Norwalk virus was the first fastidious enteric virus to be discovered,
following an outbreak affecting both children and adults in a primary school in
Norwalk, Ohio in 1968. Bacterial-free filtrates of stools collected induced
similar illness in human volunteers. Virus like particles were seen under EM by
Kapikan in 1972, immune electron microscopy was used. In subsequent years a
number of outbreaks in summer camps, cruise ships and other gatherings have been
associated with this virus. Antibodies against this virus are acquired later in
life, indicating a less widespread distribution, in the USA at least. At the age
of 50, only 50% of the population surveyed had antibodies. In a similar study in
Bangladesh, antibodies were acquired earlier and the pattern was closer to that
of rotaviruses in that the majority of the population had acquired antibody by
the age of 3. Morphologically similar viruses have been found in Europe
and elsewhere are were called small round structured viruss (SRSV). Progress in
the understanding of Norwalk-like viruses had been excruciatingly slow until
complete sequences of norovirus strains became available in 1995. Norwalk-like
viruses or SRSVs are now firmly established in the family of Caliciviridae.
Sensitive RR-PCR techniques became available and it was only then that the true
extent of Norovirus infections was realized. It is now generally accepted that
Noroviruses are the most common viral gastroenteritis agent, well surpassing
rotaviruses.
Norwalk virus was originally described as parvovirus-like and
28nm in diameter. On electron microscopy, its surface has a
ragged structured appearance. this was originally thought to be
antibody against the virus binding to the surface as the first
electron micrographs were obtained by immune electron microscopy.
However, it is mow thought that the ragged outer edge is part of
the surface of the virus particle. If so, the true mean diameter
of the Norwalk virus particle is at least 33nm. The virus is
thought to have a genome consisting of positive stranded ssRNA.
Norwalk virus has not been shown to grow in cell culture with or
without the addition of trypsin. Even partial growth has not been
described. When present in the stool, the virus is difficult to
detect by electron microscopy as the concentration of the virus
in stool culture is much lower than that of rota and
adenoviruses. The difficulties of finding Norwalk by electron
microscopy have resulted in most infections being diagnosed by
antibody tests. however, such tests will only diagnose infections
by Norwalk virus itself and other similar viruses if they share a
common group antigen. There is some cross-reactivity between
Norwalk and Montgomery County viruses but the Hawaii agent
appears to be antigenically distinct.
Noroviruses belong to the family off caliciviridae. They have similar sizes and buoyant
densities, and have a major protein of very similar molecular
weight to that of caliciviruses. There is increasing serological
evidence of cross-reaction between strains of calicivirus and
SRSV. Recently, nucleotide sequences for several NLVs have
become available which should greatly facilitate research on
these viruses. On the basis of genetic analysis, there appears to
be two main genotypes of Norwalk-like viruses, each with a large
number of subtypes. With a dramtic increase in sensitivity
compared to EM, PCR assays are now being increasingly used
for the diagnosis and investigatrion of outbreaks caused by
Norwalk-like viruses.
Electronmicrograph of Norwalk-like virus particles. Note
that unlike astroviruses, it is extremely rare to find such
as large number of virus particles congregated together like
this.
Phylogram of Norwalk-like Viruses using the capsid sequences
Epidemiology
Noroviruese are distributed
globally, with genogroup II being more common than genogroup I viruses. They are
associated with both epidemic outbreaks and sporadic cases of gastroenteritis.
Outbreaks occur regularly in institutions such as nurseries, hospitals, nursing
homes, restaurants, and cruise ships. It is now clear that noroviruses accounts
for more endemic infections than epidemic infections. Recent data suggests that
they cause at least as much or even more cases of gastorenteritis in young
children than Rotaviruses. Norovirus infections are more likely to occur in
winter months, hence the nickname "winter vomiting disease", although this
depends on the individual country. Highly infectious, aerosol spread possible as
well as faecal-oral spread. Associated with consumption of shellfish, other
contaminated foods such as salads, and faecally contaminated water.
