Although the caliciviruses were the first to be clearly associated with outbreaks of gas-troenteritis, considerably less is known about their biology than about that of the ro-taviruses. They were first associated with an outbreak in Norwalk, Ohio, in 1968, and their role was confirmed by production of disease in volunteers fed fecal filtrates. The original virus was thus called the Norwalk agent, and similar viruses have been given names such as Hawaii agent, Montgomery County agent, Ditchling agent, and so on.
The viruses are small, naked, round RNA-containing particles 27 to 38 nm in diameter; their appearance is similar to that of the DNA-containing parvoviruses and hepatitis A virus (see Fig 39–1). They are classified as members of the Caliciviridae family. At present, two genera that cause diarrhea are recognized within this family: “Norwalk-like viruses” (sometimes referred to as “Noroviruses”) and “Sapporo-like viruses.” The viruses appear to be extremely hardy; their infectivity persists after exposure to acid, ether, and heat (60°C for 30 minutes). They have not been effectively propagated in cell or organ culture.
At least four different serotypes have been demonstrated by immunoelectron mi-croscopy with convalescent sera from affected patients. Knowledge of the antigenic characteristics and biology of these viruses has been seriously hampered by the current inability to grow them in the laboratory and by their lack of known pathogenicity for animals.
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