Preliminary characterisation of torovirus-like particles of humans: comparison with Berne virus of horses and Breda virus of calves

Mechanism, structural and functional insights into nidovirus-induced double-membrane vesicles

During infection, positive-stranded RNA causes a rearrangement of the host cell membrane, resulting in specialized membrane structure formation aiding viral genome replication. Double-membrane vesicles (DMVs), typical structures produced by virus-induced membrane rearrangements, are platforms for viral replication. Nidoviruses, one of the most complex positive-strand RNA viruses, have the ability to infect not only mammals and a few birds but also invertebrates. Nidoviruses possess a distinctive replication mechanism, wherein their nonstructural proteins (nsps) play a crucial role in DMV biogenesis. With the participation of host factors related to autophagy and lipid synthesis pathways, several viral nsps hijack the membrane rearrangement process of host endoplasmic reticulum (ER), Golgi apparatus, and other organelles to induce DMV formation. An understanding of the mechanisms of DMV formation and its structure and function in the infectious cycle of nidovirus may be essential for the development of new and effective antiviral strategies in the future.

Recent Progress in Torovirus Molecular Biology

Torovirus (ToV) has recently been classified into the new family Tobaniviridae, although it belonged to the Coronavirus (CoV) family historically. ToVs are associated with enteric diseases in animals and humans. In contrast to CoVs, which are recognised as pathogens of veterinary and medical importance, little attention has been paid to ToVs because their infections are usually asymptomatic or not severe; for a long time, only one equine ToV could be propagated in cultured cells. However, bovine ToVs, which predominantly cause diarrhoea in calves, have been detected worldwide, leading to economic losses. Porcine ToVs have also spread globally; although they have not caused serious economic losses, coinfections with other pathogens can exacerbate their symptoms. In addition, frequent inter- or intra-recombination among ToVs can increase pathogenesis or unpredicted host adaptation. These findings have highlighted the importance of ToVs as pathogens and the need for basic ToV research. Here, we review recent progress in the study of ToV molecular biology including reverse genetics, focusing on the similarities and differences between ToVs and CoVs.

Genome sequence of torovirus identified from a pig with porcine epidemic diarrhea virus from the United States

Porcine torovirus (PToV) strain PToV-NPL/2013 was identified from a pig that tested positive for porcine epidemic diarrhea virus (PEDV). The spike protein-encoding gene from PToV-NPL/2013 had 92% identity with PToV-SH1, suggesting that PToV circulating in the United States is slightly different from the isolates circulating in China. To our knowledge, this is the first report of PToV in the United States.

Molecular epidemiology of Porcine torovirus (PToV) in Sichuan Province, China: 2011-2013

BACKGROUND

Porcine torovirus (PToV) is a member of the genus Torovirus which is responsible for gastrointestinal disease in both human beings and animals with particular prevalence in youth. Torovirus infections are generally asymptomatic, however, their presence may worsen disease consequences in concurrent infections with other enteric pathogens.

METHODS

A total of 872 diarrheic fecal samples from pigs of different ages were collected from 12 districts of Sichuan Province in the southwest of China. RT-PCR was done with PToV S gene specific primers to detect the presence of PToV positive samples. M gene specific primers were used with the PToV positive samples and the genes were sequenced. A phylogenetic tree was constructed based on the M gene nucleotide sequences from the 19 selected novel Sichuan strains and 21 PToV and BToV M gene sequences from GenBank.

RESULTS

A total of 331 (37.96%, 331/872) samples were found to be positive for PToV and the highest prevalence was observed in piglets aged from 1 to 3 weeks old. Through phylogenetic inference the 40 PToV M gene containing sequences were placed into two genotypes (I & II). The 19 novel Sichuan strains of genotype I showed strong correlations to two Korean gene sequences (GU-07-56-11 and GU-07-56-22). Amino-acid sequence analysis of the 40 PToV M gene strains revealed that the M gene protein was highly conserved.

CONCLUSIONS

This study uncovered the presence of PToV in Sichuan Province, and demonstrated the need for continuous surveillance PToV of epidemiology.

Detection of bovine torovirus in fecal specimens from calves with diarrhea in Turkey

Bovine torovirus (BToV), a member of the family Coronaviridae, is an established gastrointestinal infectious agent in cattle. In this study, we performed a survey to detect BToV in Turkey between 2009 and 2011 using 235 fecal samples from neonatal calves with diarrhea that were analyzed by the nested reverse transcription (RT) PCR method using primers located in the consensus sequences of the BToV membrane (M) gene. The BToV M gene was detected in 4.7 % (11/235) of the samples using the nested RT-PCR method. The nucleotide sequences of partial M fragments from the BToV isolates, including the newly identified Turkish isolates, showed more than 96 % identity. The result indicates that BToV is one of the pathogens that contribute to neonatal calf diarrhea cases in Turkey.