Association with Disease
A small series of volunteer experiments confirmed that the
virus was capable of causing disease in adults. Half the
volunteers challenged developed illness although none were
severely ill. These volunteers were rechallenged between 27 and
41 months later and those who were ill on the first occasion were
ill again. Virus was detected in the faeces of those challenged.
The volunteers developed both serum and secretory antibodies in
the gut and it is surprising that this appeared to confer no
protection. Norwalk virus has been associated almost exclusively
with outbreaks of diarrhoea and vomiting. Vomiting appears to be
a more prominent feature of infection than with rota and
adenoviruses. There is usually little or no pyrexia and the diseases is usually
mild with spontaneous recovery. Outbreaks have been associated with contaminated
seafood such as oysters. Person to person spread is probably a
component of almost all outbreaks involving SRSVs. Outbreaks in
institutions have an epidemic curve typical of person to person
spread. Faecal oral spread is probably the major means of
transmission. However other routes are possible such as through
environmental contamination through vomiting and possibly through
the airbourne route. Outbreaks of SRSV have also been associated
with exposure to recreational water such as swimming in
contaminated lakes. Occasionally SRSVs are responsible for
sporadic cases of gastroenteritis.
Laboratory Diagnosis
1. RT-PCR - have become the major means of
diagnosing norovirus infection. It is especially useful in the investigation of
outbreaks. At present, only in-house methods are available that uses primers
directed against the polymerase region of noroviruses. The amplicon could then
be sequenced for epidemilogical analysis.
2. ElectronMicroscopy
- classical means of diagnosis of Norovirus infection, the viruses are
non-cultivable. Require samples with large number of virus particles and also
highly skilled observers.
3. Antigen Detection
– a number of RIAs and EIAs had been developed using norovirus-specific
polyclonal or monoclonal IgG. However, none are shown to detect
all strains of noroviruses and are not as sensitive as RT-PCR
4. Antibody Detection – a
number of serological assays are available for the detection of rising titres of
IgG or IgM using recombinant capsid proteins.
Management and Prevention
There is no specific antiviral therapy for norovirus infections. Most cases
are mild with rapid spontaneous recovery. Severe cases may required admission of
hospital to ensure that the patient is not dehydrated. Outbreaks of noviruses in
institutions such as hospitals, schools, nurseries and cruise ships are
notoriously difficult to contain. It is essential that all vomitus and diarrhoea
be removed quickly and the surrounding areas disinfected scrupulously. There are
programs in several countries to screen for noroviruses in imported shellfish
although there is no consensus as to how useful this is. There is a possibility
of developing vaccines against noroviruses using recombinant capsid proteins.
However, this is complicated by the high variability and high mutation rate of
norovirus strains. Also, it is not clear whether there would be great demand for
a vaccine against a virus that normally causes a very mild disease.
A. Caliciviruses (Sapoviruses)
Caliciviruses have been known to infect pigs, cats, sea-lions
and fur-seals but it was not until 1976 that a calicivirus was
fond in human faeces. The human calicivirus is morphologically
indistinguishable from these animal strains. However, human
calicivirus strains are antigenically distinct from animal
strains. Human caliciviruses contain a single positive stranded
RNA genome. and are about 33nm in diameter. The virus shows
icosahedral symmetry, the face and the apices of the icosahedron
are represented by cupped hollows, of which there are 32. This
gives a virus a very characteristic appearance. However, not all
the particles in the stool show the characteristic hollows and
these particles can resemble Norwalk agents and SRSVs very
closely. 4 serotypes of human calicivirus have been recognized, 3
in the UK, and 1 in Japan. No group antigen has been detected.
