Torovirus

Toroviruses are single-stranded RNA viruses with a peplomer-bearing envelope that have been associated with enteric disease in cattle and possibly humans. Toroviruses appear to occur worldwide, and torovirus-like particles in fecal preparations have been reported from Europe, the Americas, Asia, New Zealand, and South Africa. These viruses have a unique C-shape or open torus morphology in the extracellular environment. There are four recognized species in the genus Torovirus (family Coronaviridae, order Nidovirales): Equine torovirus, Bovine torovirus, Human torovirus, and Porcine torovirus. Among them there is little genetic divergence (20–40%). Antigenic cross-reactivity has revealed a relationship between equine torovirus (EToV), bovine torovirus (BToV), and human torovirus (HToV). The torovirus genome contains six open reading frames (ORFs), which are transcribed as a 3′ co-terminal nested set of four mRNAs. ORF1a and -1b encode the replicase, and ORFs 2–5 encode the spike (S), membrane (M), hemagglutinin-esterase (HE), and nucleocapsid (N) proteins, respectively. EToV is apathogenic, and the only torovirus that has been grown in vitro. All BToV strains are pathogenic, causing diarrhea in experimentally or naturally infected calves. HToVs have been associated with gastroenteritis and diarrhea in children, as well as with nosocomial infections in infants with necrotizing enterocolitis.