The role of torovirus in nosocomial viral gastroenteritis at a large tertiary pediatric centre

OBJECTIVE

To describe the viral etiology and epidemiology of nosocomial viral gastroenteritis (NVG) at a tertiary care pediatric hospital and identify any changes over the past two decades.

METHODS

Retrospective review of all patients with laboratory-confirmed NVG at The Hospital for Sick Children (Toronto, Ontario), from January 1, 2004, to December 31, 2005.

RESULTS

One hundred forty-two episodes of NVG were found among 133 patients, occurring in 0.48 of 100 admissions. The median age was two years; 42% were <1 year of age and 41% were immunocompromised. The most commonly detected pathogen was torovirus (67% of episodes), followed by rotavirus (19%) and adenovirus (9%). Seventy-five cases (53%) were epidemiologically linked in 32 separate clusters (median cluster size two, range two to four). The NVG rate fell from 0.63 of 100 to 0.22 of 100 admissions after March 2005 (P<0.001) when enhanced infection control precautions were instituted in response to an outbreak of vancomycin-resistant Enterococcus.

CONCLUSIONS

Torovirus remains the most commonly identified cause of NVG at The Hospital for Sick Children. Most NVG cases were epidemiologically linked, and a significant reduction in cases occurred after the institution of enhanced infection control practices following an outbreak of vancomycin-resistant Enterococcus. Improved education and surveillance for NVG should lead to further reduction in this problem.

Seroprevalence of porcine torovirus (PToV) in Spanish farms

BACKGROUND

Torovirus infections have been associated with gastroenteritis and diarrhea in horses, cows, pigs and humans, especially in young animals and in children. Although asymptomatic in a large percentage of cases, however toroviruses may pose a potential threat to worsen disease outcome in concurrent infections with other enteric pathogens. Previous studies based on the analysis of limited numbers of samples indicated high seroprevalences against porcine torovirus (PToV) in various European countries. The aim of this work was to perform a seroepidemiological survey of PToV in Spanish farms in order to define the seroprevalence against this virus.

RESULTS

Serum samples (n = 2664) from pigs of different ages were collected from 100 Spanish farms coming from 10 regions that concentrate 96.1% of the 3392 farms with 80 or more sows censused in Spain. Samples were screened by means of an indirect enzyme-linked immune-sorbent assay (ELISA) based on a recombinant PToV nucleocapsid protein as antigen. The analysis of the whole serum collection yielded a total of 95.7% (2550/2664) seropositive samples. The highest prevalence (99.6%, 1382/1388) and ELISA values (average O.D. ± standard deviation) were observed in the sows (1.03±0.36) and the lowest prevalence (59.4%, 98/165) and anti-PToV IgG levels (0.45±0.16) were found amongst 3-week-old piglets. Both ELISA reactivity values and seroprevalence percentages rose quickly with piglet's age from 3 to 11 weeks of age; the seroprevalence was 99.3% (2254/2270) when only the samples from sows and pigs over 11-weeks of age were considered. Antibodies against PToV were detected in all analyzed farms.

CONCLUSIONS

This report describes the results of the largest torovirus seroepidemiological survey in farmed swine performed so far. Overall, the seroprevalence against PToV in animals older than 11 weeks of age was >99%, indicating that this virus is endemic in pig herds from Spain.

Synthesis and inhibitory activity of sialic acid derivatives targeted at viral sialate-O-acetylesterases

A series of sialosides modified at the 4- and 9-hydroxy group were synthesised and tested for inhibition of the viral haemagglutinin-esterase activity from various Orthomyxoviruses and Coronaviruses. While no inhibition of the sialate-4-O-acetylesterases from mouse hepatitis virus strain S or sialodacryoadenitis virus was found, a 9-O-methyl derivative displayed inhibitory activity against recombinant sialate-9-O-acetylesterase from influenza C virus.

