Human Calicivirus Virus - Essay Example

Type the document [Type the document sub [Pick the [Type the company pc Human Calicivirus Virus (HuCV) Article Review Introduction Calcivirus infect a wide range of animal species, causing a plethora of symptoms such as diarrhea, nausea, vomiting, fever, and urinary tract infections. The first reports of the epidemic caused by this group of virus was from Norwalk, Ohio in 1968; hence the name Norwalk virus. The pathogen was identified and described by Kapikian and associates in 1972; however lack of appropriate detection protocols prevented their significance till much later (Koo et al., 1673). Five genera are included in the Caliciviridae family; Norovirus (NoV), Sapovirus (SoV), Lagovirus, Vesivirus and the recent addition of Nebovirus. On the basis of genomic sequence and organization the human Calciviruses have been placed the general NoVs and Sapovirus in 2002 by International Committee on Taxonomy of Viruses. Prior to this certain other terms such as “Norwalk like viruses” and “Sapporo-like Viruses” were temporarily used terms for these two general respectively. Both of these agents are the causal agents of acute non-bacterial gastroenteritis, the former in all age groups, latter more commonly infects children (Schwab & Hurst, 281). Global Impact NoVses have now been recognized to be foodborne enteric pathogens of prime significance globally. Two third of the foodborne gastroenteritis incidences in US have been attributed to NoVs with approximately 23million cases reported annually. Commonly reported in children, elderly and the immunocompromised; the infection is widely prevalent in adults visiting or residing in hospitals, developing countries and areas with compact residential establishments. Next to Rotaviruses, NoVs have been identified as the major cause of gastroenteritis in children below five years of age (Koo et al., 1673). Now classified in to five genogroups (GI-GV), the strain identified to be dominant pathogen worldwide is genotype 4 belonging to genogroup II (GII.4), and has been responsible for three global pandemics in last decade (Siebenga et al., 1). Figure 1: Norwalk Virus (Sander) Figure 2: Calicivirus exhibiting the ‘Star of the David’ (Sander) Characteristics The NoVs have irregular surface characteristics and hence are termed as small-round structured viruses (SRSVs), The Sapovirus are characterized by “Star of David” morphology. These are small icosahedral and unenveloped viruses, 27 to 30 nm in diameter. The genome comprises of 7,000 to 8000 nucleotide long, single stranded, positive sense, 3’ polyadenylated RNA (Schwab & Hurst, 2006). The genome in NoVs has three open reading frames (ORFs), ORF1 encoding a non-structural protein further cleaved in to 6 proteins including an RNA dependent RNA polymerase (RdRp). ORF2 and ORF3 respectively encode a major capsid and a minor structural protein (Belliot et al., 10957). Transmission Low infection doses (18-1000 virus particle), extracellular stability and ability to resist common cleansers renders them ideal pathogens thereby giving them the Category B potential Bioterrorism Agents designation. Transmission of the NoVs is through fecal-oral route (Koo et al., 1673). Individual to individual transmission is therefore facilitated in population living in compact or congested areas, where airborne infections during vomiting has also been reported (Marks et al., 481). Transmission is further aided by poor hygiene and sanitation in such areas. Post infection an incubation period of 1-2 days is followed by symptoms of nausea, vomiting and diarrhea along with frequent occurrence of abdominal pains, anorexia and mild fever. Blood or mucous is rarely reported in fecal discharge since the NoVs are not invasive (Koo et al., 1673). Treatment Accurate diagnosis is possible through the infrequently available technique of reverse-transcription polymerase chain reaction (RT-PCR), in absence of which the Kaplan Criteria is commonly used. In absence of a cell culture model for NoVs, no precise therapeutic procedure is available. Instead symptomatic interventions such as rehydration though oral administration of electrolytes is used. Other agents being studied with no conclusive results include Bismuth Salicylate, Loperamide, Nitazoxanide etc (Koo et al., 1673). Vaccines NoV virus like particles (VLPs) have been shown to effective in mice and also humans. Attempts are being made for their transgenic expression in potatoes and tomatoes to simplify the manufacturing, storage and administration process (Tacket et al., 1866). However the major limitation of vaccine development at this stage includes lack of information regarding immunity levels that would successfully confer protection against the pathogen (Koo et al., 1673). Article Review Summary The understanding of molecular characterization of HuCV is recent since it is based exclusively on the techniques involving cloning and sequencing of virus genome. Hence despite the high prevalence of the disease caused by this pathogen, there is insufficient understanding of the molecular characteristics of the virus. This information gap is even more profound in Mexico, which is the location of the study described in the current research article. Very few studies have attempted to investigate prevalence of HuCV in Mexico. Further no reports are available regarding study of molecular characterization of HuCV. Hence the current research by Gomez-Santiago, Ribas-Aparicio and Garcia-Lozano (2012) is probably the first attempt to study the molecular characterization of various strains of NoVs across the different states of Mexico thus enabling a better understanding of their molecular