The changing epidemiology of astrovirus-associated gastroenteritis: a review

Our understanding of the epidemiology of astrovirus-associated gastroenteritis has changed markedly with each improvement in detection method. In early surveys based on electronmicroscopy (EM), astroviruses appeared to be a rare cause of gastroenteritis, being found in fewer than 1% of children with diarrhea, usually in small outbreaks of disease and primarily during the winter season. The development and use of monoclonal antibodies and enzyme immunoassays (EIA) to detect astroviruses led to reports of a higher prevalence (2.5%-9%) of astrovirus infection among patients hospitalized with diarrhea. Astroviruses appeared second only to rotaviruses as a cause of hospitalization for childhood viral gastroenteritis. Studies based on EIA detection of astroviruses indicate that astroviruses are common causes of diarrhea in children worldwide, and that most children are infected during their first two years of life. The elderly and the immunocompromised represent high-risk groups as well. The observations that newborns monitored prospectively rarely have repeat disease and that the rate of detection decreases with increasing age suggest that immunity to astroviruses, as immunity to rotaviruses, may develop early in life. The cloning and sequencing of astroviruses have led to more sensitive assays to detect the viruses by reverse transcription, polymerase chain reaction (RT-PCR). Application of RT-PCR for detection of astroviruses in children in day-care centers showed a marked increase in the detected prevalence of astrovirus-associated diarrhea, the rate of asymptomatic infection, and the duration of shedding of virus among those infected, when compared with studies that used other methods. As with rotaviruses, neither the mode of transmission nor the reservoir of astrovirus infection has been identified. Both immune and molecular-based assays to detect astrovirus serotypes indicate that serotype 1 is most common worldwide, although the predominant serotypes may vary by region and time. In the absence of obvious strategies to prevent astrovirus-associated diarrhea, vaccines might be considered if further studies establish that the disease burden would render such a vaccine cost-effective.

The isolation and characterisation of astroviruses from chickens

The isolation, cultivation and characterization of three chicken astroviruses (CAstV) isolates are described. They are antigenically related to each other but unrelated to avian nephritis virus (ANV) and duck hepatitis virus type 2 (DVH2) in neutralization, immunofluorescence and gel diffusion tests. CAstV, ANV and DVH2 all grew well in the LMH cell line, which was used for assay and serological tests. Serological surveys in 1982 and 2001 showed that antibody to CAstV virus was widespread in broiler and broiler breeder flocks and present in some turkey flocks. Infection of 1-day-old specific pathogen free chicks with one isolate in the laboratory resulted in mild diarrhoea and some distention of the small intestine. The virus could be isolated in high titres from all parts of the small intestine but rarely from other organs. Electron microscopic examination of purified particles of this agent revealed the presence of clusters of small round viruses with a diameter ranging from 25 to 30 nm. The amino acid sequence derived from the relatively conserved non-structural polyprotein region of this virus shows 62% identity with the corresponding region of turkey astrovirus 2, 58% identity with turkey astrovirus 1, 55% identity with avian nephritis virus and 33% identity with sheep astroviruses. Taken together, the results indicate that the agent is a new chicken astrovirus belonging to the family Astroviridae.

Prevalence of human astrovirus serotypes in the Oxford region 1976-92, with evidence for two new serotypes

Results of serotyping on 291 astrovirus-positive stools collected between 1976 and 1992 showed that about two-thirds (64.9%) were serotype 1. Infections were more frequent in the fourth quarter of the year and there was a suggestion that during the past 5 years serotype 1 has occurred with greater frequency in alternate years. Evidence is provided for the existence of two new serotypes, 6 and 7.

Characterization of a calici-like virus (Newbury agent) found in association with astrovirus in bovine diarrhea

A bovine calici-like virus and astrovirus, present in the same fecal sample from an outbreak of diarrhea, were separated from each other by calf passage. The calici-like virus (Newbury agent SRV-1) caused anorexia, diarrhea, and xylose malabsorption in gnotobiotic calves aged 17 to 60 days, whereas the bovine astrovirus was nonpathogenic in similar calves. The calici-like virus was shown to be antigenically distinct from a previously described isolate (Newbury agent SRV-2) by two-way cross-protection experiments in calves; calves immune to homologous challenge became clinically ill and excreted virus when challenged with the heterologous virus.

