Astrovirus and Calicivirus; Hepatitis E

Astroviruses, measuring 28-30 nm and caliciviruses, 30-35 nm, are en­teritis pathogens in small children. Human pathogens in these groups include the Norwalk virus and hepatitis E virus (HEV). The latter occurs epidemically and endemically in Asian, Central American, and African countries. It is trans­mitted by the fecal-oral route, above all via drinking water, and causes rela­tively benign infections except in pregnant women. Hepatitis E is considered a traveler's disease.

Isolated cases and minor outbreaks of enteritis are typically attributed to un­specified viral infections. Besides unidentified bacterial infections, the viral pathogens that can cause such infections include adenovirus, rotavirus, astrovirus, and calicivirus, whereby the taxonomy of the latter two have not been confirmed.

Astroviruses

Pathogen. The astrovirus is 28-30 nm in size and owes its name to its starlike appearance. It contains sense RNA with approximately 7 500 nucleotides and appears to have a replication strategy similar to that of the picornaviruses.

Pathogenesis and clinical picture. Astroviruses that are animal and human pathogens are associated with episodes of diarrhea that nearly always run a harmless course. The etiological role of these viruses has still not been clar­ified. Astroviruses appear to possess only a low level of pathogenicity. It should be mentioned at this point that the role of viruses in enteritis is fre­quently exaggerated.

Diagnosis. Detection by means of electron microscopy.

Epidemiology. Astroviruses occur worldwide. They tend to infect young chil­dren and older persons weakened by other diseases.

Caliciviruses

Pathogen. Caliciviruses are 30-35 nm, possess only one capsid protein and a polyadenylated, 7500-nucleotide RNA with a VPg at the 5' end. The surface of the viruses has a characteristic structure with small, regular, calyxlike con­cavities that give the capsid the form of a Star of David.

Caliciviruses are classified based on genomic similarities as either human caliciviruses (HuCV) or “small, round-structured viruses,” SRSV. This designa­tion stems from their initial identification under the electron microscope as “small, round, virus particles.” The SRSV are grouped in two subtypes, I and II. Type I includes the Norwalk virus and a number of similar viruses named for their geographic venues, some with antigenicity differing from the Norwalk type.

Clinical picture. Caliciviruses cause enteritis. Together with rotaviruses  and adenoviruses, they are the most frequent viral enteritis pathogens in children, often causing minor epidemics during the winter months (“winter vomiting disease”).

Diagnosis. Detection by means of electron microscopy or antigen assay in stool.

Epidemiology. Two-thirds of the adult population in the temperate zone carry antibodies to the Norwalk virus. SRSV are regularly implicated in minor epidemics and family outbreaks. The transmission route of the Norwalk virus has been described: in addition to the fecal-oral route, water and uncooked foods are involved.

Hepatitis E Viruses

Pathogen. An infectious inflammation of the liver endemic to Asia, Central America, and parts of Africa is apparently transmitted by the fecal-oral route. The RNA genome of the culprit agent has now been sequenced and the virus in question, the hepatitis E virus, has been classified with the caliciviruses. It occurs in at least 13 variants divided into three groups. In-vitro culturing of HEV has not succeeded to date.

Pathogenesis and clinical picture. The clinical course of hepatitis E infections tends to be benign and resembles that of hepatitis A. It shows no chronicity. However, infections in the third trimester of pregnancy have a lethality rate of 10-40%.

Diagnosis. The antibodies can be detected by means of an enzyme immu­noassay. Apparently due to cross-reactions with other caliciviruses, the spe­cificity of the results is uncertain. A diagnosis is often arrived at based on clinical evidence and medical history (travel to endemic areas).

Epidemiology. HEV causes repeated outbreaks of considerable dimensions in the parts of the world mentioned above. The infections can be traced to contaminated drinking water. Hepatitis E is imported to central Europe as a traveler's infection, although apparently less frequently than hepatitis A. No specific prophylactic measures exist.