Longitudinal serological and virological study on porcine torovirus (PToV) in piglets from Spanish farms

A study was performed to evaluate porcine torovirus (PToV) seroprevalence and infection in three multi-site farms from the North-eastern region of Spain. Serum samples from 120 piglets and faecal samples from 36 piglets were longitudinally collected at 1, 3, 7, 11 and 15 weeks of age. Serum samples from their dams (n = 30) were also taken 1-week post-farrowing. PToV antibodies in serum were monitored by ELISA, while viral infection was assessed by real-time RT-PCR in faeces. A high seroprevalence (about 100%) was observed in animals older than 11 weeks and in adult sows. Moreover, all 1-week-old animals were seropositive, indicating maternal antibody transference through colostrum. The antibody titers declined to close to or below the ELISA cut-off value by the age of weaning (3 weeks of age). Development of a significant antibody response to PToV occurred before 7 weeks of age in about 50% of piglets, and the remaining animals developed the response by weeks 11 or 15. These results indicate that PToV infection occurred soon after weaning. Although the prevalence of infection in suckling piglets varied among the studied farms, PToV prevalences in 7 and 11-week-old pigs were between 50–67% and 58–75%, respectively, in all farms. Sequencing results indicated that more than one PToV strains were circulating in the studied farms. Present data suggest that PToV was endemic on the studied farms, and provide new insights on the epidemiology of PToV.

Detection and molecular characterization of porcine toroviruses in Korea

This study examined the prevalence and genetic diversity of the porcine torovirus (PToV) in Korea. Of 295 samples, 19 (6.4%) samples tested positive for PToVs by RT-PCR. A low nucleotide sequence identity of the partial S gene was detected among the Korean PToVs (73.5%) and between the Korean and European PToVs (74.0%). Phylogenetic analysis of the spike and nucleocapsid genes showed that the Korean PToVs form distinct branches with clusters corresponding to the farm of origin, which were separate from the other known foreign PToVs. These findings suggest that genetically diverse Korean PToV strains cause sporadic infections in Korea.

Molecular characterization of a new PToV strain. Evolutionary implications

Toroviruses are emergent viruses, belonging to the Nidovirales order, that remain mostly ignored, despite they are able to infect different species of domestic animals and humans, causing enteric diseases and diarrhea. Thus far, only five variants of porcine torovirus (PToV) have been identified. In this report we describe the identification and partial characterization of a new strain of porcine torovirus (PToV-BRES) that was detected by RT-PCR in a swine faecal specimen from a farm in Brescia (Italy). The complete genes coding for the nucleocapsid (N), hemagglutinin-esterase (HE) and membrane (M) proteins were amplified, and sequence analysis showed that PToV-BRES is a new PToV strain that, based on the HE gene sequence, is phylogenetically related to P4 strain, that was up to now the only member of a distinct PToV lineage. The nucleocapsid protein from PToV-BRES was expressed in insect cells as a his-tagged protein, purified by affinity chromatography and used to develop an ELISA method to detect antibodies against PToV. This assay was evaluated using a serum collection including 45 samples from three commercial farms from Spain. High antibody prevalence against PToV was observed in the three farms, both in adult animals and in piglets, which could suggest that PToV might be endemic in Spanish porcine population. The ELISA method developed in this work could be useful in future epidemiological surveys about toroviruses.

Molecular epidemiology of bovine toroviruses circulating in South Korea

The prevalence of the bovine torovirus (BToV) and its genetic characterization have been reported in North America, Europe and Japan. Therefore, this study examined the prevalence and genetic diversity of the BToV in a total of 645 diarrheic fecal samples from 629 Korean native beef calf herds using RT-PCR and nested PCR with the primer pairs specific to a part of the BToV membrane (M) gene. Overall, 19 (2.9%) out of 645 diarrheic samples from 19 herds (6.9%) tested positive for BToVs by either RT-PCR or nested PCR. A comparison of the nucleotide (nt) and amino acid (aa) sequences of a part of the BToV M gene (409bp) among the BToVs showed the Korean BToVs to have comparatively higher sequence homology to the Japanese and Dutch BToVs than to the American and Italian BToVs. Generally, the Korean BToV strains clustered with the Japanese and Dutch BToV strains. However, the American and Italian BToV strains clustered on a separate major branch, suggesting that these are more distantly related to other known BToV strains. These results suggest that the BToV infections are sporadic in diarrheic calves in South Korea, and the Korean BToV strains are more closely related to the Japanese and Dutch BToVs than to the American and Italian BToVs.