A large outbreak of acute gastroenteritis associated with astrovirus among students and teachers in Osaka, Japan

In June 1991, a large outbreak of acute nonbacterial gastroenteritis occurred among students and teachers at 10 primary and 4 junior high schools in Katano City, Osaka, Japan. The outbreak affected > 4700 persons, lasted 5 days, and was believed to have been linked to contaminated food from a common supplier. Astrovirus, identified as the etiologic agent, was detected by direct electron microscopy in 10 of 38 fecal samples obtained from patients with diarrhea. Detection was confirmed by solid-phase immune electron microscopy (IEM), EIA, reverse transcription-polymerase chain reaction, and virus isolation in CaCo-2 cells. Several patients who had astrovirus in their stool also demonstrated a significant antibody response to a reference strain of astrovirus by IEM and EIA and to their own isolate by IEM. Astrovirus can be an important agent of epidemic acute nonbacterial gastroenteritis in school-aged children and adults in Japan.

Growth and characterisation of human faecal astrovirus in a continuous cell line

We report conditions for the growth of human faecal astrovirus in a continuous colonic carcinoma cell line (CaCo-2). Purified particles contained three polypeptides, one of which (24k) appeared loosely held on the exterior.

Astrovirus infection in volunteers

An extract and a filtrate prepared from feces of a child with mild gastroenteritis were shown by electron microscopy to contain numerous astrovirus particles and were given to eight volunteers by mouth. One subject developed diarrheal illness and concurrently shed large amounts of astrovirus in feces, and one other had mild constitutional symptoms with a lower level of virus shedding. Nine other volunteers were given fecal filtrate from the volunteer with diarrhea, and astrovirus shedding subsequently occurred in two of them. The syndrome accompanying virus shedding appeared distinct from that associated with the "W" agent in previous experiments. Thirteen of 16 astrovirus-inoculated subjects subsequently developed a rise in titer of the homologous antibody in serum. It was concluded that astrovirus causes a transmissible infection that is of low pathogenicity for adults. Immunofluorescence of human embryo kidney cells inoculated with astrovirus and shown by electron microscopy to contain 28 nm virus-like particles was used both to detect virus in feces and to assay astrovirus antibody.

Proteolytic processing of the astrovirus capsid

To further characterize the nature of proteolytic processing of the astrovirus capsid, we infected Caco-2 cells with a high multiplicity of astrovirus without trypsin in the presence of 5 to 10% fetal calf serum. These infections were characterized by pulse-chase labeling with [35S]Smethionine, electron microscopy, gel electrophoresis of purified viral particles, and analysis of infectivity of such particles with and without added trypsin. Pulse-chase experiments showed that the astrovirus capsid protein was initially translated as an approximately 87-kDa protein. The 87-kDa capsid protein was rapidly converted intracellularly to a 79-kDa form which was found in smaller amounts in the cell supernatant. Purification by differential centrifugation yielded particles that appeared quite similar to trypsin-grown astrovirus particles by negatively stained electron microscopy. These particles were antigenically distinct from trypsin-treated virions as demonstrated by their various reactions with monoclonal antibodies in a solid-phase immunoassay. The purified trypsin-free particles were mainly composed of the 79-kDa capsid protein which was found to have an amino terminus at residue 71 of the entire open reading frame 2 (ORF2) product. The cleavage site was identified in a highly conserved region of the astrovirus ORF2 product. These trypsin-free particles were minimally infectious in cultured Caco-2 cells but became highly infectious (10(5)-fold increase) after trypsin but not chymotrypsin treatment. This trypsin-enhanced infectivity correlated with conversion of the 79-kDa capsid protein to three smaller peptides of approximately 34, 29, and 26 kDa.

Astrovirus and Breda virus infections of dome cell epithelium of bovine ileum

A bovine enteric virus antigenically related to the United Kingdom isolate of bovine astrovirus was isolated from diarrheic feces, also containing rotavirus, of a calf in Florida. The astrovirus infected cell cultures and the epithelial cells of domes in the ileum, and there was cross-immunofluorescence with antiserum to the United Kingdom astrovirus. Calves infected with astrovirus alone did not develop clinical disease, but when astrovirus was mixed with rotavirus or Breda virus 2, the calves developed severe diarrhea and more extensive astrovirus infection of the dome epithelium. The dome epithelial cells showed degeneration associated with astrovirus infection, and a few cells showed degeneration with Breda virus 2 infection. Virions with a 30-nm diameter were seen in astrovirus-infected dome cells, and Breda virus 2 virions were also observed either in separate cells or, on occasion, with both viruses in one cell.

