Norwalk and “Norwalk-Like Viruses” in Epidemic Gastroenteritis

Predictive model for inactivation of feline calicivirus, a norovirus surrogate, by heat and high hydrostatic pressure

Noroviruses, which are members of the Caliciviridae family, represent the leading cause of nonbacterial gastroenteritis in developed countries; such norovirus infections result in high economic costs for health protection. Person-to-person contact, contaminated water, and foods, especially raw shellfish, vegetables, and fruits, can transmit noroviruses. We inactivated feline calicivirus, a surrogate for the nonculturable norovirus, in cell culture medium and mineral water by heat and high hydrostatic pressure. Incubation at ambient pressure and 75 degrees C for 2 min as well as treatment at 450 MPa and 15 degrees C for 1 min inactivated more than 7 log10 PFU of calicivirus per ml in cell culture medium or mineral water. The heat and pressure time-inactivation curves obtained with the calicivirus showed tailing in the logarithmic scale. Modeling by nth-order kinetics of the virus inactivation was successful in predicting the inactivation of the infective virus particles. The developed model enables the prediction of the calicivirus reduction in response to pressures up to 500 MPa, temperatures ranging from 5 to 75 degrees C, and various treatment times. We suggest high pressure for processing of foods to reduce the health threat posed by noroviruses.

Calicivirus 3C-like proteinase inhibits cellular translation by cleavage of poly(A)-binding protein

Caliciviruses are single-stranded RNA viruses that cause a wide range of diseases in both humans and animals, but little is known about the regulation of cellular translation during infection. We used two distinct calicivirus strains, MD145-12 (genus Norovirus) and feline calicivirus (FCV) (genus Vesivirus), to investigate potential strategies used by the caliciviruses to inhibit cellular translation. Recombinant 3C-like proteinases (r3CL(pro)) from norovirus and FCV were found to cleave poly(A)-binding protein (PABP) in the absence of other viral proteins. The norovirus r3CL(pro) PABP cleavage products were indistinguishable from those generated by poliovirus (PV) 3C(pro) cleavage, while the FCV r3CL(pro) products differed due to cleavage at an alternate cleavage site 24 amino acids downstream of one of the PV 3C(pro) cleavage sites. All cleavages by calicivirus or PV proteases separated the C-terminal domain of PABP that binds translation factors eIF4B and eRF3 from the N-terminal RNA-binding domain of PABP. The effect of PABP cleavage by the norovirus r3CL(pro) was analyzed in HeLa cell translation extracts, and the presence of r3CL(pro) inhibited translation of both endogenous and exogenous mRNAs. Translation inhibition was poly(A) dependent, and replenishment of the extracts with PABP restored translation. Analysis of FCV-infected feline kidney cells showed that the levels of de novo cellular protein synthesis decreased over time as virus-specific proteins accumulated, and cleavage of PABP occurred in virus-infected cells. Our data indicate that the calicivirus 3CL(pro), like PV 3C(pro), mediates the cleavage of PABP as part of its strategy to inhibit cellular translation. PABP cleavage may be a common mechanism among certain virus families to manipulate cellular translation.

Viruses causing gastroenteritis

Acute gastroenteritis is one of the most common diseases in humans worldwide. Viruses are recognized as important causes of this disease, particularly in children. Since the Norwalk virus was identified as a cause of gastroenteritis, the number of viral agents associated with diarrheal disease in humans has steadily increased. Rotavirus is the most common cause of severe diarrhea in children under 5 years of age. Astrovirus, calicivirus and enteric adenovirus are also important etiologic agents of acute gastroenteritis. Other viruses, such as toroviruses, coronaviruses, picobirnaviruses and pestiviruses, are increasingly being identified as causative agents of diarrhea. In recent years, the availability of diagnostic tests, mainly immunoassays or molecular biology techniques, has increased our understanding of this group of viruses. The future development of a safe and highly effective vaccine against rotavirus could prevent, at least, cases of severe diarrhea and reduce mortality from this disease.

