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Astroviridae
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Members of the Astroviridae family contain species of astroviruses that infect a wide range of mammals and birds including: cows, turkeys, ducks, cats, pigs, sheep and humans. There are eight serotypes of Human Astrovirus, of which Astrovirus 1 is the most common. Astroviruses are recognized as the second most common cause of gastroenteritis in children worldwide. Epidemiological studies have demonstrated that astrovirus has a global geographic distribution and that infections are especially common in community centers, hospitals and day-care centers. Astroviruses were once responsible for a bulk of viral gastroenteritis-causing agents with unknown etiology. The identification and classification of astroviruses into genotypes and serotypes stems from advances in the use of immunoassays and molecular analysis.
Astroviruses are small RNA viruses found in a variety of animal species including humans (Mandel). The virions are non-enveloped, and show a cubic symmetry. They measure about 28 to 30 nm in diameter. When viewed with electron microscope (EM), the star-shape appearance is the defining characteristic of this virus, an observation made in maternity hospitals in UK in 1975 (Palombo). EM reveals a characteristic morphology that consists of round smooth edges with multiple triangular lucent areas and also an electron dense area in the center, which gives the virus the five or six pointed star appearance, from which the virus derives its name (Mandel). Cell cultured analysis of the virus has shown that virus particles exhibit a 10 nm layer of spikelike projections on the surface. Human astroviruses have positive sense, single stranded RNA genome with a 3' polyadenylated tail. The genomic organization includes three open reading frames: ORF1a encodes the protease region, ORF1b encodes the polymerase, and ORF2 encodes the capsid proteins. At least seven serotypes of human astroviruses have been recognized and are denoted as HastV-1 to HastV-7 and all of these seven serotypes are widely distributed across the world.
The pathogenesis of astroviruses isnot well understood. However, astrovirus infections in animals have been associated with instestinal villus shortening with decreased intestinal disaccharidase activity and subsequent diarrhea. Astroviruses are less pathogenic than Norwalk virus in adults.
Unlike other gastroenteritis
viral agents, astroviruses are usually shed in large amounts in the
stool and can readily be detected with EM. Techniques used to characterize
astrovirus include immune electron microscopy, immunofluorescence and
also other immunoassay techniques to detect the astrovirus group antigen.
In addition, nucleic acid hybridization and RT-PCR techniques are employed.
Diseases associated with astrovirus are usually not serious. The principle
goal in the treatment of astrovirus infection is the maintainance of
hydration and electrolyte balance. Oral reydration therapy is one effective
method of treatment for dehydrated patients, itravenous fluids may also
be administered in more severe cases. Because there are no antiviral
medications or vaccines for astrovirus, prevention of person- to- person
contamination is especially important. Frequent and thorough hand-washing
is critical in containing the disease.
References: Mandel, Douglas and Bennett's. Principle and Practice of Infectious Diseases. 5th Edition, vol.2, 1956-1957 The Biology and Epidemeology of Astroviruses: Reviews in Medical Microbiology. 12(2): 75-85, April 2001. Mustafa, Huseyin; Bishop, Ruth F. *; Palombo, Enzo A Role of Astroviruses
in Childhood Diarrhea: Walter JE*, Mitchell DK. The role of astroviruses
in childhood diarrhea. Current Opinion in Pediatrics: 2000;12:275-279 |
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