Diagnosis of astrovirus gastroenteritis by antigen detection with monoclonal antibodies

An enzyme-linked immunosorbent assay (ELISA), based on monoclonal antibodies to the astrovirus group antigen, was designed for the detection of astroviruses in stools of patients with gastroenteritis. Compared to immune electron microscopy used as the standard test, the sensitivity of the astrovirus ELISA was 91% (31/34) and the specificity was 96% (54/56). All five of the known astrovirus serotypes could be detected in 16 samples on which serotyping was done. In tests on 155 stools containing other enteric viruses, including adenoviruses, rotaviruses, caliciviruses, Hawaii virus, Snow Mountain virus, and Norwalk virus (30, 20, 70, 24, 4, and 7 samples, respectively), only 3 were positive in the astrovirus ELISA. The combined specificity for all astrovirus immune electron microscopy-negative samples was 98% (206/211). The results demonstrate that the new ELISA provides a sensitive and specific means for the diagnosis of astrovirus gastroenteritis.

Comparison of the features of astroviruses and caliciviruses seen in samples of feces by electron microscopy

Astroviruses and caliciviruses may both be observed in the feces of babies by electron microscopy and are two of the viruses whose relationship to infantile diarrhea is still being established. Although similar in size and in their star-shaped surface configurations, they differ in detailed structure. The astrovirus has a round unbroken edge, a six-pointed star with a white center, and surface hollows that are triangular. The calicivirus has a feathery edge, a six-pointed star with a dark hollow in the center (Star-of-David) appearance), and surface hollows that appear round or oval. Because only a minority of the virions of both viruses exhibit a completely characteristic structure, careful searching may be necessary to distinguish between the two viruses.

Cytopathic astrovirus isolated from porcine acute gastroenteritis in an established cell line derived from porcine embryonic kidney

A cytopathic astrovirus was isolated from pigs with acute diarrhea in an established cell line that was derived from porcine embryonic kidneys with the aid of trypsin. The virus showed a distinct cytopathic effect characterized by an enlargement of cells and the appearance of fine granules in the cytoplasm. Porcine astrovirus was shown to have an RNA genome, as determined by the effect of 5-iodo-2'-deoxyuridine on its replication, and five polypeptides with molecular masses of 13,000, 30,000, 31,000, 36,000, and 39,000 daltons; and it was shown to be stable to lipid solvents and heating at 50 degrees C for 30 min but somewhat labile to acid (pH 3.0). The buoyant density of the isolate determined in CsCl was 1.35 g/ml. Seroconversion to the virus was evident in the paired serum specimens obtained from pigs with diarrhea that were housed at the farm where the disease occurred. The neutralization test on serum specimens collected randomly from 128 adult pigs of eight herds revealed that 50 of the serum specimens were positive for antibody to porcine astrovirus, although there was considerable variation in the prevalence among herds, ranging from 0 to 83%. Hysterectomy-produced, colostrum-deprived, 4-day-old pigs developed mild diarrhea after oral exposure to porcine astrovirus propagated in the cell culture; and the virus was isolated again from diarrheal stool specimens.

Astrovirus epidemiologically linked to pre-weaning diarrhoea in mink

Diarrhoea and excessive secretion from the cervical apocrine glands in young, suckling mink kits is a well-known, but poorly defined, syndrome often referred to as "sticky", "greasy", or "wet" kits. We have performed a case-control study, at farm level as well as at mink kit level, in Denmark and Sweden to investigate whether enteric virus infections may be a risk factor in the development of pre-weaning diarrhoea. Tissue samples from the enteric tract of 180 sacrificed mink kits were analysed histologically. Faecal contents were examined by electron microscopy (EM). Astrovirus was detected in abundance and found to be a significant risk factor both at farm level (OR=21.60, p<0.001) and at mink kit level (OR=7.95, p<0.001). Other factors, i.e. low body weight, coccoid bacteria adherent to the enteric villi, and presence of calicivirus were also shown to increase the risk of pre-weaning diarrhoea, although with less impact than astrovirus.

Detection of astroviruses in feces of a cat with diarrhea. Brief report

Astroviruses were detected by electron microscopy in the feces from a 4 month old kitten with diarrhea. The mean diameter of the viral particles was 28.7 nm, and they showed characteristic five- or six-pointed star-shaped surface configurations. The clinical disease manifested by the cat and the observed morphology of the viral particles are consistent with previous reports on astroviruses of other species.

