Rotaviruses and other agents of gastroenteritis

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ROTAVIRUSES

Classification

Family Reoviridae, which includes genus Rotavirus, Reovirus and Colorado Tick Fever virus.

Rotavirus was first identified by electron microscopy in 1973 from duodenal biopsies of children with diarrhea.

Human and animal rotaviruses are known.

Structure

Rotaviruses are non-enveloped, icosahedral, with double capsid. Their electron microscopic appearance shows a 60-80nm wheel with radiating spokes (Latin, rota = wheel).

The rotavirus genome contains double stranded (ds) RNA in 11 segments that can be separated by polyacrylamide gel electrophoresis (PAGE).

Major structural proteins

Outer structural proteins - VP7 and VP4. VP4 is the viral hemagglutinin and forms spikes from the surface.

Inner core structural proteins - VP 1, 2, 3, and 6. VP6 is an important antigenic determinant

Groups

There are six different groups (A to F) based on the antigenicity (each group shares common antigens) and the electrophoretic mobility of their RNA segments. Groups D, E and F have not been found in humans. Only group A rotaviruses cause human disease in the United States, primarily in the young (under two years of age - infantile gastroenteritis). However, group A rotaviruses can also cause milder diarrhea older children and adults. Group B has been found to cause human disease in China where there may be annual outbreaks of severe adult and infant diarrhea. More characteristically, group B rotaviruses cause diarrhea in pigs.

Serotypes

There are at least 15 different serotypes of rotaviruses. Six are based on antibodies to VP7 (P type) while nine serotypes, based on VP4 (G type), are known.

Group A viruses are most common, with subgroups that are defined by antigenic differences in VP6 and with 4 different serotypes that affect humans.

Pathogenesis

Affected host cells are mature enterocytes lining the middle and upper end of the intestinal villi. In laboratory animals, hepatocytes are also infected. The infectious particle is thought to be an "intermediate sub-viral particle" (ISVP).

Viral attachment protein is probably exposed after protease digestion in the GI tract removes some or all of the outer capsid (VP4). Virus replicates in the host cell cytoplasm. Virions enter the host cell by endocytosis and viral mRNA is transcribed using the viral RNA polymerase that is already present in the virion to form structural protein units of the capsid. The mRNA segments are assembled into the immature capsid and then replicated to form the double stranded RNA genome. 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].

Cell dysfunction and death results in a net secretion of intestinal fluid, hence the watery diarrhea. Activation of the enteric nervous system may also play a role.

Repopulation with immature secretory cells may contribute to the secondary lactose intolerance that is sometimes seen.

Properties

Rotavirus is stable in the environment and is relatively resistant to handwashing agents. Is susceptible to disinfection with 95% ethanol, ‘Lysol’, formalin and in environments with pH<2.

Epidemiology

Distribution - Worldwide, causing 600,000-850,000 deaths per year.

Seroprevalence studies show that antibody is present in most infants by age 3 years.

In the U.S., there are 20 - 40 deaths per year with 50,000 hospitalizations per year

Dehydration=1-2.5%

Seasonality

In the britain., rotavirus infections occur in the winter months (November through May). The disease spreads across North America from the warmer climates, starting from Mexico and SW britain and gradually progressing N/NE to reach East Coast and Canada in spring. As might be expected, rotavirus infections are seen year round in the tropics.

Incubation period - thought to be <4 days

Contagious Period - Before onset of diarrhea to a few days after end of diarrhea

Age - Rotaviruses infect children at a young age. Older infants and young children (4 months - 2 years) tend to be more symptomatic with diarrhea. Young infants may be protected due to trans-placental transfer of antibody.

Asymptomatic infections are common, especially in adults. Many cases and outbreaks are nosocomial

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.

Spread is mainly person to person via fecal - oral route and through fomites. Spread by food and water is also possible. Spread via respiratory route is speculated.

High numbers of viral particles are shed in diarrheal stools (1010/gm). Infective dose is only 10-100 pfu.

Clinical Features

Fever- can be high grade (>102° F in 30% of patients)

Vomiting, nausea precedes 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 contributor to mortality.

Secondary malabsorption of lactose and fat, and chronic diarrhea are possible

Diagnosis

Rapid diagnosis - antigen detection in stool by ELISA (uses a monoclonal antibody) and LA. Several kits are commercially available. These detect only Group A rotavirus.

Electron microscopy - also detects non-Group A viruses

Group A rotaviruses can be cultured in monkey kidney cells.

Epidemiologic studies use patterns of viral RNA migration by gel electrophoresis (electropherotyping). Different genetic strains may circulate in a given community.

