,” he says. “This was determined based on blood analysis or seroprevalence, which tests for the presence of antibodies against ECoV showing either current infection or previous exposure to the virus.”

Data from Japan also confirm that draft horses appear more susceptible to developing ECoV than other breeds, such as Thoroughbreds.

“There’s variation in reports of disease, but that may simply be testing bias,” adds Weese. “We don’t tend to see ECoV in Ontario, despite some testing, so I suspect there is still some true regional variation.”

The Importance of Diagnosing ECoV

Equine coronavirus occurs in fewer than one in 10 horses, most cases remain free of clinical signs of disease, and most horses that do develop clinical infections typically recover uneventfully even without treatment. So why do we care about what appears to be a relatively benign viral disease? Further, why bother spending money on testing rather than simply treating suspected cases if needed?

“While there are no specific treatments for ECoV, a timely diagnosis can still be useful to exclude other causes that might have specific treatments, understand disease trends, and guide infection control practices on the farm,” Weese says.   

Just as you wouldn’t take an equine herpesvirus outbreak lightly, don’t treat ECoV outbreaks as benign. Like equine herpesviruses, ECoV is highly infectious, spreading rapidly between horses. Also similar to herpesviruses, apparently healthy horses can shed ECoV—a population referred to as inapparent shedders.

“Further, if one considers the extensive list of colitis/enteritis (small intestine inflammation) causes, the need for testing to discriminate between them is clear,” says Weese.

Causes of diarrhea in adult horses that can cause signs similar to ECoV include Salmonella spp; Clostridium difficile and Clostridium perfringens; Potomac horse fever; parasitism; antibiotic use; sand impaction; dietary imbalances; inflammatory bowel disease; and more.

“Diarrhea is always a concern in adult horses because it can be severe and because outbreaks can occur,” says Weese. “Regardless of the underlying cause, the wall of the horse’s colon can secrete large amounts of fluid and proteins into the intestinal tract. This results in loss of valuable electrolytes and nutrients from the intestinal content.

“The inflamed wall also allows bacteria and their toxins that usually remain in the lumen of the intestine to gain access to the bloodstream, resulting in endotoxemia,” he adds. “These bacteria and toxins negatively impact various internal organs and can also contribute to complications such as laminitis.”

The ECoV Test

Several laboratories in the United States offer ECoV testing.

“The test currently offered by our laboratory is referred to as a quantitative PCR test,” says Mittel. “This test allows us to quickly detect even very small amounts of viral genetic material in the submitted fecal samples,” which must be sent in by your veterinarian. Turnaround time for results is typically three business days

Since 2013 the AHDC has received approximately 2,000 fecal samples for equine coronavirus testing. Of those, about 18% were ECoV-positive.

Note, however, that horses can test negative during the very early stages of clinical disease because viral shedding peaks three to four days after clinical signs appear.

“In other words, if a test is performed too early it may come back as a false negative,” says Mittel. 

Weese also suggests not hanging your hat on testing alone.

“ECoV testing is useful but may not have a large impact on case care—how the horse is treated—in sporadic cases,” he says. “Testing for something like Salmonella, where there are greater herd and public health implications, should also be performed.” 

Preventing ECoV Infection

Like many equine infectious diseases, particularly those caused by viruses, the most important strategy for avoiding infection is appropriate disease control and biosecurity strategies (TheHorse.com/biosecurity-tips).

“Standard strategies that would be used for any type of equine disease outbreak, such as equine herpesvirus outbreaks, are advocated,” says Weese. “These include isolating all new horses introduced onto the premises; isolating sick animals and handling them last out of the groups; and using hand-washing, protective outerwear, foot baths, and disinfectants to minimize spread of disease.” 

Unfortunately, unlike the equine herpesviruses, a vaccine for ECoV is not currently available anywhere in the world. Researchers in Japan (Nemoto et al.) did, however, recently test a coronavirus vaccine for use in cows—the virus belongs to the same Betacoronavirus genus as ECoV—in horses. They conducted this study to determine if the BCoV vaccine could induce the horse’s body to produce ECoV infection-fighting antibodies. This strategy is similar to using the canine Lyme vaccine in horses.

The research team found that the BCoV vaccine did induce antibody production against ECoV in young Thoroughbreds. Each horse received the vaccine intramuscularly on two occasions, 28 days apart. The authors noted, however, that blood antibody levels, or titers, were not particularly elevated.

“It is unclear whether the ­antibodies provided by the BCoV vaccine are sufficient to be effective against ECoV and, therefore, ECoV challenge studies (exposing healthy horses to the virus following vaccination) in horses are needed to evaluate the efficacy of the vaccine in the future,” they wrote.

“In addition to efficacy, additional studies on the safety of administering the BCoV vaccine to horses must be conducted prior to recommending the widespread use of the bovine vaccine in at-risk horses,” adds Pusterla.

Take-Home Message

Pusterla, colleagues, and other veterinarians and researchers continue making progress in understanding this disease. Nonetheless, there remain certain voids. Nearing the top of the list includes preventing infection in the first place. At present, prevention relies on the standard infection control and biosecurity strategies mentioned earlier, until additional data regarding vaccination and epidemiological information that can help identify at-risk horses become available.