Identification of astrovirus serotypes from children treated at the Hospitals for Sick Children, London 1981-93

An enzyme immunoassay (EIA) for astrovirus type 1 together with immune electronmicroscopy (IEM) was used to type a collection of 162 astroviruses obtained from 1981-93 from children with diarrhoea. The EIA was found to be specific for astrovirus type 1. Astrovirus types 2-4 were typed by IEM. Astrovirus type 1 was the prevalent serotype 107/125 (86%), followed by type 3 (8%), type 4 (6%) and type 2 (1%). Six samples containing astrovirus could not be typed or detected by EIA because they were coated with coproantibodies; 11 others were not identified. Virus particles could no longer be detected in 15/162 (9%) samples following storage for > or = 2 years. Selected samples containing astrovirus types 1-4 were passaged in CaCO2 cells and their identity confirmed by one or both assays. One sample was shown to have remained viable for 10 years when stored as an aqueous suspension at -20 degrees C. Two patients with severe combined immune deficiency disease (SCID) were shown to be excreting astrovirus type 1 for 32 and 102 days respectively. One child was simultaneously shedding rotavirus and the other child was excreting adenovirus.

Application of electronmicroscopy, enzyme immunoassay, and RT-PCR to monitor an outbreak of astrovirus type 1 in a paediatric bone marrow transplant unit

During 1997, an extensive outbreak of astrovirus occurred in a unit where paediatric patients were being treated for leukaemias and inherited immune deficiency disorders. Prolonged shedding of virus for many months following infection was demonstrated in three patients who had undergone bone marrow transplantation. Comparison of reverse transcription-polymerase chain reaction (RT-PCR), enzyme immunoassay (EIA), and electronmicroscopy (EM) to monitor the outbreak showed that many subclinical infections, mainly in children aged > 3 years could only be detected by RT-PCR. Use of RT-PCR revealed that several patients were infected earlier and shed virus for longer than by using EM or EIA. The virus responsible for the outbreak was identified as HAstV-1 and was shown to have a sequence that differed from a strain obtained in 1988.

Six-year retrospective surveillance of gastroenteritis viruses identified at ten electron microscopy centers in the United States and Canada

To identify the prevalence, seasonality and demographic characteristics of patients with viral gastroenteritis, we reviewed 6 years of retrospective data on viral agents of gastroenteritis screened by electron microscopy at 10 centers in the United States and Canada. From 52,691 individual electron microscopic observations, a virus was detected in 16% of specimens, and the yearly positive detection rate among centers ranged from 8 to 34%. Rotavirus was the agent most commonly observed (26 to 83%), followed by adenoviruses (8 to 27%, respiratory and enteric combined), and small round viruses (SRVs) (0 to 40%) which were second most common at two of the centers. Rotavirus and astrovirus detections occurred more often in the winter but seasonal trends in detection were not apparent for the other viruses. Of all astroviruses detected 64% were found in infants (less than 1 year); unlike the other agents studied SRVs were detected in a large percentage of infants (48%) and older children and adults (20%). Among hospitalized patients a majority of all astroviruses, caliciviruses and SRVs were detected 7 days or more after admission in contrast to both rotaviruses and adenoviruses which were more likely to be detected earlier. The data suggest that SRVs are common agents of gastroenteritis and may be important causes of nosocomial infections. Because of the relative insensitivity of direct electron microscopy as a screening method for SRVs, astroviruses and caliciviruses, these data probably underestimate the true prevalence of disease caused by these agents.

Virus and virus-like particles in the faeces of cats with and without diarrhoea

Negative staining electron microscopy was used to identify viruses in 166 normal and 62 diarrhoeal faecal samples from 208 cats admitted to an animal shelter during a 16-month period (March 1984 to June 1985). On the basis of size and shape 7 distinct viral types were detected: 24 nm parvovirus-like particles, 30 nm astrovirus, 30 nm picornavirus-like particles, reovirus, rotavirus, coronavirus and a 75 nm "togavirus-like" particle. The incidence of these particles in the 208 cats was 11%, 7%, 6%, 0.4%, 5%, 1% and 1% respectively. Virus isolation studies using 40 of the faecal samples succeeded in isolating reovirus 1 in 2 cases. Immune electron microscope studies demonstrated the presence of antibody in a human serum to cat astrovirus, but failed to clarify the identity of the parvovirus-like particles and picornavirus-like particles, other than showing that some of the parvovirus-like particles were not related to feline panleukopenia virus. It was found that parvovirus-like particles, astrovirus, picornavirus-like particles, reovirus and rotavirus could be excreted by cats with normal faeces as well as cats with diarrhoeal faeces. Parvovirus-like particles, astrovirus, picornavirus-like particles and rotavirus could be excreted in high concentration in normal faeces. There was no simple relationship between age and diarrhoea in the population of cats studied. Age was not a critical factor in the excretion of parvovirus-like particles, astrovirus, picornavirus-like particles and rotavirus. The incidence of diarrhoea was not clearly associated with the seasons.

