Astrovirus

Astrovirus is a type of virus that infects mammals and birds. Astroviruses belong to the virus family Astroviridae. They were first described in the year 1975 using electron microscopes during an outbreak of diarrhoea.^[1] Astrovirus has a non-segmented, single stranded, positive sense RNA genome within a non-enveloped icosahedral capsid.^[2]

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Taxonomic structure

The family Astroviridae contains two genera: Mamastroviruses which infect mammals, and Avastroviruses which infect birds. Within each genus, there are species of astroviruses, each of which is named after the host in which they replicate. The astroviruses are further subclassified within each species into serotypes.^[3]

Virus structure

Astroviruses have a star like appearance with 5 or 6 points and their name is derived from the Greek word “astron” meaning star. They are non-enveloped RNAviruses with cubic capsids, approximately 28-30nm in diameter.^[4]

Genome

Astroviruses have a genome composed of a single strand of positive sense RNA. The single strand of RNA 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 3 open reading frames (ORFs), with an overlap of approximately 70 nucleotides between ORF1a and ORF1b. The remaining ORF is known as ORF2.^[5]

Human disease

Members of a relatively new virus family, the astroviridae, astroviruses are now recognised as a cause of gastroenteritis in children and adults. The main symptoms are diarrhoea, followed by nausea, vomiting, fever, malaise, anorexia, and abdominal pain. Some research studies have shown that the duration of the symptoms are approximately three to four days.Astrovirus infection is not usually a severe situation and only in some rare cases leads to dehydration. Infected people do not need hospitalization because symptoms reduce by themselves, after a small period of time.^[6]

Diagnosis

Electron microscopy, enzyme-immunoassay EIA, immunofluorescence, and PCR have all been used. These detect virus particle, antigens or viral nucleic acid in the stools of infected people.^[7]

Epidemiology

A study of intestinal disease in the UK, published in 1999 determined incidence as 3.8/1000 patient years in the community (95%CI, range2.3-6.4), the fourth most common known cause of viral gastroenteritis.^[8] Studies in the USA have detected astroviruses in the stools of 2-9% of children presenting symptoms; illness is most frequent in children of less 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, and 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.^[9]

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.^[10][11]

Astroviruses cause disease worldwide, and they are second only to rotaviruses as the cause of childhood diarrhoea. 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.^[9]

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

Prevention

There is no vaccine or anti-viral treatment against the Astrovirus infection but personal hygiene can reduce the incidence of the illness.

References