Torovirus- An Overview


Torovirus is a genus of viruses that belong to the order Nidovirales, the family Coronaviridae, and the subfamily Torovirinae . The name torovirus comes from the Latin word "torus", meaning a ring or a doughnut, because of the distinctive shape of the virus particles. Torovirus is the only genus in the monotypic subfamily Torovirinae, and it also contains only one subgenus, Renitovirus .

There are four recognized species of torovirus: bovine torovirus (BToV), equine torovirus (EToV), porcine torovirus (PToV), and human torovirus (HToV) . These viruses primarily infect vertebrates, especially cattle, pigs, horses, and humans, and are associated with gastroenteritis and diarrhea . Among the four species, there is little genetic divergence (20-40%), and antigenic cross-reactivity has been observed between EToV, BToV, and HToV .

The first torovirus was discovered in 1972 from a rectal swab of a horse with diarrhea and was named Berne virus (later renamed as EToV) . EToV is the only torovirus that can be grown in cell culture, using equine kidney or dermis cells . In 1979, another torovirus was isolated from an outbreak of neonatal gastroenteritis in calves from a dairy farm in Breda, Iowa, and was named Breda virus (later renamed as BToV) . In 1984, torovirus-like particles were detected by electron microscopy in human fecal samples from patients with gastroenteritis and were named human toroviruses (HToVs) . Porcine toroviruses (PToVs) were identified later in 1998 from pigs with diarrhea.

Toroviruses are classified as group IV viruses according to the Baltimore classification system, which is based on the nature and polarity of the viral genome. Toroviruses have a single-stranded RNA genome with positive polarity, meaning that it can act as a messenger RNA and be directly translated by the host cell ribosomes . The genome is about 25-30 kilobases long and has a 5` cap and a 3` polyadenylated tail . The genome contains six open reading frames (ORFs) that encode for the replicase proteins (ORF1a and ORF1b) and the structural proteins (ORF2 to ORF5), which are spike (S), membrane (M), hemagglutinin-esterase (HE), and nucleocapsid (N) proteins .

Toroviruses are enveloped viruses with a lipid bilayer derived from the host cell membrane . The envelope contains three types of glycoproteins: S, M, and HE . The S protein forms club-shaped spikes on the surface of the virus that mediate attachment and fusion with the host cell receptors . The M protein is the most abundant protein in the envelope and plays a role in virus assembly and budding . The HE protein is a receptor-destroying enzyme that cleaves sialic acid residues from glycoproteins and glycolipids on the host cell surface and prevents virus aggregation . The N protein is the major component of the nucleocapsid, which is a helical structure that encloses the genomic RNA . The nucleocapsid has a unique C-shape or open torus morphology that gives the virus its name .