Etiology

Coronaviruses are members of the Coronaviridae family, order Nidovirales, all of which are positive-sense ribonucleic acid (RNA) viruses.^3–6 The family Coronaviridae contains two genera, Torovirus and Coronavirus.^3,4,6–8 The coronaviruses were so named because the unusually large, club-shaped peplomers projecting from the envelope give the viral particle the appearance of a solar corona.^4,6,9 The tubular nucleocapsid is composed of a phosphorylated nucleoprotein and seems to be connected directly to a transmembrane protein, M, which spans the lipid bilayer three times and has only a small fraction of its mass exposed to the external environment.^4,6,8,9 Two types of prominent spikes line the outside of the virion. The long spikes, which consist of the S (spike) glycoprotein, are present on all coronaviruses and give them their characteristic “corona” appearance. The short spikes, which consist of the HE (hemagglutinin-esterase) glycoprotein, are present in only some coronaviruses.^4,6–8 Based on antigenic relationships and genetic homologies, the coronaviruses are subdivided into three antigenic groups. ^1–3,7,10 ECV is a member of the group 2 mammalian coronaviruses.⁷

Toroviruses are established agents of gastroenteritis in animals, and the type species of the genus is Berne virus (BEV), a chance isolate from a diarrheic horse in 1972.^5,8,11–13 Torovirus is discussed in more detail later in this chapter.

Epidemiology and Clinical Findings

Coronaviruses are a common cause of disease in humans and domestic animals.⁶ Coronaviruses have been identified in mice, rats, chickens, turkeys, swine, dogs, cats, rabbits, horses, cattle, and humans.^1,4 They cause respiratory, gastrointestinal, neurologic, and generalized infections.⁸ In horses, it is believed that coronavirus infection spreads through fecal-oral transmission; however, other routes of transmission, such as respiratory and mechanical, may also be possible.^8,14 Most coronaviruses infect only the cells of their natural host species and a few closely related species.⁶ In their natural host species, coronaviruses exhibit marked tissue tropism. Virus replication in vivo can be either disseminated, causing systemic infections, or restricted to a few cell types, often the epithelial cells of the respiratory or enteric tracts and macrophages, causing localized infections. Coronavirus replication takes place in the cytoplasm of infected cells.⁶

Anzai et al.¹⁴ investigated the effect of long-distance transport of 29 racehorses (age 2 years) on serologic evidence of infection with potential respiratory pathogens, including coronavirus. Serum antibody titers to coronavirus were evaluated by serum neutralization (SN) test using bovine coronavirus (BCV), which is closely related antigenically to ECV.^7,14,15 Two horses were seropositive for BCV 1 month before transportation (dilution titers 1:10 and 1:40). These horses were transported in the same vehicle as four horses that were seronegative to coronavirus. The four seronegative horses seroconverted after transportation (titers between 1:10 and 1:20 within 1 month), but none developed clinical signs, and a direct relationship between disease and coronavirus infection could not be confirmed.¹⁴ This study suggests that ECV may spread among horses while they are stabled together or during transport. This conclusion is consistent with serologic evidence that BCV or its related virus is widely prevalent in horses in Japan.^14,15

Coronavirus-like particles have been observed by negative-contrast electron microscopy (EM) in fecal samples from healthy and diarrheic foals,^16–21 from one foal with combined immunodeficiency syndrome and diarrhea,²² and from adult horses with Potomac horse fever.²³ Concurrent infections with rotavirus^18,19 and Cryptosporidium²² have also been reported.

Davis et al.²

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