Infection of horses with Equine Coronavirus (ECoV)

Abstract

Coronaviruses have always been part of our biozoenosis. Different biological properties are part of their evolutionary strategies, such as the change of host systems, pathogenicity, organotropism, antigenicity and immunogenicity. In addition to humans and domesticated animals, a wide range of susceptible animal host systems exists. Coronaviruses have been divided into former group 1 and group 2 based on the different antigenic relationships. The new taxonomy now divides these viruses into the genera alpha-, beta- (man and animals) and gammacoronaviruses (poultry). The first isolation of typical coronavirus particles was found by electron microscopy in faecal samples from foals in Kentucky. They died or were killed in the acute stages of gastroenteritis, described by Bass and Sharpee (1974). Neutralizing antibodies against calf diarrhoea coronavirus were identified in equine sera. Durham et al. (1979) investigated foals in New Zealand with profuse watery diarrhoea and detected coronavirus in their faeces. They described a wide range of ages of the animals infected and serological cross-reaction with calf coronavirus. More virus-positive cases of enteric foal disease were reported in the following years. Eugster and Jones (1988) reported on coronavirus in an aborted equine foetus and Ward et al. (1983) described the presence of coronavirus in foals with diarrhoea. Sato et al. (1981) demonstrated neutralizing antibodies to calf diarrhoea coronavirus also in horses in Japan. All these reports deal with cases of severe gastroenteritis and foal diarrhoea. The first coronavirus-associated diarrhoea in mature horses was documented by Huang et al. (1983). There were 113 clinical cases of an acute form of diarrhoea in Maryland, USA, in the summer of 1982. The isolated virus was not related to bovine coronavirus, swine transmissible gastroenteritis or avian infectious bronchitis based on fluorescent antibody tests. Coronavirus could be isolated not only from faeces but also from plasma samples of the sick horses. The classic virological diagnostic techniques have been now replaced by molecular tests using qPCR. Reports on disease associated with equine coronavirus (ECoV) infections mainly in mature horses in the U.S.A. have been based on qPCR positive faeces since about 1990. The description of the clinical signs, virus transmission and diagnosis of the ECoV disease could be summarized as follows. It is spread via oral investigation of qPCR positive faeces and faecal-oral transmission can also occur when horses make oral contact with surfaces or objects that have been contaminated with infected faeces. The disease is diagnosed most commonly in winter. Subclinically infected horses (11–93%) can spread ECoV depending on their carrier status. This pertains to foals and adult horses. Incubation is two to four days and the clinical signs are fever up to 40.5°C, anorexia, depression, sometimes colic, diarrhoea may or may not be present, and frequent lying down. Low white blood cell count (leucopenia, lymphopenia), hyperammonemia could be responsible, with signs of encephalopathic disease. The faecal virus load (ECoV genome equivalents per gram of faeces) might be of prognostic value in the horses affected. Prognosis is good in most cases. Exposure to the virus can result in up to 85% infection rate, however, most horses do not show clinical signs. A horse with fever and no respiratory clinical signs may have ECoV in the faeces. Therefore, the qPCR of their faeces is recommended. Epidemiological studies of ECoV infections are still limited. The seroprevalence has been investigated by an indirect enzyme-linked immunosorbent assay (ELISA) utilizing ECoV spike S1 protein since 2017. Using this test, Zhao et al. (2019) demonstrated seroconversion by analysing 27 paired sera from horses involved in an ECoV outbreak. Seropositivity was carried by 25.9% (young horses) to 82.8% (adult horses) from 1084 sera from healthy Dutch horses. Sera of Icelandic horses reacted positive in 62%. This ELISA demonstrated the surprisingly high percentage of seropositive European horses. Seroprevalence in 9.6% of 5247 sera of healthy horses from different geographic regions of the U.S.A. was demonstrated by Kooijman et al. (2017). Summarizing the results available, ECoV could be demonstrated in high percentages in faecal material and nasal secretions (Pusterla et al., 2019) and plasma and blood (Huang et al. 1982, Goodrich et al. 2020) from healthy and/or diseased horses. Therefore, it seems to be questionable whether faecal-oral transmission is be the only infection chain. Further investigations are necessary. This is also part of the recommended methods of prevention. The latter are based on the following: high standards of sanitation in all equine facilities; disinfection methods; isolation of horses for three weeks, especially those with fever and no evidence of respiratory illness and horses moving to new facilities, especially in cases where they moved from a facility with ECoV; and positive faeces test are recommended.