Equine coronavirus (ECoV) is classified within the Betacoronavirus 1 genus, along with human coronaviruses OC43, 4408 and HKU1, bovine coronavirus (BCoV), porcine hemagglutinating encephalomyelitis virus, canine respiratory coronavirus, mouse hepatitis virus, bubaline coronavirus and sialodacryoadenitis rat coronavirus. ECoV is genetically distinct from the human SARS-CoV-2 and there is no evidence to indicate that horses could contract SARS-CoV-2 or that horses may be involved in the spread of SARS-CoV-2 to other animals or humans.
Over a decade ago, a Japanese research group investigated an unusual outbreak of fever and enteric signs in 2- to 4-year-old racing draft horses. It is of interest to notice that enteric signs were only reported in 10% of the horses, and a total of 132/600 horses (22%) became diseased. Additional outbreaks have since been observed and reported in the USA and Europe. Collectively, these outbreaks have been able to refine the clinical presentation of ECoV, one that is still perplexing considering the inconsistent development of enteric signs. The lack of enteric signs such as colic and/or changes in fecal character may specifically relate to the intestinal section affected by the virus. ECoV has been shown to cause enteritis in both foals and adult horses. While enteritis is consistently associated with diarrhea in foals, this condition may not affect the fecal character of infected adult horses. Horses infected with ECoV generally recover with minimal to no medical treatment within 2-4 days post-onset of clinical signs. While ECoV infection is often self-limiting, adult horses may occasionally require intensive care to resolve leukopenia, systemic inflammation and metabolic disturbances. Morbidity rates have been reported to range between 17-57% and there are still yet undetermined host, viral, and environmental factors that impact susceptibility and outcome of ECoV infection. Of interest is the observation that clinical expression of ECoV infection is age-dependent with foals rarely developing clinical disease. Given the lack of documented outbreaks of ECoV at large breeding farms, it is possible that virus circulating between foals and breeding stock confers protection against clinical disease.
It is the lack of gastrointestinal signs that often misleads the equine veterinarian into ruling out an enteric pathogen. Leukopenia due to neutropenia and/or lymphopenia is a consistent hematological abnormality and, although not specific for ECoV, should direct the diagnostic work-up toward a viral disease. The laboratory support of ECoV infection is based on the detection of the virus in feces. Historical detection modalities such as electron microscopy and antigen capture ELISAs have been supplanted by quantitative real-time PCR (RT-qPCR). RT-qPCR has the advantage of being highly sensitive and specific, has a quick turn-around-time and is cost-effective. Further, the quantitative capability of RT-qPCR allows the study of viral kinetics and the ability to determine the horse’s contagious nature and prognosis. Experimental studies have shown that ECoV RNA can be detected as early as 72-96 hours post-inoculation and continues to be detected until 10-14 days post infection. In naturally infected horses, ECoV can be detected by RT-qPCR for 3-9 days with a detection time occasionally extending up to 25 days from onset of clinical disease.
Histological changes have only been reported in a very small number of horses and showed morphological changes similar to BCoV infection. The histological hallmarks of ECoV infection were diffuse necrotizing enteritis, marked villus attenuation, epithelial cell necrosis of the tips of the villi, neutrophilic and fibrin extravasation into the small intestinal lumen, as well as crypt necrosis, microthrombosis and hemorrhage. Post-mortem diagnosis of ECoV can be achieved by RT-qPCR on feces or small intestinal contents, and ECoV can be detected in intestinal tissue by electron microscopy, immunochemistry and direct fluorescent antibody testing using BCoV reagents.
Specific preventive measures are scarce, and there are yet no licensed vaccines against ECoV. The cornerstone of ECoV prevention resides in strict biosecurity measures aimed at reducing the risk of introducing and disseminating ECoV on equine premises. It is important to be vigilant when working-up horses presenting with fever, anorexia and lethargy, with or without concurrent enteric signs. Such horses should be isolated until ECoV, as well as other potential infectious pathogens, have been ruled in or out. ECoV qPCR-positive horses should be isolated and stable- or herdmates closely monitored until the outcome of past-exposure has been determined. Outbreaks of ECoV are generally short lasting, especially when strict biosecurity measures have been followed, and quarantine can routinely be lifted 2-3 weeks following the resolution of clinical signs in the last affected horse. While common disinfectants inactivate ECoV, it is unknown as to how long ECoV remains infectious in the environment. Severe acute respiratory syndrome (SARS)-CoV has been shown to persist up to 2 days in wastewater and dechlorinated tap water, 3 days in feces and 17 days in urine at room temperature. The survival of the virus is even longer at lower temperatures.