Enteric infections with coronaviruses and toroviruses.

Many enteric viruses are difficult or impossible to propagate in tissue culture. Coronaviruses and toroviruses are large, enveloped, plus-strand RNA viruses in the order Nidovirales that cause enteric disease in young pigs, cows, dogs, mice, cats and horses. Two different serogroups of mammalian coronaviruses cause frequent respiratory infections in humans, and coronaviruses and toroviruses have been implicated in human diarrhoeal disease by immunoelectron microscopy. However, there is as yet no consensus about the importance of these enveloped viruses in human diarrhoea, and little is known about their genetic variability. The large spike (S) glycoprotein is an important determinant of species specificity, tissue tropism and virulence of coronavirus infection. To infect enterocytes, both S glycoproteins and the viral envelope must resist degradation by proteases, low and high pH, and bile salts. One specific site on the S glycoprotein of bovine coronavirus must be cleaved by an intracellular protease or trypsin to activate viral infectivity and cell fusion. S glycoprotein binds to specific receptors on the apical membranes of enterocytes, and can undergo a temperature-dependent, receptor-mediated conformational change that leads to fusion of the viral envelope with host membranes to initiate infection. Analysing spike-receptor interactions may lead to new ways to propagate these enteric viruses as well as new strategies for development of novel antiviral drugs.