106. Virion buoyant density in CsCl is 1.36-1.39 g/cm3. Virion S20w is about 160S. Virions are resistant to pH 3, 50°C for 1 hour, 60°C for 5 minutes, chloroform, lipid solvents and non-ionic, anionic and zwitterionic detergents.
|
Type Species |
Human astrovirus |
(HAstV) |
Virions are 28-30 nm in diameter, spherical in shape and non-enveloped. A distinctive five- or six-pointed star is discernible on the surface of about 10% of virions (Fig. 1).
Physicochemical and Physical Properties
Virion Mr is about 8 106. Virion buoyant density in CsCl is 1.36-1.39 g/cm3. Virion S20w is about 160S. Virions are resistant to pH 3, 50°C for 1 hour, 60°C for 5 minutes, chloroform, lipid solvents and non-ionic, anionic and zwitterionic detergents.
Virions contain a single molecule of infectious, positive sense, ssRNA, 6.8-7.9 kb in size. A poly(A) tract is located at the 3
-end of the genome. The structure of the 5-end of the genome is not known.
Virion protein composition remains unclear; however, all isolates have at least two, possibly three, major proteins with a Mr between 24-39 103. Several isolates also contain smaller proteins with Mr 13-36 103. It has been reported that a smaller protein is removed from virions following purification in SDS.
Virions do not contain a lipid envelope. No information exists concerning fatty acid modification of any capsid protein.
No information exists concerning carbohydrate modification of any capsid protein.
Genome Organization and Replication
The genome organization and replication strategy of two human astroviruses have been determined (Fig. 2). A polyadenylated, sub-genomic RNA (about 2.8 kb) has been detected in the cytoplasm of infected cells. Viral RNA replication is resistant to actinomycin D. Post-translational processing of viral proteins has not been examined in detail. A presumed precursor of capsid protein (CP) with Mr of 86-90 103 has been detected in the cytoplasm of infected cells. Microsequencing of the VP26 and VP29 proteins of Human astrovirus 2 (HAstV-2) suggests that these proteins are overlapping and may arise via alternative processing. Early in infection, proteins have been detected in the cell nucleus. Mature virus is often seen in crystalline arrays in the cytoplasm of infected cells.
At least eight serotypes of human astroviruses have been defined by immune electron microscopy and neutralization tests. They share at least one common epitope recognized by monoclonal antibody. Neutralization epitopes have been mapped to the VP26 and VP29 proteins of HAstV-1 and HAstV-2. At least two distinct serotypes of bovine astroviruses have been described by neutralization.
Astroviruses appear to be host restricted, and have been detected in stool samples from humans, cats, cattle, deer, dogs, ducks, mice, pigs, sheep and turkeys. Transmission is by the fecal-oral route and no intermediate vectors have been described. Astroviruses are distributed worldwide and have been associated with about 2-8% of acute, non-bacterial gastroenteritis in children. Astroviruses have also been associated with gastroenteritis in immunocompromised adults. The predominant feature of astrovirus infection in humans and animals is a self-limiting gastroenteritis. In humans, astrovirus has been detected in duodenal biopsies in ephithelial cells located in the lower part of villi. In experimentally infected sheep, astrovirus was found in the small intestine in the apical two-thirds of villi. In calves, astrovirus infection was localized to specialized M cells overlying the Peyer’s patches. An often fatal hepatitis has been described in ducklings. The duck astrovirus, (Duck hepatitis virus 2) (types 1 and 3 are considered picornaviruses) is distinct from astrovirus isolates from turkeys and chickens in cross-protection and transmission studies.
Human, bovine, feline and porcine astroviruses have been isolated in primary embryonic kidney cells, but only the human and porcine viruses have been adapted to growth in established cell lines. Trypsin is required in the growth medium for serial propagation of the virus. Duck astrovirus 1 grows in embryonated chicken eggs following blind passage in the amniotic sac. Few infected embryos die in less than seven days. Infected embryos appeared stunted and have greenish, necrotic livers in which astrovirus-like particles have been identified.
List of Species Demarcation Criteria in the Genus
Species are distinguished on the basis of host origin. Serotypes are defined on the basis of twenty-fold, or greater, two-way cross-neutralization titers. Serotypes assigned to the species are given numbers (e.g., HAstV-1, HAstV-2, etc). At least seven serotypes of human astroviruses, two serotypes of bovine astroviruses and one serotype each of duck, ovine, porcine, and turkey astrovirus are recognized. Their relationships to each other and those observed in other hosts have not been defined.
Official virus species names are in italics. Tentative virus species names, alternative names ( ), strains or serotypes are not italicized. Virus names, genome sequence accession numbers [ ], and assigned abbreviations ( ) are:
|
Bovine astrovirus (2 serotypes) |
(BAstV) | |
|
Bovine astrovirus 1 |
(BAstV-1) | |
|
Bovine astrovirus 2 |
(BAstV-2) | |
|
Duck astrovirus (1 serotype) |
(DAstV) | |
|
Duck astrovirus 1 |
(DAstV-1) | |
|
Feline astrovirus (1 serotype) |
(FAstV) | |
|
Feline astrovirus 1 |
(FAstV-1) | |
|
Human astrovirus (7 serotypes) |
[Z25771; L13745] |
(HAstV) |
|
(HAstV-1) | ||
|
Human astrovirus 2 |
(HAstV-2) | |
|
Human astrovirus 3 |
(HAstV-3) | |
|
Human astrovirus 4 |
(HAstV-4) | |
|
Human astrovirus 5 |
(HAstV-5) | |
|
Human astrovirus 6 |
(HAstV-6) | |
|
Human astrovirus 7 |
(HAstV-7) | |
|
Human astrovirus 8 |
(HAstV-8) | |
|
Ovine astrovirus (1 serotype) |
(OAstV) | |
|
Ovine astrovirus 1 |
(OAstV-1) | |
|
Porcine astrovirus (1 serotype) |
(PAstV) | |
|
Porcine astrovirus 1 |
(PAstV-1) | |
|
Turkey astrovirus (1 serotype) |
(TAstV) | |
|
Turkey astrovirus 1 |
(TAstV-1) |
Tentative Species in the Genus
None reported.
List of Unassigned Viruses in the Family
None reported.
Phylogenetic Relationships within the Family
See Fig. 3.
None reported.
Astro: from Greek astron, “star”, representing the star-like surface structure on virions.
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