First isolation of cytopathogenic bovine torovirus in cell culture from a calf with diarrhea

A cytopathogenic virus (designated the Aichi/2004 strain) was isolated in a human rectal adenocarcinoma cell line (HRT-18) from the ileum contents of a calf with diarrhea. Oval and elongated particles, approximately 100 to 170 nm in diameter, with club-shaped projections were seen in the infected culture supernatant, and torovirus-like (tubular and torus nucleocapsid) structures were seen in the infected cells by electron microscopy. An antiserum against bovine torovirus (BToV) reacted with the infected cells by immunofluorescence and neutralized the isolate. However, antisera against bovine coronavirus (BCV) failed to react with the infected cells by immunofluorescence or did not neutralize the isolate. Further, the isolate was positive for BToV by reverse transcription-PCR (RT-PCR) targeting fragments of the nucleocapsid (N), membrane (M), and spike (S) genes. Comparison of the nucleotide sequences of the PCR products with those of the published N, M, and S genes (476 to 497, 672, and 687 to 690 nucleotides, respectively) of toroviruses showed high sequence identities (up to 99.4%, 98.7%, and 94.9% for the N, M, and S genes, respectively) between the isolate and BToVs. In contrast, the isolate was negative for BCV by RT-PCR. In a serological survey of serum samples from 355 calves at 33 farms, 92% of calves were positive for neutralizing antibodies to the isolate. These results indicate that the isolate in this study was BToV and that BToV infection might be common in cattle in Japan. To our knowledge, this is the first isolation of BToV in tissue culture.

Gastroenteric Viruses

In recent years, viruses have been recognized increasingly as an important cause of foodborne infections. More than 160 enteric viruses are excreted in the feces of infected individuals, and some may also be present in the vomitus. Food and water are directly contaminated with fecal material, through the use of sewage sludge in agriculture, sewage pollution of shellfish culture beds, or may be contaminated by infected food-handlers.

Supramolecular architecture of severe acute respiratory syndrome coronavirus revealed by electron cryomicroscopy

Coronavirus particles are enveloped and pleomorphic and are thus refractory to crystallization and symmetry-assisted reconstruction. A novel methodology of single-particle image analysis was applied to selected virus features to obtain a detailed model of the oligomeric state and spatial relationships among viral structural proteins. Two-dimensional images of the S, M, and N structural proteins of severe acute respiratory syndrome coronavirus and two other coronaviruses were refined to a resolution of approximately 4 nm. Proteins near the viral membrane were arranged in overlapping lattices surrounding a disordered core. Trimeric glycoprotein spikes were in register with four underlying ribonucleoprotein densities. However, the spikes were dispensable for ribonucleoprotein lattice formation. The ribonucleoprotein particles displayed coiled shapes when released from the viral membrane. Our results contribute to the understanding of the assembly pathway used by coronaviruses and other pleomorphic viruses and provide the first detailed view of coronavirus ultrastructure.

Supramolecular architecture of severe acute respiratory syndrome coronavirus revealed by electron cryomicroscopy

Coronavirus particles are enveloped and pleomorphic and are thus refractory to crystallization and symmetry-assisted reconstruction. A novel methodology of single-particle image analysis was applied to selected virus features to obtain a detailed model of the oligomeric state and spatial relationships among viral structural proteins. Two-dimensional images of the S, M, and N structural proteins of severe acute respiratory syndrome coronavirus and two other coronaviruses were refined to a resolution of approximately 4 nm. Proteins near the viral membrane were arranged in overlapping lattices surrounding a disordered core. Trimeric glycoprotein spikes were in register with four underlying ribonucleoprotein densities. However, the spikes were dispensable for ribonucleoprotein lattice formation. The ribonucleoprotein particles displayed coiled shapes when released from the viral membrane. Our results contribute to the understanding of the assembly pathway used by coronaviruses and other pleomorphic viruses and provide the first detailed view of coronavirus ultrastructure.

Bovine torovirus (Breda virus) revisited

Bovine torovirus (BoTV) is a pleomorphic virus with a spike-bearing envelope and a linear, non-segmented, positive-sense single-stranded RNA genome. This kidney-shaped virus is associated with diarrhea in calves and apparently has a worldwide distribution. This review provides details of the history and taxonomy of BoTV since its discovery in 1979. Information about virion morphology and architecture, antigenic and biological properties, viral genome, protein composition, thermal and chemical stability, and pH and proteolytic enzymes resistance is also summarized. A major focus of this review is to postulate a possible epidemiological cycle for BoTV, based on epidemiological data obtained in our studies and other published data, and progressing from the newborn calf to the adult animal. The distribution, host range, pathogenesis, disease and clinical signs (under experimental and natural exposure), pathology, diagnosis, prevention, treatment and control of BoTV infections are also described. In addition, a discussion of the zoonotic implications of torovirus-like particles detected in patients with gastroenteritis that resemble and cross-react with BoTV is presented. Hopefully, the findings described here will alert others to the existence of BoTV in cattle and its contribution to the diarrheal disease complex. This review also highlights the need for continual vigilance for potential zoonotic viruses belonging to the order Nidovirales, such as the SARS coronavirus.

