All human astroviruses, members of the family Astroviridae,
are included in the same genus, Astrovirus. However, there are five
serotypes of human astrovirus that have been identified by immunofluorescence,
neutralization, and immunoabsorbent electron microscopy since the 1980s.
Each of these reference strains was isolated from natural infections in
the United Kingdom and adapted to cell growth in cell culture. However,
serotypes 6 and 7 have recently been identified in the UK in 1989 and 1991-1992,
respectively, bringing the total number of human serotypes up to seven.
Astrovirus serotypes 6 and 7 are rare (only one case of serotype 6 infection
and only two cases of serotype 7 infection have been reported) and are
indistinguishable from serotypes 1 to 5 by electron microscopy, cultivated
in CaCo-2 cells in media supplemented with trypsin, and recognized by the
group-reactive monoclonal antibody.
Of the first five serotypes, type 1 is by far the most common; in Oxford, UK, from 1975 to 1992, 65-72% of all astroviruses encountered were serotype 1. Serotypes 2, 3, 4, and 5 each accounted for 6-8% of the astrovirus strains.