Characterization of an enteropathogenic bovine calicivirus representing a potentially new calicivirus genus

Bovine enteric caliciviruses (BEC) are associated with diarrhea in young calves. The BEC strains detected in Europe form a third genogroup within the genus "Norwalk-like viruses" (NLV) of the family Caliciviridae. In this report, we present sequence, clinical, and histological data characterizing a novel enteropathogenic BEC strain, NB, detected in fecal specimens from calves in the United States. The complete RNA genome of the NB virus is 7,453 bases long and is organized into two open reading frames (ORFs). ORF-1 is 2,210 amino acids long and encodes a large nonstructural polyprotein contiguous with the major capsid protein (VP1), similar to the lagoviruses and "Sapporo-like viruses" (SLV). The conserved calicivirus motifs were identified in the nonstructural proteins. ORF-2 is located at the 3' end of the genome and encodes a small basic protein (VP2) of 225 amino acids. The 5' and 3' untranslated regions are 74 and 67 bases long, respectively. Among caliciviruses, NB virus shows amino acid identities of 14.1 to 22.6% over the entire ORF-1 nonstructural-protein sequence with NLV, SLV, vesivirus, and lagovirus strains, while the overall sequence identity of the complete NB VP-1 with other caliciviruses is low, varying between 14.6 and 26.7%. Phylogenetic analysis of the complete VP1 protein, including strains from all four calicivirus genera, showed the closest grouping of NB virus to be with viruses in the genus Lagovirus, which cause liver infections and systemic hemorrhage in rabbits. In gnotobiotic calves, however, NB virus elicited only diarrhea and intestinal lesions that were most severe in the upper small intestine (duodenum and jejunum), similar to the NLV BEC strains. The tissues of major organs, including the lung, liver, kidney, and spleen, had no visible microscopic lesions.

Substrate specificity of the Norwalk virus 3C-like proteinase

The Norwalk Virus (NV) is the prototype strain of human caliciviruses that cause epidemic outbreaks of foodborne and waterborne gastroenteritis. These viruses do not grow in cell culture and the mechanisms of virus replication are obscure. The NV genome is a 7.7 kb ssRNA molecule that encodes three open reading frames (ORFs). The first ORF is a 1789 amino acid polyprotein that is processed into nonstructural proteins by a viral protease similar to the picornavirus 3C protease. Primary cleavage sites in the ORF1 polyprotein of two Norwalk-like viruses have been identified as QG dipeptides. We studied primary cleavage sites in the NV polyprotein and residues surrounding the scissile bond that are important in substrate recognition. A series of mutations were made at amino acids occupying positions implicated as important in cleavage site recognition for chymotrypsin-like viral proteases. We determined that effective processing at amino acid 398 to release the N-terminal p48 protein is necessary for proteolytic release of the p41 protein, that the P4 position N-terminal to the scissile bond is important for efficient processing, and that substitution of large hydrophobic residues were tolerated at this position. Finally, we defined the acidic residue of the 3CL(pro) catalytic site.

Viral causes of diarrhea

Viruses are important causes of diarrhea. In healthy adults, the main clinical manifestation is acute, self-limited gastroenteritis. Advances in molecular diagnostics have shown that epidemics of acute gastroenteritis most frequently are due to caliciviruses spread through contaminated food or through person-to-person contact. Application of similar technology is needed to make a definitive statement about the role of such candidate viruses as rotavirus, astrovirus, and adenovirus as the cause of nonepidemic acute gastroenteritis in adults. Rarely a previously healthy adult gets acute CMV colitis. CMV and EBV mainly cause diarrhea in immunocompromised patients, however. Advances in prophylaxis and treatment have reduced the frequency and severity of these diseases. Acute infantile gastroenteritis is caused by rotavirus, calcivirus, astrovirus, and adenovirus. These viral diseases of the gut are seen by the physician as routine and rare clinical problems.

Laboratory approaches to infectious diarrhea

Future applications of advanced molecular diagnostics in clinical laboratories will enhance significantly capabilities to diagnose bacterial, parasitic, and viral agents in the early course of disease through enhanced assay sensitivities and specificities and improved turnaround times, theoretically leading to more timely and directed therapeutic intervention. Until such time, clinicians must continue to rely on clinical judgment and the diverse battery of traditional culture techniques, direct examination (including light microscopy and electron microscopy), and immunoassays that are available. Cost considerations and the ever-increasing array of infectious agents responsible for infectious gastroenteritis will continue to drive the development of practice guidelines to assist practitioners with reasoned and reasonable approaches to management of diarrheal illnesses.

An improved method for the detection of Norwalk-like caliciviruses in environmental samples

An original magnetic beads RNA capture was developed for the detection of Norwalk-like virus by RT-PCR. The same oligonucleotide was used both for capture and reverse transcription of the viral RNA. The optimization studies showed that the most important parameter for sensitivity is the biotin-binding capacity of the beads. This method was found to be efficient for eliminating inhibitors in sewage samples compared with the classic RT-PCR. Moreover, the sensitivity was greatly enhanced, allowing the detection of 42% positive sample after gel electrophoresis, which is fourfold greater than classic RT-PCR (11%). Beads-RT-PCR sensitivity is the same as classic RT-PCR and hybridization. Thus, this method, which is easy to perform, should be of particular interest for developing quantitative RT-PCR and sequencing.