Phylogenetic and evolutionary relationships among torovirus field variants: evidence for multiple intertypic recombination events

Toroviruses (family Coronaviridae, order Nidovirales) are enveloped, positive-stranded RNA viruses that have been implicated in enteric disease in cattle and possibly in humans. Despite their potential veterinary and clinical relevance, little is known about torovirus epidemiology and molecular genetics. Here, we present the first study into the diversity among toroviruses currently present in European swine and cattle herds. Comparative sequence analysis was performed focusing on the genes for the structural proteins S, M, HE, and N, with fecal specimens serving as sources of viral RNA. Sequence data published for animal and human torovirus variants were included. Four genotypes, displaying 30 to 40% divergence, were readily distinguished, exemplified by bovine torovirus (BToV) Breda, porcine torovirus (PToV) Markelo, equine torovirus Berne, and the putative human torovirus. The ungulate toroviruses apparently display host species preference. In phylogenetic analyses, all PToV variants clustered, while the recent European BToVs mostly resembled the New World BToV variant Breda, identified 19 years ago. However, we found ample evidence for recurring intertypic recombination. All newly characterized BToV variants seem to have arisen from a genetic exchange, during which the 3' end of the HE gene, the N gene, and the 3' nontranslated region of a Breda virus-like parent had been swapped for those of PToV. Moreover, some PToV and BToV variants carried chimeric HE genes, which apparently resulted from recombination events involving hitherto unknown toroviruses. From these observations, the existence of two additional torovirus genotypes can be inferred. Toroviruses may be even more promiscuous than their closest relatives, the coronaviruses and arteriviruses.

Detection of bovine torovirus and other enteric pathogens in feces from diarrhea cases in cattle

The objectives of this study were to determine the prevalence of bovine torovirus (BoTV) in bovine fecal samples from diarrhea cases submitted to the Ohio Animal Disease Diagnostic Laboratory (ADDL) and to assess if a relationship exists between BoTV and the other enteric pathogens detected. From November 1999 to May 2001, 259 specimens from 53 calves (< or = 6 months old), 27 young adults (52 years), 125 adults (> or = 2 years), and 54 animals of unknown age were examined by an antigen-capture enzyme-linked immunosorbent assay (ELISA) and reverse transcriptase-polymerase chain reaction (RT-PCR) assay developed to detect BoTV. Testing for other enteric pathogens was performed by ADDL, and the results were analyzed with the BoTV data. The BoTV was detected using ELISA or RT-PCR in 9.7% (25/259) of the clinical samples, 56% (14/25) of which were from calves (P < 0.001) representing 26.4% (14/53) of the calves tested. Of the BoTV-positive calves, 71% (10/14) were less than 3 weeks of age. In 11/25 positive specimens, BoTV was the only pathogen detected among those examined. Other enteric organisms detected alone or in combination with BoTV in calf samples were rotavirus, coronavirus, Salmonella spp., Cryptosporidium spp., and Giardia spp.; but no consistent association between BoTV and these organisms was observed. In summary, BoTV was detected in fecal samples from cattle with diarrhea, principally in young calves less than 3 weeks of age. Future studies of infectious diarrhea in cattle should also include assays for this etiologic agent.

Comparison of ELISA and RT-PCR versus immune electron microscopy for detection of bovine torovirus (Breda virus) in calf fecal specimens