Electromicrograph of classical calicivirus particles. Note
the charcteristic cupped shaped depressions on the surface
3 of the 4 human serotypes described are associated with
outbreaks of diarrhoea and vomiting. Vomiting being a more
prominent feature than diarrhoea and in this way and the epidemic
nature of the viruses, infection by human caliciviruses resemble
that by Norwalk agents. Caliciviruses have also been reported
infrequently in the stools of normal babies. However it is not an
easy virus to find and considerable under reporting may have
taken place. Antibody against caliciviruses may be no more
protective than with Norwalk, a further point of similarity with
Norwalk agents. Volunteer studies in the UK and USA have shown
the viruses to be capable of inducing disease in adult
volunteers. The incubation period being 24 - 72 hours with
symptoms of N+V, abdo pain and diarrhoea. Virus is excreted in
the faeces during the illness. A serological survey carried out
in Japan showed that the majority had acquired antibody in the
first three years of life. The pattern of acquisition of antibody
closely parallels the acquisition of antibody to rotaviruses and
astroviruses, suggesting this virus is widespread although few
actual infections are diagnosed. With much of the attention focused on noviruses,
research on sapoviruses have been comparatively lacking. Recent studies that had
been carried out with sensitive RT-PCR shows that like noroviruese, sappovirus
infections are much more common than previously thought. Hopefully, more
definitive date will become available in the near future.
B. Small Round Viruses
Small round virus-like objects are often seen in human faeces.
SRVs show consideration variation in size. Some of the smaller
SRVs have a diameter of 22nm and are often hexagonal in outline
so that they resemble parvoviruses. In some instances,
adenoviruses may also be found in the same stool and thus these
are probably defective adenovirus-associated virus. However,
adenoviruses are not always detectable and it may well be in
those instances that one is dealing with an autonomous
parvovirus. Indeed, certain animal parvoviruses e.g. canine
parvovirus, can cause severe diarrhoeal disease in animals. SRVs
in the next larger size band are about 25nm in diameter and
resemble enteroviruses. It is uncertain whether these are new
strains of enteroviruses but these viruses do not grow in cell
culture. Larger SRVs may occasionally be seen but it is doubtful
whether these are viruses. Occasionally, cubic bacteriophages may
be misdiagnosed as SRVs. SRVs may be present in a considerable
number of stools of infants. However, when detectable, the
numbers are relatively small. They are more likely to be found in
common source outbreaks. Both SRVs and SRSVs have been associated
with common source outbreaks, of which a considerable proportion
is associated with the consumption of shellfish, particularly
oysters. The incubation period is 36-48 hours, following which
the patient develops vomiting and diarrhoea. At present, there
are no known animal parallels.
C. Coronaviruses
Coronavirus-like particles are occasionally seen in stool
extracts. The particles seen in the stools differ from the human
respiratory coronaviruses in morphology. The particles in the
stools have projections that resemble pins with a narrow shaft
whereas those seen in the respiratory tract have club-shaped
surface projections. The identities of these particles as true
viruses have yet to be proved. Most attempts to culture
coronaviruses have failed although there was a single unconfirmed
report of growth in fetal intestinal cells. These particles have
been seen in the faeces of adults as well as children. Prolonged
excretion following recovery is common. Most reports associate
these viruses with endemic rather than epidemic diarrhoea. No
volunteer experiments have been reported to date. Coronaviruses
are well established as causes of diarrhoea in animals,
particularly in swine.
D. Other_Virus-like_Particles_Seen_in_Faeces
Breda-like Agents ;- Virus-like particles resembling
those previously described in the faeces in cows have been seen
in the faeces of a few children. Although an association with
diarrhoea in man has been reported, further information is
needed.
Bacteriophages ;- The human gut contains vast numbers
of bacteria which are potential hosts for bacteriophages. Like
animal viruses, bacteriophages come in a variety of sizes and
shapes. However, the tailed bacteriophage has a structure unique
to this class of virus. It is possible for cubic bacteriophages
to be mistaken for SRVs. Moreover, a possible role for these
phages in the causation of human diarrhoea should not be totally
overlooked.