epidemiology and genomic characteristics. Further the study also attempts to determine the prevalence of NoV GII.4 which has been reported to be responsible for the most incidences of acute diarrhea in children below 5 yrs of age. The study was based on 414 fecal samples taken from 14 out of 31 states of Mexico. The samples were taken from children in the age group of 0-5 yrs and suffering from acute gastroenteritis during the period spanning October 2005 and December 2006. All samples had been tested negative for bacterial enteric pathogens and were pretested by RNLSP (Spanish acronym for National Network of Public Health Laboratories) against presence of Rotavirus using polyacrylamide gel electrophoresis (PAGE), and had undergone quality assurance and reference using ELISA (Enzyme linked immunosorbent assay) and RT-PCR. The samples were then tested using ELISA for NoV infection which was then followed by RT-PCR genotyping for ensuring presence of NoV and another RT-PCR using another set of specific primer for identification of NoV GI and NoV GII. Finally the amplification products were sequenced and analyzed. Result The genotyping tests showed the presence of SoVs in 3 of 414 samples (1%), while NoV presence was detected in 128 (31%) of the samples. Further genotyping to identify the specific strains of NoV in the 128 samples NoV positive samples used primers targeting the 319 base pair RdRp gene and provided results indicating that 72 of the 128 samples had NoV GII virus strain. Of the remaining 56, 46 were identified again as NoV GII by primers specific for capsid proteins. None of the specimens showed presence of NoV GI either exclusively or in combination with NoV GII. The results are compiled in table 1 below. A Phylogenetic tree was finally prepared on the basis of the sequencing of the amplification products of 89 of the 131 samples using 319 bp long RdRp gene. These samples with HuCV, comprised of 88 NoV GII and one SaV GI strain. Of the 88 NoV strains, 86 showed close resemblance to NoV GII.4 while the remaining two were found to be similar to two strains of NoV GII.2 group namely Kuenzelsau 3870 and Melksham. Phylogenetic analysis based on VP1 capsid gene using 63 NoV samples of 131 HuCV showed that of the 63, 61 samples were of NoV GII.4 strain, the remaining two could be categorized as NoV GII.2 due to their clustering with OsakaN2004, Point de Roide673 and E397Crete strains. Another trend observed in the results based on monthly incidences of disease indicated two phases of peak infection, in November-December and in April-June. Moreover much higher infection rates were observed in children in the age group of 7-8months accounting for approximately 60% of the incidences. The children in the age group of 18 months to 5 years formed a much smaller proportion (figure 3 A & B). Figure 3 Conclusions The major conclusion derived in the article from the results summarized above is the extreme significance of RdRp and capsid genes in facilitating the molecular characterization of HuCV. Further it indicates the prominent role of NoV GII.4 in causing acute gastroenteritis in children in Mexico. Finally it underlines the high prevalence of HuCV caused acute diarrhea in Mexico and thus stresses the need for further and exhaustive study for the management, prevention and control of human calicivirus in Mexico and worldwide. Critique NoVs form a genetically diverse group. Attempts at classifying the group were based on cross reactivity analysis using immune electron microscopes. These classifications were inaccurate and could rarely be reproduced. Further in absence of cell culture system rendered neutralization based direct stereotyping impossible. Thus the authors rightly claim that RT-PCR and sequencing can be ideal methods of classifying and identifying this group of viruses. The same have been used in many recent studies (Zheng et al., 312). This technique requires use of conserved sequences such as RdRp as primers. The RdRp gene is forms the region A of the ORF1. Earlier studies have made use of other regions such as region B of ORF1 and region D of ORF2 (Castilho et al., 3947). The study by Castilho and associates was based on fecal samples derived from children in Brazil. Using the VP1 gene as primer it was able to detect the presence of NoV in 33.3% of the sample. In the current study however using primers derived from both RdRp gene and VP1 capsid gene, the authors were able to detect 32% of the samples as positive for HuCV and were further able to detect the strain type in 94% of the samples found positive for HuCV. 92% were found to be NoV and remaining 2% as SoV. Thus the authors rightly claim the significance of RdRp gene in facilitating identification of strain type in HuCV pathogens. The capsid gene VP1 has been used in many studies for the molecular characterization and classification of HuCV. Zheng and associates (2006) have designated a similarity of 55% and 85% in the amino acid sequence of strains to be classified as genogroup and genotype respectively. Capsid genes form the basis of presently accepted classification of HuCV in to five groups namely GI, GII, GIII, GIV and GV. Of these GI, GII and rarely GIV are human pathogens, with GII being infectious in pigs as well. GIII viruses are pathogenic in cattle and sheep, and GV in mice. However this classification based on VP1 is neither conclusive nor precise. Several new genotypes are regularly being added and have led to confusions in classification which are further aggravated by recombinant strains. Hence identification strategies based on both ORF1 and ORF 2 are required and have been speculated to be more effective. The current study can be considered to