Treatment

Supportive - rehydration (oral / intravenous)

Antiviral agents not known to be effective

Prevention of spread

Handwashing with good technique

Disinfection of surfaces, toilets, toys

Adequate chlorination of water

Vaccine

Live tetravalent rhesus-human reassortant vaccine (Rotashield) was first licensed for use in infants in August 1998. Contained human G types 1, 2, 4, and simian G type 3.

Post-licensure surveillance indicated a possible relationship between the occurrence of intussusception 3-20 days after the vaccine especially the first dose (15 cases/1.5 million doses were reported).

Use of the vaccine was suspended and it was eventually removed from the market in October 1999, when studies confirmed the link between vaccination and intussusception.

Other vaccine candidates are still being tested.

SMALL ROUND RNA VIRAL AGENTS CAUSING GASTROENTERITIS

This group of RNA viruses morphologically is subdivided in to 2 sub-groups:

Structured - Small round structured viruses (SRSV), Calicivirus, Astrovirus

Other small viruses that are relatively structureless or featureless - W (Wollan) and Ditchling.

NORWALK VIRUS AND NORWALK-LIKE VIRAL AGENTS

SRSV- Norwalk virus and Norwalk-like agents

Non-enveloped, single stranded RNA viruses. 27-35 nm in size (figure 7 and Cool

Contain a single capsid protein.

Norwalk virus was first detected in stools of patients with gastroenteritis in Norwalk, Ohio in 1972.

27 - 32nm in size with a ragged surface.

Norwalk-like viruses are similar in size and are named after the place where they were first isolated in relation to outbreaks of diarrhea. Different serotypes are-

Hawaii
Snow Mountain (CO)
MC (MD)
Taunton (England)
Otofuke (Japan)
Sapporo (Japan)

Clinical Features

Adults and children are affected

Relatively short incubation period: <24 hours

Illness is short (<3 days)

Nausea, vomiting, abdominal cramping and watery diarrhea accompanied by headache, fever and malaise

Outbreaks often occur in institutions, cruise ships, etc. through contaminated food or water

Feco-oral spread, perhaps also spread through vomitus

Diagnosis

Immune EM

Serology for epidemiologic purposes

CALICIVIRUS

Described in 1976

Calciviruses are non- enveloped single stranded RNA viruses, 30-45nm in size (figure 9). They appear round in shape with icosahedral symmetry and contain a single capsid protein. The viral surface has 32 cup-shaped depressions (‘calici’= chalice or calyx i.e. cup-like) described as the ‘Star of David’ appearance. Otherwise they are similar to Norwalk group of agents and belong to family caliciviridae

The 5 antigenic types in humans are UK 1 to 4, and Japan/Sapporo

Epidemiology

Worldwide distribution

Outbreaks can involve infants and school-age children

Viral excretion in stool can continue beyond the cessation of symptoms

Transmission-fecal-oral, through contamination of food and person to person (even asymptomatic persons)

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.

Most people have had infections by age 4 years (by seroprevalence studies).

Clinical Manifestations

Incubation period - 12hrs. to 4 days

Most infections are asymptomatic

Mild diarrhea, vomiting, anorexia and fever

1/3 have respiratory (flu-like) symptoms (esp. UK3 and UK4strains)

Diagnosis

EM - not widely available, requires higher level of viral excretion for detection

Immune EM, ELISA

ASTROVIRUS

Described in relation to an outbreak of gastroenteritis in 1975.

Small single stranded RNA, non-enveloped virus, about 27 - 32nm in size. They are round with an unbroken surface (unlike indented surface of calicivirus)

EM appearance of a 5 or 6 pointed star; contain 3 structural proteins.

Astroviruses are immunologically distinct from Norwalk and Caliciviruses - belong to family Astroviridae

7 human serotypes are known

Clinical Features

Diarrhea, headache, nausea, low-grade fever, vomiting (Iess common)

Epidemiology

Worldwide, mainly in children <7 years of age.

Transmission person-to-person via fecal-oral route

Outbreaks due to fecal contamination of sea-food/water

Diagnosis

EM and Immune EM are especially useful since the virus is often shed in large amounts in stool. Immunfluoresence detects all serotypes.

GASTROENTERITIS DUE TO ADENOVIRUSES

Adenovirus serotypes implicated in gastroenteritis are 40, 41, 31, 3, 7. They cause diarrheal disease in infants and children <4 years of age. The virus is spread by the feco-oral route and is not shed in the nasopharynx

Incubation period is 8-10 days

Diarrhea lasts 5-12 days, prolonged diarrhea often seen with type 40 infections

Diagnosis - Latex agglutination, ELISA, EM