Astrovirus is a type of virus that was first discovered in 1975 using electron microscopes following an outbreak of diarrhea in humans. In addition to humans, astroviruses have now been isolated from numerous mammalian animal species (and are classified as genus Mammoastrovirus) and from avian species such as ducks, chickens, and turkey poults (classified as genus Aviastrovirus). Astroviruses are 28–35 nm diameter, icosahedral viruses that have a characteristic five- or sixpointed star-like surface structure when viewed by electron microscopy. Along with the Picornaviridae and the Caliciviridae, the Astroviridae comprise a third family of nonenveloped viruses whose genome is composed of plus-sense, single-stranded RNA. Astrovirus has a non-segmented, single stranded, positive sense RNA genome within a non-enveloped icosahedral capsid. Human astroviruses have been shown in numerous studies to be an important cause of gastroenteritis in young children worldwide.
Members of a relatively new virus family, the astroviridae, astroviruses are now recognised as a cause of gastroenteritis in children, whose immune systems are underdeveloped, and elderly adults, whose immune systems are generally somewhat comprised. Presence of viral particles in fecal matter and in epithelial intestinal cells indicate that the virus replicates in the gastrointestinal tract of humans. The main symptoms are diarrhoea, followed by nausea, vomiting, fever, malaise and abdominal pain. Some research studies have shown that the incubation period of the disease is approximately three to four days. Astrovirus infection is not usually a severe situation and only in some rare cases leads to dehydration. The severity and variation in symptoms correlates with the region the case develops in. This could be due to climatic factors influencing the life cycle or transmission method for that particular strain of Astrovirus. Malnutrition and immunodeficiency tend to exasperate the condition, leading to more severe cases or secondary conditions that could require hospital care. Otherwise, infected people do not need hospitalization because symptoms will reduce by themselves, after 2 to 4 days.
Electron microscopy, enzyme-immunoassay (ELISA), immunofluorescence, and polymerase chain reaction have all been used for detecting virus particle, antigens or viral nucleic acid in the stools of infected people. A method using real-time RT-PCR, which can detect all human astrovirus genotypes, has been reported.
A study of intestinal disease in the UK, published in 1999, determined incidence as 3.8/1000 patient years in the community (95% CI, range 2.3–6.4), the fourth most common known cause of viral gastroenteritis. Studies in the USA have detected astroviruses in the stools of 2–9% of children presenting symptoms; illness is most frequent in children younger than two years, although outbreaks among adults and the elderly have been reported. Early studies carried out in Glasgow demonstrated that a significant proportion of babies excreting virus particles, 12%, did not exhibit gastrointestinal symptoms; seroprevalence studies carried out in the US have shown that 90% of children have antibody to HastV-1 by age 9, suggesting that (largely asymptomatic) infection is common. There is, as with most viral causes of gastroenteritis, a peak of incidence in the winter.
Humans of all ages are susceptible to astrovirus infection, but children, the elderly, and those that are immunocompromised are most prone. The majority of children have acquired astrovirus antibodies by the age of 5 and, looking at the pattern of disease, it suggests that antibodies provide protection through adult life, until the antibody titre begins to decline later in life.
Astroviruses are associated with 5–9% of cases of gastroenteritis in young children. The occurrence of astrovirus infection varies depending on the season. In temperate climates infection is highest during winter months. This is in contrast to tropical regions where prevalence is highest during the rainy season. This seasonal distribution of infection in temperate climates is rather puzzling. But the seasonal distribution in tropical climates can be explained by the impact of the rain particularly on breakdown of sanitation in developing countries.
The main mode of astrovirus transmission is by contaminated food and water. Young children in childcare backgrounds or adults in military barracks are most likely to develop the disease.
Inactivated vaccines are in use for certain strains of Chicken Astrovirus (CastV).
There are no anti-viral treatment against infections but personal hygiene can reduce the incidence of the illness.
The family Astroviridae contains two genera: Mamastroviruses which infect mammals and Avastroviruses which infect birds. In each genus, there are species of astroviruses, each named after the host in which they replicate. The astroviruses are further subclassified within each species into serotypes.
There are at least three genotypes of bovine astroviruses. These appear to be related to the Capreolus capreolus astrovirus.
Astroviruses have a star-like appearance with five or six points. Their name is derived from the Greek word "astron" meaning star. They are non-enveloped RNA viruses with cubic capsids, approximately 28–35 nm in diameter.
Human astrovirus structure: Human astroviruses are part of the Mammastrovirus genus and contains 8 serotypes. The human astrovirus capsid spikes have a distinct structure. The spike domain in particular has a 3-layered beta-sandwiches fold and a core, 6-stranded beta-barrel structure. The beta-barrel has a hydrophobic core. The triple-layered beta-sandwich is packed outside the beta-barrel. The spike also forms a dimer. This unique structure was found to be similar to the protein projections found on the capsid of the hepatitis E virus. The projection domain of the human astrovirus contains a receptor binding site for polysaccharides. The amino acid sequence of the astrovirus capsid protein does not have similar homology to other known viral proteins, but the closest would be hepatitis E virus.
Viral replication is cytoplasmic. Entry into the host cell is achieved by attachment to host receptors, which mediates endocytosis. Replication follows the positive stranded RNA virus replication model. Positive stranded RNA virus transcription, using an unknown model of subgenomic RNA transcription is the method of transcription. Translation takes place by -1 ribosomal frameshifting. Vertebrates serve as the natural host. Transmission routes are fecal-oral.
Astroviruses have a genome composed of a single strand of positive sense RNA. The strand has a poly A tail at the 3' end, but no 5' cap. With the exclusion of polyadenylation at the 3' end, the genome is between 6.8–7.9 kb long.
The genome is arranged into three open reading frames (ORFs), with an overlap of approximately 70 nucleotides between ORF1a and ORF1b. The remaining ORF is known as ORF2. ORF2 encode the structural proteins, which are -at least- VP26, VP29 and VP32, the most antigenic and immunogenic of these being VP26. This protein is probably involved in the first steps of viral infection, being a key factor in the biological cycle of astroviruses.
The mutation rate has been estimated to be 3.7×10−3 nucleotide substitutions per site per year with the synonymous changes rate of 2.8×10−3 nucleotide substitutions per site per year.
The Astroviridae capsid is related to those of the Tymoviridae. The non-structural region is related to the Potyviridae. It appears that this group of viruses may have arise at some point in the past as a result of recombination event between two distinct viruses and that this even occurred at the junction of the structural and non structural coding regions.
In addition to humans, goats, cattle, pigs, wild boar, sheep, roe deer, bats, sea lions, rats, and mice are known to be infected with these viruses.