be first step in this direction. It has attempted to use both RdRp and VP1 genes from ORF1 and ORF2 respectively and has been successful in identifying NoV.GII. RdRp is one of the conserved sequences commonly used for diagnosis of caliciviruses. However; this sequence too exhibits high genetic diversity with variations as high as 40% observed within the nucleotide sequence of RdRp gene from the same genogroup (Wang et al., 5982). Hence rather than being dependent on RdRp gene, other regions of the genome need to be explored and other more conserved sequences need to be identified to enhance accuracy in Calicivirus diagnosis. Till this is made possible the best possible use of sequencing of RdRp gene would be in epidemiological studies. It can be used to identify the source of the genome it forms a part of thus facilitating understanding of the molecular epidemiology of the disease. The region D primers have been found to be useful in earlier studies in the identification of GI and GII Norviruses (Zheng, 313) and as in current study region A has been used to identify GII. However a complete sequence for complete strain classification and identification is still lacking. Hence the current study can provide for only one aspect of understanding of HuCV molecular characterization and cannot be considered as conclusive. NoV GII.4 strain was detected to be primarily responsible for the incidences of HuCV caused acute gastroenteritis in Mexico. Considering that 97% of the HuCV isolates from the sample positive for HuCV belonged to NoV GII.4 group, the conclusion of the authors can be considered to hold good. Further the samples were collected by the researchers from regions across North, Central and South Mexico, and during a period spanning across the complete seasonal cycle. Hence seasonal or regional variations in the frequency of the infectious agent have been accounted for. Finally the large sample size also adds to the credibility of the result. Earlier studies have also attributed GI, GII and GIV to be responsible for infections in humans. The first major outbreak associated strain identified was also GII.4. In 1995-96, 55% of NoVs epidemic outbreaks were attributed to strain named as US95/96. This strain was also identified to be responsible for 85% of outbreaks in Netherlands during the same period. In the period spanning 2000-2004, the strain US95/96 was replaced by GII.4. Simultaneously in Europe GII.4b strain was identified to be responsible for outbreaks (Lindesmith et al., 0269). Gomes and associates (1703) have made similar conclusions with respect to GII group on the basis of studies in Argentina, though only two of the 13 NoVs isolated were identified as GII.4 by them. Castilho and associates (3947) too in his study on samples derived from Brazil has attributed 40 of the 66 isolated strains i.e. 60.6% to GII.4 strain of NoVs on the basis of Phylogenetic analysis. The final conclusion derived by the authors that HuCV is one of the leading causes of acute gastroenteritis in children in Mexico cannot be fully substantiated on the basis of results. Absence of other similar studies for Mexico prevent validation of this conclusion derived by the authors. Studies based on other areas however have provided results on the basis of which similar conclusions can be derived. An important study based in France, comparing the prevalence of group A Rotavirus, HuCV, Astrovirus and adenovirus Type 40 and 41 infections among children with acute gastroenteritis presented similar results. HuCV was found to be the second most prevalent cause of acute gastroenteritis next to Rotavirus (Bon et al., 3055). The conclusions derived on the basis of this study form the major points of discussion in the article. The use of RT-PCR for the identification and molecular characterization of HuCV in children with acute gastroenteritis and the nationwide implications of the available data remain the major goals and points of discussion. Further the use of RdRp along with CV1 enabling higher rates of detection too has been stressed by the authors. Further the seasonal pattern observed in the incidences of disease with peaks occurring in November-December and April-June is a unique observation in this study. However the validity of this observation is not undisputed since the sample size was insufficient for each of the seasons. Besides this another major limitation of the study is the dependence on RdRp gene which itself has been reported to exhibit variations within the subgroup as well. Finally RT-PCR is not a technology suitable and affordable for developing countries where these HuCV infections are most prevalent. Further studies need to be designed to attain the twin targets of genomic characterization of HuCV and developing more convenient methods of diagnosis. Works Cited Belliot, G, et al. "In vitro proteolytic processing of the MD145 NoVs ORF1 nonstructural polyprotein yield stable precursors and products similar to those detected in calicivirus-infected cells." Journal of Virology (2003): 10957-74. Bon, F, et al. "Prevalence of group A Rotavirus, HuCV, Astrovirus and adenovirus Type 40 and 41 infections among children with acute gastroenteritis in Dijon, France." JOurnal of clinical microbiology (1999): 3055-8. Castilho, J. G, et al. "Genetic diversity of NoVs among children with acute gastroenteritis in Sao Paulo state, Brazil." Journal of clinical virology (2006): 3947-53. Gomes, K. A, et al. "Molecular characterization of calicivirus strains detected in outbreaks of gastroenteritis in Argentina." Journal of medical virology (2007): 1703-9. Gomez-Santiago, F, R. M Ribas-Aparicio and H. 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