Bovine Torovirus (BoTV) is an uncultivable enteric pathogen of cattle. Its failure to grow in vitro limits epidemiological studies, characterization of the virus, and development of diagnostic techniques. The objectives of this study were to develop and standardize an antigen-capture enzyme-linked immunosorbent assay (ELISA) and a reverse transcriptase-polymerase chain reaction (RT-PCR) assay for the detection of BoTV in fecal specimens. These assays were compared with immunoelectron microscopy (IEM) to evaluate their sensitivity, specificity, and efficiency as well as their advantages and limitations. Additionally, several methods to calculate ELISA cutoff values were used and compared using a statistical approach to obtain the optimal cutoff value for the ELISA. A plate cutoff ELISA value was determined to be the best method to calculate the cutoff value. The ELISA and RT-PCR assays developed in this study identified BoTV antigen and viral nucleic acids in feces without cross-reactions with the other calf enteric viruses examined. Both assays showed good agreement with IEM, with a Kappa value of 0.86 for ELISA and 0.85 for RT-PCR. The latter exhibited the higher analytical sensitivity. On the basis of the results obtained in this study, it is recommended that no single test should be used alone in an epidemiological survey because of the observed limitations of each assay. The fast and inexpensive ELISA combined with the highly specific and sensitive RT-PCR are a practical approach for future epidemiological studies of BoTV. These results should provide other researchers with the information needed to develop similar diagnostic assays for the study of BoTV.

Diagnosis of noncultivatable gastroenteritis viruses, the human caliciviruses

Gastroenteritis is one of the most common illnesses of humans, and many different viruses have been causally associated with this disease. Of those enteric viruses that have been established as etiologic agents of gastroenteritis, only the human caliciviruses cannot be cultivated in vitro. The cloning of Norwalk virus and subsequently of other human caliciviruses has led to the development of several new diagnostic assays. Antigen detection enzyme immunoassays (EIAs) using polyclonal hyperimmune animal sera and antibody detection EIAs using recombinant virus-like particles have supplanted the use of human-derived reagents, but the use of these assays has been restricted to research laboratories. Reverse transcription-PCR assays for the detection of human caliciviruses are more widely available, and these assays have been used to identify virus in clinical specimens as well as in food, water, and other environmental samples. The application of these newer assays has significantly increased the recognition of the importance of human caliciviruses as causes of sporadic and outbreak-associated gastroenteritis.

Bovine torovirus: sequencing of the structural genes and expression of the nucleocapsid protein of Breda virus

Breda virus (BRV), a member of the genus torovirus, is an established etiological agent of diarrhea of cattle, which is found as two separate serotypes, BRV-1 and BRV-2. In this study, a 7.5 kb fragment of the BRV-1 genome that bracketed the genes for the structural proteins of BRV was amplified by long RT-PCR and the amplicon purified and sequenced directly. Sequence analysis revealed the presence of four open reading frames (ORF) corresponding to the peplomer (S), envelope (M), and nucleocapsid (N) genes, and an ORF for a novel 1.2 kb gene located between the M and N genes. This new gene was identical in nucleotide sequence to the hemagglutinin-esterase (HE) gene of BRV-2. With the exception of this new ORF, BRV-1 manifests 80% nucleotide sequence identity with the torovirus prototype, Berne virus (BEV) in the 7.5 kb region from the 3' end of the genome that contains the genes for the structural proteins. A 504 base segment containing the ORF for the BRV-1 N gene was amplified by RT-PCR, and cloned into an Escherichia coli expression system. The resulting protein was purified by SDS-PAGE and used to immunize guinea pigs. Hyperimmune serum was reactive with bovine torovirus (BTV) and human torovirus (HTV) antigens. By immunoelectron microscopy, it was shown to aggregate broken but not intact torovirus particles from BTV-positive fecal specimens. By immunoblot, the hyperimmune serum reacted specifically with the 20 kD N proteins of both BTV and HTV, as well as with the expressed N protein. BRV-1 and BRV-2 immune sera from gnotobiotic calves, but not human convalescent sera from HTV-infected patients, reacted with the expressed N protein by immunoblot. These findings were applied to the design of a dot blot assay that could specifically detect BTV and HTV from fecal specimens.

Identification and characterization of a porcine torovirus

A porcine torovirus (PoTV) was identified and characterized; it is a novel member of the genus Torovirus (family Coronaviridae, order Nidovirales), closely related to but clearly distinct from the already recognized equine torovirus (ETV) and bovine torovirus (BoTV) representatives. Immunoelectron microscopy of feces from piglets revealed elongated, 120- by 55-nm particles which were recognized by a torovirus-specific antiserum. Amplification by reverse transcriptase (RT) PCR with primers designed to detect conserved regions (on the basis of the genomes of BoTV strain Breda and ETV strain Berne) resulted in the identification of the 489-bp nucleocapsid gene, encoding a 18.7-kDa protein. The sequence identity in this region between PoTV and both ETV and BoTV was only about 68%, whereas the latter two show 81% identity. Neutralizing antibodies directed against ETV were found in sera of adult and young pigs. In all 10 herds sampled, seropositive animals were present, and 81% of randomly selected adult sows possessed antibodies. A longitudinal study with RT PCR showed that piglets shed virus in the feces for 1 or more days, starting 4 to 14 days after weaning.