Ultrastructure of human astrovirus serotype 2

The ultrastructure of human astrovirus serotype 2 (H-Ast2) grown in cell culture was analysed by electron microscopy of thin sections and negatively stained preparation. Infected LLCMK2 cells, as visualized in thin sections, contained cytoplasmic aggregates of dense or hollow-cored particles that aggregated in quasicrystalline arrays and were specifically labelled using a rabbit polyclonal anti-Ast2 antiserum. H-Ast2 particles from the supernatant of infected LLCMK2 cells in thin sections after flat- embedding were similar in size to intracellular virions. In negatively stained preparations, these virus particles had an external diameter of 41 nm and exhibited a well defined layer of surface spikes. Pentagonal and hexagonal contours were occasionally visible, and probably correspond to the projections of icosahedral structures. Star-like morphologies and particles with surface triangular hollows were seen in dark areas of the preparations only after a short treatment of the viruses of pH 10. Incubation of the viruses at pH 10.5 induced a rapid disassembly of the virus particles. The finding that the particles with icosahedral geometry and surface spikes are fully infective allows an alternative morphological model to the traditional one for astroviruses to be proposed.

Ultrastructure of the small intestine in astrovirus-infected lambs

The ultrastructure of the small intestine of gnotobiotic lambs infected with lamb astrovirus was studied. The virus was observed from 14 to 28 h p.i. in mature columnar epithelial cells covering the apical two-thirds of villi. Crystalline arrays of virus particles with a centre to centre distance of approx. 29 nm were seen in the cytoplasm and virus particles were also observed in apical pits and tubules and in lysosomes. Macrophages containing virus particles in lysosome-like organelles were seen in the lamina propria. Virus particles were released by desquamated cells disintegrating in the gut lumen. Cuboidal cells lining villi appeared from 38 to 70 h p.i., and by 120 h p.i. the villi appeared normal.

Viruses and virus-like particles in the faeces of dogs with and without diarrhoea

Negative staining electron microscopy was used to identify viruses in 157 normal and 29 diarrhoeal faecal samples collected from 156 dogs admitted to an animal shelter during an 8 month period (March to October) in 1982. Seven distinct viral types were detected: 21-26 nm parvovirus-like particles, 28-31 nm astrovirus-like particles, a previously undescribed 34-35 nm "round" virus particle, coronavirus, coronavirus-like particles ( CVLP ), rotavirus and papova-like virus. Parvovirus-like particles alone were detected in 14 diarrhoeal and 50 normal faeces, astrovirus-like particles in 3 normal faeces, "round" viruses in 4 normal faeces, coronavirus in 2 diarrhoeal and 5 normal faeces, CVLP in one diarrhoeal and one normal faeces, rotavirus in 2 normal faeces, papova-like virus in one normal faeces, both parvovirus-like particles and coronavirus in 2 diarrhoeal and 2 normal faeces, parvovirus-like particles and rotavirus in one normal faeces and parvovirus-like and papova-like virus in one normal faeces. The significance of these findings in canine and human disease is discussed.

Association of the astrovirus structural protein VP90 with membranes plays a role in virus morphogenesis

VP90, the capsid polyprotein precursor of human astrovirus Yuc8, is assembled into viral particles, and its processing at the carboxy terminus by cellular caspases, to yield VP70, has been correlated with the cell release of the virus. Here, we characterized the effect of the VP90-VP70 processing on the properties of these proteins, as well as on their intracellular distribution. VP90 was found in membrane-enriched fractions (mVP90), as well as in fractions enriched in cytosolic proteins (cVP90), while VP70 was found exclusively in the latter fractions. Upon trypsin activation, infectivity was detected in all VP90-containing fractions, confirming that both mVP90 and cVP90 are able to assemble into particles; however, the two forms of VP90 showed differential sensitivities to trypsin, especially at their carboxy termini, which in the case of mVP90 was shown to remain membrane associated after protease digestion. Structural protein oligomers were detected in purified VP70-containing viruses, as well as in membrane-enriched fractions, but they were less evident in cytosolic fractions. Ultrastructural studies of infected cells revealed different types of viral particles, some of which appeared to be associated with membranes. By immunoelectron microscopy, structural proteins were shown to form virus particles in clusters and to associate with the edges of vesicles induced during infection, which also appear to contain subviral particles inside. Nonstructural proteins and viral RNA colocalized with mVP90, but not with cVP90, suggesting that mVP90 might represent the form of the protein that is initially assembled into particles, at the sites where the virus genome is being replicated.