Norovirus is a single stranded RNA virus of the Caliciviridae family. It is a common cause of acute gastroenteritis worldwide. Symptoms occur 24 to 72 hours after infection and include diarrhea, nausea, vomiting, and abdominal cramping.1 The virus is transmitted by consumption of contaminated foods and liquids, contact with infected surfaces and/or people.
The aim of our study was to determine factors associated with positive norovirus samples and analyze the seasonality of this virus in the Charleston, WV area.
The Virology Laboratory at the Charleston Area Medical Center (CAMC) receives rectal swab and stool samples from patients suspected of having norovirus gastroenteritis. Samples are obtained at 1 of the 4 CAMC hospitals and selected other hospitals and physician offices. The samples are analyzed using a commercially available IDEIA (TM) norovirus assay kit (Cambridgeshire, United Kingdom).
Data were extracted from the Virology Laboratory database for the period of January 1, 2007 through December 31, 2007. Additional information when necessary was obtained from electronic administrative data of patients.
The following variables were recorded: patient identification number, gender, ethnicity, birth date, date of the sample collected, source of the sample (rectal swab sample or stool sample), and test result. When duplicate samples were obtained from the same patient, only the first test result was recorded. Only samples with a complete data set were used in analysis.
We grouped patients into the following categories: neonate (<1 month), infancy (1 month–1 year), early childhood (1–4 years), later childhood (5–9 years), adolescents (10–19 years), and in 10-year increments thereafter. Data were analyzed with SAS 9.1 (Cary, NC). χ2 testing and logistic regression analysis were used to evaluate data. A P < 0.05 was considered significant.
The study protocol was approved by the CAMC Institutional Review Board.
After exclusion of duplicates and incomplete records, there were 2687 samples. Of these, 1424 (53%) were from females and 1263 (47%) were from males. The percentage of females positive for norovirus was 37.7% and the percentage of males positive for the virus was 34.6%. The ethnicity of our sample was White 90.7%, African American 6.3%, Hispanic 0.2%, other 1.6%, and unknown for 1.2% of the samples.
Samples were obtained from patients as young as 1 week of age up to individuals 99 years of age. However, 60.2% of the samples (n = 1617) were from subjects who were less than 19 years of age, whereas only 5.5% of samples (n = 149) were from individuals 80 to 99 years of age (similar age range).
To determine whether there was seasonality, samples were grouped into months: January through July and August through December (Fig. 1). The January through July period contained 74.1% of the total positive cases, while only 25.9% of the positive cases occurred from August through December (P = 0.0001). At the same time, the proportion of positive cases was also higher during January through July (44.1%) compared with August through December (24.0%; P < 0.0001).
The data were analyzed to determine the proportion of positive samples in each age group. The age spectrum was divided into 3 groups: neonate through early childhood (n = 1143 with 449 positive), late childhood through 39 years of age (n = 699 with 220 positive), and those 40 years through 99 years of age (n = 845 with 305 positive). The largest number of positive cases was seen in the neonate through 4 years of age group (n = 449), whereas the lowest positive caseload was seen in those aged 5 to 39 years of age (n = 220). The proportion of positive cases to negative samples was highest in the neonate through early childhood group with 39.3% of the samples being positive, while it was lowest for the late childhood through 39 years of age group (31.5%). Those 40 through 99 years of age had a 36.3% chance of their sample being positive for norovirus antigen.
The type of sample was evaluated (rectal swab sample vs. stool sample) and the percentage of positive tests determined for each type of sample. The percentage of positive rectal swab and stool samples were 34.7% and 43.0%, respectively (P = 0.0005).
With 974 positive tests and 1713 negative tests, a logistic regression model was developed to predict positivity of the norovirus antigen test. Variables included were: season of the year (January through July or August through December), source of the sample (rectal swab sample or stool sample), age groups (neonate through 4 years, or 5 through 39 years, or 40 through 99 years), and gender. Those tested from January through July were 2.51 (95% confidence interval [CI]: 2.11–2.98) times more likely to be positive than those tested from August through December. Stool samples were 1.59 (95% CI: 1.29–1.9) times more likely to be positive than rectal swab samples. Those in the neonate though 4 years of age group were 1.35 (95% CI: 1.14–1.60) times more likely to be positive compared with the other 2 groups. Females were 1.21 (95% CI: 1.03–1.42) times more likely to be positive than males.
Recently, a large study of children hospitalized for acute gastroenteritis (n = 1840) during a 2-year period revealed that 156 stool samples were positive for the Caliciviridae virus and 131 of these were norovirus confirmed by sequencing.2 This implicates norovirus as an important pediatric pathogen. Although many studies have documented outbreaks of norovirus on cruise ships, in prisons, and at eating establishments, we are not aware of studies that have tracked the presence of norovirus antigen in samples submitted to 1 laboratory in a 1-year period.