Characterization of torovirus from human fecal specimens

The toroviruses, Berne virus (BEV) and Breda virus (BRV), are recognized pathogens of horses and cattle, respectively. Torovirus-like particles (TVLPs) that are immunologically related to BRV have been reported as etiological agents of gastroenteritis in humans. Of the toroviruses, only BEV has been shown to replicate in cell culture. Hence, these agents can be routinely detected only by electron microscopy (EM), although serological testing has been used as well. Our studies have provided supporting evidence that the TVLPs detected in the stool specimens of pediatric patients with gastroenteritis are human toroviruses. By EM, these particles are morphologically similar to BEV and BRV. Thin-section electron microscopy revealed that TVLPs contain toroidal-shaped nucleocapsids. Viruses purified from human fecal specimens agglutinate rabbit erythrocytes. BRV antiserum as well as convalescent sera from patients with gastroenteritis whose stools contain TVLPs were shown to contain antibodies that react with purified TVLPs as demonstrated by hemagglutination inhibition, immunoelectron microscopy, and immunoblotting. RNA extracted from partially purified TVLP preparations is amplifiable by RT-PCR using primers bracketing a 219-base region at the 3' end of the Berne virus genome. Sequence analysis of amplicons from five isolates showed a high degree of identity with the corresponding BEV sequence.

Association of torovirus with acute and persistent diarrhea in children

OBJECTIVE

To study the etiologic role of toroviruses as a cause of gastroenteritis in humans.

METHODS

The design was a case-control study. We compared the rate of torovirus detection in fecal specimens from a selection of children with acute or persistent diarrhea and controls without diarrhea from a study of childhood diarrhea in an urban Brazilian slum. Stool samples were coded and tested in a blinded fashion for the presence of torovirus antigen by enzyme-linked immunosorbent assay, other enteropathogens, toxins and fecal leukocytes.

RESULTS

Thirty-three children with acute diarrhea, 41 children with persistent diarrhea and 17 controls were enlisted in the study. Torovirus antigen was detected in 9 (27%) samples from children with acute diarrhea, 11 (27%) samples from children with persistent diarrhea and none of the samples from controls (P < 0.05). In addition the presence of enteroaggregative E. coli was associated with persistent diarrhea and the presence of Cryptosporidium oocysts was common although not significant (P = 0.08); torovirus and Cryptosporidium occurred in different subsets of samples, whereas torovirus and enteroaggregative Escherichia coli were commonly found in combination.

CONCLUSIONS

These data indicate that toroviruses, alone or in combination with enteroaggregative E. coli, may play a pathogenic role in acute and possibly persistent diarrhea. Further studies are warranted to determine the etiologic role of toroviruses in gastroenteritis.

Toroviruses of animals and humans: a review

Toroviruses are a group of enveloped positive-stranded RNA viruses that cause enteric, respiratory, and perhaps generalized infections in animals and humans. Their name refers to their unique morphological features: an elongated bacilliform core with two rounded ends is surrounded by a membrane that may either tightly adhere to or “shrink-wrap” it, without respecting the capsid's rod shape; in the first instance, straight or curved rhabdovirus-like particles are formed, whereas in the latter a biconcave disk results. Torovirus history is brief: the first representative, Berne virus (BEV), was isolated in Berne, Switzerland, in 1972 from a rectal swab taken from a horse with diarrhea 1 week before it died. BEV is the only equine torovirus isolate that replicates in cell culture; since most molecular data have been obtained with this isolate, BEV has been acknowledged as the torovirus prototype. Recognition of toroviruses as a new group of potentially pathogenic viruses came seven years after the discovery of BEV, when morphologically similar particles were discovered by electron microscopy (EM) in stool specimens from calves with severe diarrhea in a dairy herd in Breda, Iowa. Despite repeated attempts, BRV has not been adapted to the growth in cell or tissue culture, a problem which has hampered its biochemical, bio-physical, and molecular characterization. However, its pathogenesis and pathology have been studied in the experimentally infected gnotobiotic calves, showing that BRV infections may cause gastroenteritis. Recently, Vanopdenbosch et al. reported the isolation of a torovirus-like virus from the respiratory tract of calves with pneumonia, suggesting that both enterotropic and pneumotropic bovine toroviruses exist. Besides the established toroviruses of horses and cattle, torovirus-like particles (TVLPs) have been found by EM in different animal species; torovirus antibodies appear to be widespread in higher vertebrates, indicating that these viruses infect a broad range of animal hosts. The possibility of a torovirus infecting humans was first reported in 1984 and has become more likely in view of the recent data. This chapter is intended to update the information about toroviruses, and to describe the similarities and differences with the related coronaviruses.

