Dept.of Microbiology/Virology Assist.prof. Shatha F. Abdullah
VIRAL GASTROENTERITIS AGENTS
The following are well-established causal agents of viral gastroenteritis
in humans:
a. Rotavirus
b. Enteric adenoviruses
c. Human Caliciviruses
d. Astrovirus.
e. Torovirus
f. HSV, CMV in immunocompromised patients (not discussed here)
ROTAVIRUS
Rotavirus was first identified by electron microscopy in 1973 from
duodenal biopsies of children with diarrhea and was later detected in
stool samples by negative EM.
Human and animal rotaviruses are known.
CLASSIFICATION
Family Reoviride includes genus Rotavirus, Reovirus and Colorado Tick
Fever virus
STRUCTURE
♦ Non-enveloped, icosahedral, 60-80nm in size
♦ Electron microscopic appearance of a wheel with radiating spokes
(Latin, rota=wheel)
♦ Double capsid (outer capsid, inner capsid) and a core.
♦ Major structural proteins-
1
. Outer
capsid structural proteins- VP7 (G protein) and VP4 (the viral
hemagglutinin or P protein). Both elicit neutralizing antibodies.
2
. Inner
capsid structural proteins- Mainly VP6, also VP1, 2, 3.
♦ Inner core
contains the genome that has double stranded (ds) RNA in
11 segments. Genome codes for 6 virus proteins (VP1-6) and 6 non-
structural proteins (NSP1-6). RNA segments can be separated by gel
electrophoresis; segmented genome can reassort with co-infection of
cells with different strains.
CLASSIFICATION
♦ Groups:
Seven different groups (A to G) and 2 subgroups based on
antigenic specificity of VP6.
Most human pathogens belong to groups A, B, and C; group A viruses
with either subgroup I or II, are the most common.
♦ Serotypes:
14 human G serotypes (specified by VP7) and 20 P
serotypes (specified by VP4)
♦ E-types:
based on differences in relative migration rates of genome
segments in PAGE
PROPERTIES
Virus is acid-labile but rather stable in the environment.
It is susceptible to disinfection with 95% ethanol, ‘Lysol’ and formalin.
PATHOGENESIS
Affected host cells are mature enterocytes lining the middle and upper
end of the intestinal villi in the small intestine.
The infectious particle is thought to be an
“intermediate sub-viral
particle”
(ISVP)
that is generated after removal of some of the outer
capsid. The viral attachment protein is probably exposed after protease
digestion in the GI tract removes some or all of the
outer capsid (VP4)
.
Virions enter the host cell by endocytosis. Virus replicates in the host cell
cytoplasm. Large amounts of viral particles are shed in diarrheal stools.
Histopathology of infected intestines shows villous atrophy and blunting,
due to death of the mature enterocytes and infiltration of lamina propria
with mononuclear cells. Subsequently there is repopulation of the villous
tips with immature secretory cells (crypt hyperplasia).
Mechanism
of watery diarrhea is
due to
cell dysfunction/death,
activation of the enteric nervous system, and by NSP4, a non-structural
protein, that may play a role as a viral enterotoxin and secretory agonist.
During convalescence, repopulation with immature secretory cells may
contribute to the secondary lactose intolerance that is sometimes seen.
EPIDEMIOLOGY
♦ Distribution
Worldwide: 20% of diarrhea-related deaths under age 5 years and 40%
of severe diarrhea cases
♦ Seasonality
Winter months -spring.
Seen year round in the tropics.
♦ Age
Infects children at a young age. Older infants and young children (4mo-2
years) tend to be more symptomatic with diarrhea. Young infants may
be protected due to transplacental transfer of maternal antibody.
♦ Spread
-mainly person to person via fecal-oral route and through
fomites.
Spread by food and water is also possible.
♦ Contagiousness
-
Contagious period starts before onset of diarrhea to a few days after end
of diarrhea.
Highly contagious
among susceptible individuals due to the following: -
a) Small infective dose (only 10 pfu)
b) High numbers of viral particles being shed in diarrheal stools
(1010/gm)
c) Shedding of virus from asymptomatic hosts
♦ Nosocomial cases and outbreaks are known to occur.
♦ Asymptomatic infections are common, especially in adults.
♦ Severe infections are seen in young, elderly, immune compromised
hosts.
♦ Group A infections are most common.
Group B has been associated with outbreaks in adults in China
Group C is responsible for sporadic cases of diarrhea in infants around
the world.
CLINICAL FEATURES
♦ Incubation period is < 4 days
♦ Fever- can be high grade
♦ Nausea and vomiting precede diarrhea.
♦ Diarrhea is usually watery (no blood or leukocytes), lasting 3-9 days,
but longer in malnourished and immune deficient individuals.
Necrotizing enterocolitis and hemorrhagic gastroenteritis is seen in
neonates.
Dehydration is the main cause of death.
Secondary malabsorption of lactose and fat, and chronic diarrhea are
possible
DIAGNOSIS
♦ Antigen-detection in stool-by ELISA (uses a monoclonal antibody) and
LA is used for rapid diagnosis.
♦ Electron microscopy
♦ Culture- group A rotaviruses can now be cultured in monkey kidney
cells.
♦ Epidemiological studies use patterns of viral RNA migration by
polyacrylamide gel electrophoresis. Different genetic strains may
circulate in a given community.
♦ RT-PCR
TREATMENT:
Supportive-rehydration
PREVENTION OF SPREAD
♦ Hand washing- partly effective
♦ Alcohol-based hand gels
♦ Disinfection of surfaces, toilets, toys etc.
