| J Clin Microbiol. 2006 August; 44(8): 2997–3000. doi: 10.1128/JCM.00065-06. | PMCID: PMC1594604 |
Copyright © 2006, American Society for Microbiology Asymptomatic Norovirus Infection in Mexican Children Coralith García, 1,2,3 Herbert L. DuPont, 1,2,4 Kurt Z. Long, 5 Jose I. Santos, 6 and GwangPyo Ko 7*Center of Infectious Diseases, The University of Texas, School of Public Health, Houston, Texas,1 Baylor College of Medicine, Houston, Texas,2 Instituto de Medicina Tropical “Alexander von Humboldt,” Universidad Peruana Cayetano Heredia, Lima, Peru,3 St. Luke's Episcopal Hospital, Houston, Texas,4 Harvard School of Public Health, Boston, Massachusetts,5 Hospital Infantil de Mexico “Federico Gomez,” Mexico City, Mexico,6 Institute of Health and Environment, Department of Environmental Health, School of Public Health, Seoul National University, Seoul, Korea7 Received January 11, 2006; Revised February 17, 2006; Accepted May 22, 2006. |
Statistical methods. Frequencies and measures of the central tendency were used. Categorical and continuous variables were analyzed with a χ2 test or the Fisher exact test (two tailed) and an unpaired t test, respectively. A t test was used to compare the mean age between the of NoV-positive and NoV-negative groups, and a χ2 test was used to compare the sex between the NoV-positive and -negative groups. The monthly incidence was calculated by using the number of asymptomatic infections as the numerator and the total number of children followed up that month as the denominator. The statistic software used was STATA version 8. The mean age of children studied was 14 months (range, 6 to 22 months); 56% were females. The average number of analyzed stool specimens per child was 2.5 (total of 161 stool specimens collected from 63 children). The overall rate of NoV detection in stools was 48 of 161 (29.8%). No significant differences in the percentage of NoV-positive stools and age (P = 0.85) or sex (P = 0.78) of the children were found. The incidences of asymptomatic NoV infection were 0.36, 0.39, and 0.25 episodes per child in June, July, and August, respectively. A total of 31 children of 63 (49.2%) had at least one NoV stool that was positive during the study period. Eight of the thirty-one (25.8%) had two or more different strains. One child had four consecutive positive stool samples with the same strain during July and August, and this was considered as one infection when the incidence was calculated. Of 48 (56%) NoV-positive stool specimens, 27 were positive for NoV GII and two children were positive for both strains GI and GII in the same specimen. In all, nine different genotypes were observed. Identified genotypes include five GI genotypes (GI 1, GI 3, GI 5, GI 7, and GI 14) and four GII genotypes (GII 1, GII 2, GII 7, and GII 17) ( 12). One of the NoV GII strains was detected during the entire 3-month period and also was the most frequently identified in seven stool samples. The remaining NoV strains were detected during 1 or 2 months. The phylogenetic tree based on the partial capsid sequences (250 bp) of NoV isolated and the reference NoV strains shows a high genetic diversity, including strains from the GI and GII genotypes (Fig. 1). | FIG. 1.Phylogenetic tree construct based on partial sequences of the capsid gene of norovirus isolated in asymptomatic Mexican children. The accession numbers (DQ220748 to DQ220783) of our strains (CG1 to CG34) are indicated. The reference strains and their (more ...) |
The study showed a high rate of asymptomatic NoV infection in children living in a poor periurban Mexican community. Only a few past studies focused on asymptomatic NoV infection. In a community-based study in The Netherlands, NoV was detected in fecal samples from control subjects at a rate of up to 5% ( 3). A recent study in Japan reported that 6% of fecal specimens collected from asymptomatic infants were infected with NoV ( 1). Approximately 8% of human calicivirus was detected in nondiarrhea stool specimens collected from a birth cohort of Mexican children ( 5). Based on enzyme-linked immunoassay and immunosorbent electron microscopy, 11 of 14 children (78.6%) infected with human calicivirus were found to be asymptomatic in a day care center study carried out in the United States ( 18). In the present study, almost 30% of the stool specimens collected from asymptomatic children were infected with NoV. Almost half (49.2%) of asymptomatic children had at least one stool positive for NoV during the summer. This rate was much higher than found in other previously reported studies. The children in the present study were residents of a low-class community of the periurban area of Mexico City. The living conditions and poor hygienic measures could facilitate the transmission of enteric infections. Higher rates could be caused by other factors, such as the season or year of the study, or be related to the sensitive RT-PCR methods used. Ideally, NoV shedding from asymptomatic children should be divided into viral shedding from a latest symptomatic NoV gastroenteritis versus primary asymptomatic NoV infection. In our study setting, children in a poor community in Mexico commonly suffered from diarrhea throughout the year. In a previous challenge study, infected adult were found to excrete NoV in their stool specimens for up to 22 days ( 21). In the present study, asymptomatic stool specimens were collected from children without clinical symptoms (diarrhea, vomiting, or fever, etc.) for at least 8 days prior to and at least 2 days after the sample was collected. Information as to whether the patients were symptomatic with NoV gastroenteritis prior to the summer study is not available. We cannot therefore be certain in our study whether NoV was shed from a recent symptomatic NoV gastroenteritis or whether the subjects were experiencing primary asymptomatic infections. High rate of asymptomatic infection in the children described here is surprising since the summer is typically not the season one expects to see heavy rates of NoV infection ( 19). Recent studies have identified summer peaks of NoV illness ( 2, 17). Our finding suggests that young children could be an important reservoir for summer transmission of NoV. A recent study found high rates of asymptomatic excretion among adult patients and staff in hospital settings in England, but asymptomatic excreted NoV was not responsible for nosocomial outbreaks ( 6). Previous volunteer studies indicated that a significant proportion of infected persons after NoV challenge remained asymptomatic ( 10). It is not understood what factors would determine the development of clinical symptoms. Recent studies suggest that human histo-blood group antigens, such as secretor factor or blood type, could determine the human susceptibility to NoV ( 15, 22). In addition, host immunity response, the genotypes and virulence of NoV strain, or a combination of both are likely related to host susceptibility and asymptomatic infection of NoV ( 9, 14, 15). Our results are supported by a previous seroepidemiologic study that showed a high percentage of anti-norovirus antibodies in Mexican infants (85%) in 2-year-old infants, indicating widespread early life exposure to NoV in Mexico ( 11). Prior exposure could result in the development of protective immunity against symptomatic disease later in life. However, in that earlier study a significant association between serum antibodies and resistance to NoV infection was not found, suggesting the role for innate immunity and antigenic diversity of NoV influencing patterns of infection ( 15). Our study also demonstrated a high number of NoV strains circulating in this specific community of children living in the outskirts of Mexico City. This result coincides with other studies based on NoV associated with sporadic cases of acute gastroenteritis in children ( 8, 20). Future studies are needed to understand the importance of asymptomatic NoV infection in children as it relates to the transmission of infection and gastroenteritis. |
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