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.
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
Types of Astroviruses
The genus "Astrovirus" within the familiy of Astroviridae
is divided into eight human species:
- human astrovirus
- human astrovirus 2 (HAstV-2)
- human astrovirus 3 (HAstV-3)
- human astrovirus 4 (HAstV-4)
- human astrovirus
- human astrovirus 6 (HAstV-6)
- human astrovirus 7 (HAstV-7)
- human astrovirus 8 (HAstV-8)
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.
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
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.
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.
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.
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.