Astroviridae

Mamastrovirus Avastrovirus

The Astroviridae are a family of viruses, including the following genera:
 * Genus Mamastrovirus; type species: Human astrovirus
 * Genus Avastrovirus; type species: Turkey astrovirus

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. These viruses were first seen by electron microscopy over 30 years ago, after examination of stool samples from infants with gastroenteritis. Information on this virus family is limited, because of a restrictive tissue culture system (foetal tissue). But since their observation in 1975, certain characteristics have been described, these viruses: are non-enveloped, spherical in shape and have a diameter of 28-30nm.

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.

Transmission and pathogenesis
The main way 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

Human disease
An important infection of Astrovirus is called gastroenteritis, and 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.

Target populations
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.

Seasonal distribution
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.

Prevention
There is no vaccine or anti-viral treatment against the Astrovirus infection but personal hygiene can reduce the expression of the illness. In the case that children are in high risk of disease, prevention can be successful by oral rehydration therapy.