Abstract
Introduction. The Coronaviridae is a monogeneric family comprising 11 viruses which infect vertebrates. Members of the group are responsible for diseases of clinical and economic importance, in particular respiratory and gastrointestinal disorders (Table 1). The group was originally recognized on the basis of a characteristic virion morphology (Tyrrell et al., 1968), but can now be defined by biological and molecular criteria. Various aspects of coronavirus biology have been dealt with in recent reviews (Robb & Bond, 1979; Siddell et al., 1982; Wege et al., 1982).
Structure. Morphology. Coronavirions are pleomorphic, although generally spherical, 60 to 220 nm in diameter and bear widely spaced, club-shaped surface projections about 20 nm in length. Complete virions have a density in sucrose of about 1.18 g/ml. In thin sections the virion envelope may be visualized as inner and outer shells separated by a translucent space. In negatively stained preparations of avian infectious bronchitis virus (IBV) an inner tongue-shaped membrane is visible (Bingham & Almeida, 1977; Fig. 1).