A total of 16 persons inoculated with Norwalk virus met the criteria for having Norwalk virus infection. Of these, 11 (69%) met the predefined definition for viral gastroenteritis. The 5 who did not meet this predefined definition had >3 symptoms that did not include vomiting or >200 g of watery diarrhea. All 11 participants with viral gastroenteritis had abdominal cramps, nausea, and vomiting; 5 of these participants also had >200 g of watery diarrhea, and 1 had <200 g of watery feces. Other signs and symptoms in the 11 participants were malaise (n = 9), anorexia (n = 8), headache (n = 7), myalgia (n = 4), temperature >37.6oC (n = 4), and chills (n = 3). The 5 participants who did not fulfill the criteria for gastroenteritis had nausea (n = 5), anorexia (n = 5), malaise (n = 4), abdominal cramps (n = 3), myalgia (n = 3), headache (n = 3), temperature >37.6oC (n = 2), chills (n = 2), and watery diarrhea <200 g (n = 2). Although the number of infected participants in each dosage group was relatively small, no differences in signs and symptoms based on inoculum dosage were apparent. The median duration of signs and symptoms was 23 (range 10–61) hours and was similar for both groups of participants.
All infected participants shed virus as measured by RT-PCR and had a >4-fold rise in serum antibody level, and all but 2 also shed virus as measured by antigen ELISA (). Virus shedding as measured by IMC RT-PCR was first detected a median of 36 hours (range 18–110 hours) after inoculation and lasted a median of 28 days after inoculation (range 13–56 days). Norwalk virus was detected in fecal samples of 7 participants 3–14 hours before onset of any clinical signs or symptoms. Presymptomatic shedding was more common in persons who did not meet the definition of clinical gastroenteritis than in those who did (4/5 vs. 3/11, respectively, p = 0.11, 2-tailed Fisher exact test). Virus shedding as measured by antigen ELISA was first detected ≈33 hours (median 42 hours) after inoculation and was last detected 10 days (median 7 days) after inoculation. Median values of the onset and resolution of virus shedding, as measured by IMC RT-PCR or antigen ELISA, were similar for the participants who had clinical gastroenteritis compared with those of persons who did not meet the definition of gastroenteritis ().
Fecal virus shedding among 16 participants inoculated with Norwalk virus*
Norwalk virus concentration in feces, as measured by qRT-PCR, peaked a median of 4 days after inoculation; the time of peak shedding was similar for participants who did and did not meet the definition of viral gastroenteritis (). The highest fecal concentrations of virus were detected in 11 (69%) participants after their clinical signs had resolved. The median peak amount of virus shedding was 95 × 109 (range 0.5–1,640 × 109) genomic copies/g feces, and 5 participants shed >100 × 106 copies/g until at least day 14 (). Persons who met the clinical definition of gastroenteritis had a higher median peak of virus shedding than those who did not have gastroenteritis (250 × 109 vs. 12 × 109 genomic copies/g feces, p = 0.08, Wilcoxon rank sum), and the average total number of viral genomic copies measured in the feces over the first 2 weeks after inoculation also was higher in the clinical gastroenteritis group (1013.3 vs. 1012.4, p = 0.056, Student t test). The virus concentrations in feces collected later after inoculation were low (range 225,000–40 × 106 genomic copies/g). Correlation between virus titer in feces and optical density results obtained in the antigen ELISA was strong (r = 0.823, Pearson correlation, p<0.001; ).
Figure 1 Shedding of Norwalk virus in feces. The quantity of viral RNA measured by quantitative reverse transcription–PCR (qRT-PCR; black line) and of virus antigen measured by ELISA (optical density; blue line) is shown for 2 participants: no. 703, who (more ...)
Figure 2 Correlation of viral RNA titer with antigen ELISA (optical density [OD]). Titers of viral RNA are correlated with the OD measured by antigen ELISA for the 148 fecal samples with positive quantitative reverse transcription–PCR (qRT-PCR) results (more ...)