Enzyme-linked immunosorbent assay reactivity of torovirus-like particles in fecal specimens from humans with diarrhea

Toroviruses are recognized enteric pathogens of cattle and horses; in humans, similar pleomorphic particles have been described, but doubt has been raised concerning their identity as viruses. We screened fecal samples from humans with diarrhea for the presence of torovirus-like particles (TVLPs) by electron microscopy and subsequently used an enzyme-linked immunosorbent assay (ELISA) with bovine torovirus reference reagents to test for the presence of torovirus antigens. To add another selection criterion to this heterologous ELISA, we enriched the TVLPs from the stool specimens by using sucrose density gradients before testing. The results of ELISA and EM correlated significantly, the ELISA having a sensitivity of 68% and a specificity of 86% (chi-square, P < 0.0001). In the gradient, peaks of ELISA reactivity were found at a buoyant density of 1.16 g/ml and were parallel to those found when using bovine torovirus. Furthermore, in 50% of the ELISA-positive gradients, a hemagglutinin for human group O erythrocytes comigrated with the peaks of ELISA reactivity. We were unable to isolate human TVLPs in human colonic tumor or rectal tumor cells. We cloned and sequenced amplification products obtained by low-stringency polymerase chain reaction amplification using consensus primers mapping to the 3' end of the genome of animal toroviruses, but found no significant homologies with animals torovirus sequences. Rabbits were inoculated with material from the gradient peak fractions of human stool specimens, and their sera were assayed for immunologic comparison with bovine torovirus as a reference. A two-way antigenic cross-reactivity was seen between human TVLP and bovine torovirus reagents when tested by ELISA. The rabbit antisera to human TVLP detected a higher number of electron microscopy-positive stool specimens than did the rabbit antisera to bovine torovirus. The application of these assays and reagents should help to elucidate the roles of TVLPs and toroviruses in diarrheal disease in humans.

Prevalence of neutralising antibodies to Berne virus in animals and humans in Vellore, South India. Brief report

In Southern India the prevalence of neutralising antibody to Berne virus was high in sera obtained from cattle (49%), horses (38%), and sheep (36%). Neutralising antibody was not detected in sera from humans and monkeys.

Detection of Breda virus antigen and antibody in humans and animals by enzyme immunoassay

Enzyme immunoassays were developed for the detection of Breda virus antibody and antigen. Cattle sera collected in the United Kingdom were found to have a high prevalence of antibody (55%) to Breda virus when examined in a competitive enzyme-linked immunosorbent assay. A low prevalence of antibody was found in pigs (2.2%), and no antibody was found in sheep or goat sera. No antibody to either Breda virus or Berne virus was detected in human sera collected from veterinarians and farm workers. Only 1 of 430 human fecal specimens (0.2%) contained Breda virus antigen detectable by enzyme-linked immunosorbent assay.

Characteristics of Australian human enteric coronavirus-like particles: comparison with human respiratory coronavirus 229E and duodenal brush border vesicles

The polypeptide profiles of highly purified coronavirus-like particles (CVLPs) proved to be very different from that of human respiratory coronavirus 229E and showed the particles not to be coronaviruses. Differences in polypeptide profiles and morphology between the CVLPs and duodenal brush border vesicles suggested that the CVLPs were also not such vesicles. Although they shared some basic overall similarity, the polypeptide profiles of three different but possibly antigenically identical CVLP preparations from Central Australian Aborigines were very dissimilar in detail. At least 38, 39 and 48 bands respectively were observed on the three profiles. At least 46 bands were visible on the polypeptide profile of CVLPs from a Vietnamese immigrant to Australia, and it also differed in detail from those of the Central Australian CVLPs. Indications of antigenic difference were obtained between Central Australian CVLPs and CVLPs from India, Kiribati, South Africa and Vietnamese immigrants to Australia. Antigenic difference was also suggested between the Central Australian CVLPs and those from one distant location within Australia, but antigenic similarity with those from another was indicated.