What are Norwalk-like viruses

Discovered in 1975, Astroviruses are uniformly small, round viruses with a characteristic starlikeappearance in the electron microscope. Animal and human strains of astroviruses have recently been placed in a separate Astrovirus family. Seven different serotypes of human astroviruses have now been described, causing gastroenteritis mainly in children.

 

Importance / Illness
Astroviruses cause gastroenteritis, predominantly diarrhoea, mainly in children under five years old although it has been reported in adults. Seroprevalence studies show that more than 80% of children between 5 and 10 years old have antibodies to astroviruses. Occasional outbreaks in schools, nurseries and families have been reported. The illness is self-limiting, of short duration and has a peak incidence in the winter. The illness is self-limiting, of short duration and has a peak incidence in the winter.

However, the number of infections may be under-estimated, since the illness is usually mild, and many cases will go unreported.

Types of Astroviruses
The genus "Astrovirus" within the familiy of Astroviridae is divided into eight human species:

- human astrovirus 1 (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)

 

Morphology
The astroviruses are spherical particles about 28 nm in diameter without an envelope. Nucleocap-sids are isometric with an obvious regular surface structure (star-like with five or six points).Virions contain one molecule of positive-sense single stranded RNA. Total genome length is about 6800-7900 nt, excluding the poly (A) tract at the 3' end. Currently, three of the seven human serotypes have been fully sequenced. The full details of the replication cycle are not known, but it is suspected that replication occurs primarily in the cytoplasm, with a possible nuclear step.The genome is composed of three open reading frames. They encode both a full genomic and a subgenomic RNA. ORF 1a is approximately 2,842 nucleotides long and overlaps with ORF 1b (approximately 1,557 nucleotides long) by 70 nucleotides. There is a (-1) ribosomal frameshifting event between the two frames. The length of ORF 2 varies according to strain, but is between 2,358 and 2,388 nucleotides long. ORF 1a is thought to encode the viral protease, ORF 1b an RNA dependent RNA polymerase, and ORF 2 non structural proteins. This is consistent with the observation that it is a generally effective strategy for viruses to encode nonstructural proteins at the 3' end and structural proteins at the 5' end.

Physicochemical properties
Astroviruses have a single-stranded RNA genome with a molecular weight of about 2500 kD. Their buoyant density has been reported as within the range 1.35-1.40 g.ml-1 in caesium chloride. Astroviruses are acid stable and resistant to a range of detergents and lipid solvents. Astroviruses are heat resistant for short periods above 56°C and survive for long periods below -20°C.

Sources of exposure / Transmission routes
Person to person spread by the faecal-oral route is thought to be the most common route of transmission. Recent work with sensitive assay techniques has shown the prevalence of this virus to be much higher than previously thought: i t is endemic all over the world, and second only to Rotavirus as a cause of childhood diarrhea.

Detection methods
Electron microscopy is one usual method of identification of these viruses in clinical specimens but this is not practical for environmental samples. Culture is possible, as described in the previous section. This method has recently been used in a few environmental studies which have demonstrated the presence of virus in sewage and water. Molecular techniques using the polymerase chain reaction (PCR) method have also been utilised to detect the astrovirus genome directly from processed water samples and from cell culture after limited incubation. ELISA tests have been developed for clinical detection of astroviruses in faeces but as significant quantities of virus are needed for a positive reaction, the test is unlikely to be useful for environmental studies.

Occurrence in water sources
Little information is available on the environmental occurrence of human astroviruses. However, since infected individuals may excrete large numbers of viruses in the faeces, they will be present in sewage, and occurrence in sewage-polluted water can be inferred. Since viruses only replicate in living host cells no increase in numbers will occur in the environment.

Susceptibility t o removal or inactivation by conventional water treatment processesin water sources
No direct information is available on removal of astroviruses by water treatment processes. However, information gained using other culturable viruses and bacteriophages indicates that normal water treatment processes (coagulation-sedimentation, filtration and disinfection), if properly applied, can produce water which is essentially virus-free. No specific information is available on the susceptibility of astroviruses to disinfectants such as chlorine and ozone.


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