The ethnicity we noted in our study parallels to that of the Charleston metropolitan area. There was an increase in likelihood of samples being positive for viral antigen earlier in life (<5 years of age) compared with older age groups. Our findings are interesting in view of a recent multicenter study of more than 600,000 persons of all ages residing in Bangladesh, China, Pakistan, Indonesia, Vietnam, and Thailand, who were included in a shigellosis surveillance program.3Shigella was isolated from 2927 (5%) of 56,958 diarrhea episodes during the study, which was conducted between 2000 and 2004. The overall incidence of treated shigellosis was 2.1 episodes per 1000 residents per year in all ages, and 13.2/1000/y in children less than 5 years of age. However, the incidence of shigellosis increased after age 40 years.3 These findings are similar to our observations and may reflect the immaturity of the immune system in younger children and/or an ignorance or inability of young children to practice principles of sanitation. The increased likelihood of positive samples in older patients may be a reflection of a waning immune surveillance system in older individuals.
Our finding of norovirus being more common in winter months is strengthened by 2 recent reports suggesting this same trend.4,5 However, we found a high rate of positive samples into May and through July. A summer time peak has been noted in other studies.6,7
To our knowledge, the current study is the first to compare positive samples of norovirus in rectal swab specimens versus stool specimens. A total of 2192 rectal swab samples were analyzed and 34.7% of them were positive for norovirus. On the other hand, 495 stool samples were analyzed and 43.0% were positive. The difference is significant. It should be noted that the Centers for Disease Control suggests that as large a stool quantity as can be obtained is the preferred specimen for enteric viral isolation.8 We believe the appropriate technique for rectal swab samples was not always practiced and suggest that stool samples, and not rectal swab samples, be used for identifying norovirus antigen.
Our study is limited by the fact that no data were collected concerning clinical findings or outcome. We presume that samples were most likely obtained from individuals with diarrhea or from patients who did not have diarrhea but did have other signs and/or symptoms suggestive of gastroenteritis, such as abdominal pain, nausea, and vomiting. In addition, there was no control over how a sample was collected or the time interval from sample collection to when a sample arrived at the laboratory for testing. Finally, others have studied the IDEIA (TM) norovirus assay kit used by the Virology Laboratory at CAMC. The assay has a sensitivity of 62% to 66% and a specificity of 85% to 90%.9 It has been suggested that RT-polymerase chain reaction be used if increased sensitivity and specificity are necessary or if norovirus genogroups are being studied.10
In conclusion, our data suggest that: (1) norovirus is a common pathogen isolated from rectal swab samples and stool samples in patients living in the Charleston, WV area, (2) there are seasonal trends, (3) samples are more likely to be positive in young patients (<5 years of age), and (4) rectal swab samples are less likely to be positive when compared with stool samples.
2. Zintz C, Bok K, Parada E, et al. Prevalence and genetic characterization of caliciviruses among children hospitalized for acute gastroenteritis
in the United States. Infect Genet Evol
3. von Seidlein L, Kim DR, Ali M, et al. A multicentre study of shigella diarrhea in six Asian countries: disease burden, clinical manifestations, and microbiology. PLOS Med
4. Lopman BA, Adak GK, Reacher MH, et al. Two epidemiologic patterns of norovirus
outbreaks: surveillance in England and Wales, 1992–2000. Emerg Infect Dis
6. Lopman BA, Reacher M, Gallimore C, et al. A summertime peak of “winter vomiting disease”: surveillance of noroviruses in England and Wales, 1995 to 2002. BMC Public Health
7. Marshall JA, Hellard ME, Sinclair MI, et al. Incidence and characteristics of endemic Norwalk-like virus-associated gastroenteritis
. J Med Virol
8. Lew JF, LeBaron CW, Glass RI, et al. Recommendations for collection of laboratory specimens associated with outbreaks of gastroenteritis
. MMWR Recomm Rep
9. Dimitriadis A, Bruggink LD, Marshall JA. Evaluation of the Dako IDEIA norovirus
EIA assay for detection of Norovirus
using faecal specimens from Australian gastroenteritis
10. Rohayem J, Berger S, Juretzek T, et al. A simple and rapid single-step multiplex RT-PCR to detect norovirus
, astrovirus and adenovirus in clinical stool samples. J Virol Methods
Keywords:© 2009 Lippincott Williams & Wilkins, Inc.
norovirus; gastroenteritis; newborns; elderly; Charleston; WV