Astroviruses and Picobirnaviruses

Classification

Astroviruses are members of the Astroviridae family, which consists of two genera: Mamastrovirus , which infects mammals and includes the human astroviruses, and Avastrovirus , which includes avian viruses. Astroviruses were first detected by their characteristic starlike appearance on electron micrographs ( Fig. 177.1 ) and have subsequently undergone extensive molecular virologic characterization. The ability to grow astroviruses in cell culture, including primary HEK cells and a variety of monkey cell lines, has facilitated their study.

Mamastroviruses are subdivided into two genogroups (GI and GII), which have at least 19 genotypes. The first clade of these includes eight human astroviruses (HAstV1 to HAstV8) that correspond to the eight serotypes first noted in human astrovirus infections. Genotypes in two additional clades have been subsequently identified: HAstV-MLB and HAstV-VA/HMO. HAstV serotypes can be distinguished with immunofluorescence or plaque neutralization techniques, whereas considerable cross-reactivity exists with enzyme immunoassays (EIAs).

Structure

Astroviruses are positive-stranded RNA viruses that are found in a wide variety of animal species, in addition to humans. The virions are nonenveloped, display icosahedral symmetry, and are about 28 to 30 nm in diameter. Under the electron microscope, the particles in stool samples have a characteristic morphologic appearance that consists of round smooth edges with multiple triangular electron-lucent areas and an electron-dense center that results in the appearance of a five- or six-pointed star from which the virus derives its name (see Fig. 177.1 ). Analysis of virus grown in cell culture has shown the virus particles to exhibit a layer of 41-nm spikelike projections on the surface. The human astroviruses have a density of 1.35 to 1.37 g/mL in cesium chloride and contain a positive-sense, single-stranded 35S RNA genome with a 3′ polyadenylated tail.