♦ Vaccines
Rotashield
, a live tetravalent rhesus-human reassortant vaccine,it was
removed from the market( possible relationship between the occurrence
of intussusceptions)
RotaTeq
is a new live pentavalent human-bovine reassortant vaccine
(2006 /US). Vaccine appears to confer protection that lasts for at least 2
years.
Rotarix
(Avant Immunotherapeutics) is a live attenuated monovalent
vaccine.
HUMAN CALICIVIRUSES
CLASSIFICATION
♦ Members of the family Caliciviridae
♦ Now classified into genera Norovirus and Sapovirus, based on
antigenic differences.
♦ Norwalk virus is the prototypic member of the Genus Norovirus. It
was first detected in stools of patients with gastroenteritis in Norwalk,
Ohio in 1972.
♦ Examples of Sapoviruses include Manchester virus, London/92 virus,
etc.
STRUCTURE
♦ Non-enveloped, ss RNA viruses with a +ve sense genome.
♦ 27-35 nm in size.
♦ Two morphologic groups are based on electron microscopic
appearance:-
Atypical,
Small round structured viruses (SRSVs) –smaller, approx. 27
nm, they have a rough, feathery edge, but no internal pattern. (These
are the members of Norovirus group.)
Typical
, spiked appearing viruses- are 31-35nm and have 32 cup-shaped
depressions on surface
(calici= i.e. cup-like).
CLINICAL FEATURES
♦ Causal agent referred to for illnesses commonly known as- “stomach
flu” or viral gastroenteritis.
♦ Relatively short incubation period (Range 12hrs. to 4 days)
♦ Illness is brief (3-5 days) and usually self-limited
♦ GI symptoms -nausea, vomiting, abdominal cramping and watery
diarrhea (fecal leukocytes are absent).
♦ Other symptoms-headache, fever, malaise, myalgia,
♦ Outbreaks in institutions, cruise ships, etc. through contaminated
food or water (feco-oral)
EPIDEMIOLOGY
♦ Worldwide distribution. Higher seroprevalence in developing
countries.
♦ Established as the most frequent etiology of infectious GE in some
countries.
♦ Infections occur year-round with an apparent predominance in winter
season.
♦ Transmission-direct fecal-oral (person-to-person) spread or indirect
fecal-oral spread via contamination of food/water, and through fomites
contaminated by vomitus or stool.
♦ Possible spread through aerosolization of vomitus and ingestion
(rather than inhalation) of infective particles.
♦ Low infective dose (~10 virions).
♦ Viral excretion in stool can continue beyond the cessation of
symptoms.
♦ Asymptomatic infections (>30% cases) can result in transmission of
infection
♦ Outbreaks can involve infants and school-age children
♦ Outbreaks related to consumption of contaminated oyster and
shellfish have been described.
It is thought that the seafood or water or ice becomes contaminated
with fecal material from sewage or food handlers. Estimated to cause
>50% of food-borne gastroenteritis outbreaks in the US.
♦ Clinical and microbiologic features as well as epidemiologic criteria
suggest viral etiology of gastroenteritis outbreaks.
DIAGNOSIS
♦ Analysis of stool or suspected food material; also environmental
samples (outbreak investigations)
♦ State public health laboratories mainly use realtime RT-PCR of stool
and emesis samples, as well as environmental samples during outbreak
investigations.
♦ Serology is used for epidemiological purposes, with paired sera.
♦ ELISA is thought to have poor sensitivity
CONTROL
CDC recommends environmental disinfection of non-porous surfaces
with bleach solution (1 part bleach: 50 parts water)
ASTROVIRUS
Genus Astrovirus, family Astroviridae, contains 8 human species
(antigenic types) that are known as HuAst 1 to 8.
STRUCTURE
♦ Small ss RNA, non-enveloped virus 27-32nm in size.
♦ EM appearance of a smooth, round structure with an unbroken
surface (unlike indented surface of calicivirus) and a 5 or 6 pointed star
within.
♦ Immunologically distinct from Human Caliciviruses.
CLINICAL FEATURES
Incubation period 1-4 days
Diarrhea, headache, nausea, low-grade fever, vomiting
(Similar to other viruses)
EPIDEMIOLOGY
♦ Worldwide
♦ Mainly affects children infants and young children <7 years of (clinical
illness is less common in adults)
♦ More common in winter season
♦ Transmission by direct person-to-person, via fecal-oral route or
ingestion of aerosolized vomitus
♦ Transmission indirectly through fecal contamination of sea-
food/water (outbreaks described) and contaminated fomites
♦ Low infective dose
♦ Outbreaks have been seen in child-care centers.
DIAGNOSIS
♦ EM especially useful since the virus is often shed in large amounts in
stool
♦ Enzyme Immunoassay (EIA) test
♦ RT-PCR in used by research laboratories.
ENTERIC ADENOVIRUSES
♦ Serotypes implicated are
40
and
41
(enteric serotypes). These are
difficult to culture and require special cell lines
♦ Cause diarrheal disease in infants and children<4 years of age
♦ Feco-oral spread
♦ Incubation period is 8-10 days
♦ Diarrhea lasts 5-12 days, longer than other viral gastroenteritis
♦ Diagnosis- Latex agglutination, ELISA, EM
TOROVIRUS
♦ Family Coronaviridae
♦ Genus Torovirus
♦ Human and animal pathogen
♦ Pleomorphic, coated ss (+) RNA virus
♦ Core “doughnut-shaped” (torus)
♦ Watery diarrhea in 2 – 12 months old
♦ Diagnosis: EM