Astroviruses

The human pathogens in this group have been poorly studied since for the most part, they do not grow in culture. Only recently has molecular information on the nature of their genomes become available. The taxonomy of these viruses is still somewhat confused and they have previously been grouped on the basis of their appearance in the EM (unreliable). Recently, nucleotide sequences of members of each group have been determined, allowing genome organization to be used as a basis for future classification:

Group IV: (+)sense RNA Viruses
Family	Genus	Type Species	Hosts
Astroviridae	Avastrovirus	Turkey astrovirus	Vertebrates
Astroviridae	Mamastrovirus	Human astrovirus	Vertebrates

Astroviruses were first described in 1975 as a result of electron microscope (EM) studies of an outbreak of diarrhoea in a maternity unit and a survey into the etiology of diarrhoea in childhood in a Scottish city. Subsequently astroviruses were experimentally transmitted to adult volunteers and adapted to growth in human foetal cells. Restrictions on the availability and use of foetal tissue limited the cultivation of astroviruses and therefore studies were confined to a few laboratories that access to electron microscopes.

Surveys conducted using EM showed that astrovirus infection occurs world-wide and accounts for 2-8% of cases of diarrhoea in infants. One study investigating the causes of infectious intestinal disease in England based on surveillance in GP practices showed that astroviruses were the most frequent viral cause (Roderick et al, Epidemiol Infect 1995:114: 277-288). A major advance in the ability of laboratories to diagnose astrovirus came as the result of the finding that they could be propagated in a continuous line of colon carcinoma cells CaCO₂ (Willcocks et al, Arch Virol, 1990; 113: 73-81). The development of enzyme immunoassays (EIA) for detecting viruses in stools showed that astrovirus is a significant cause of diarrhoea in developing countries. In Thailand, 8.6% of cases of diarrhoea were associated with astrovirus and a similar rate was found in Guatemala (7.3%). Application of EIA to study outbreaks in day care centres in the USA showed that many more cases were detected by EIA than by electron microscopy, the most widely used diagnostic technique.

The name of the family is derived from their distinctive appearance in electron microscopy - a smooth or slightly indented outer electron-dense shell with an inner, negatively staining 5 or 6 pointed star shaped core. To view a negatively-stained electron micrograph of astrovirus particles, click here.

Clinically, these viruses cause similar symptoms to caliciviruses. Astroviruses have been isolated from birds, cats, dogs, pigs, sheep, cows & man. There are at least 7 human astrovirus serotypes. Like rotaviruses, astrovirus infections occur through the year with peaks in the winter months. Infections have been shown to occur mainly in childhood. A serological survey of 87 with children for antibody prevalence to astrovirus in England found that 67% of the children became seropositive by the age of 4 years and 87% by age 10. Other studies showed that most of the cases of infection were detected in children under five years of age with the majority of the children being under 1 year of age. Outbreaks of astrovirus infection involving children and elderly patients have been described and prolonged excretion documented in immunosuppressed, immunodeficient and AIDS patients.

Walter JE, Mitchell DK. Astrovirus infection in children. Curr Opin Infect Dis. 16: 247-253, 2003.

Studies have shown that astrovirus type 1 is the prevalent strain in the UK accounting for 65% of the cases. However this situation may be changing with the emergence of more cases of astrovirus type 4. In contrast, type 2 was the most prevalent in Mexico (31%) and type 1 relatively rare (6%) and there are reports of outbreaks of foodborne astrovirus infection in Japan involving thousands of children and adults associated with astrovirus type 6.