Coronaviruses: a comparative review.

The coronaviruses have been recently classified as a separate virus genus on the basis of several fundamental characteristics, which include their nucleic acid type, the presence of a lipid envelope, and, in particular, their distinctive morphology (Tyrrell et al., 1968a). Members of the genus infect a number of different animal species, and until their reclassification were considered to belong to the myxovirus group although they possessed many atypical features. It was through detailed studies of their morphology in negatively stained preparations that they were finally differentiated and set out as a separate genus. When properly prepared, Coronavirus particles appear medium-sized, round, and moderately pleomorphic, and bear characteristic widely-spaced club-shaped surface projections. Coronaviruses naturally infect man, chickens, pigs, mice and rats, causing a wide variety of disorders involving a number of different organ systems. Indeed, new species are being added at frequent intervals as the techniques of electron microscopy and modern virology are applied to diseases which have often been clinically recognized for decades. A tentative scheme of the Coronavirus genus is shown in Table 1, with a listing of serotypes and strains. The list of types is not complete, but those of importance to this review are shown.