## 4 Discussion  
In this prospective study of children who presented to the ED with acute infectious diarrhea, we examined the kinetics of viral shedding in stool samples of three major gastroenteritis viruses--norovirus, adenovirus, and rotavirus. All three viruses were shed in high levels at the time of presentation for ED care when the first specimen was obtained, and demonstrated unique fecal shedding kinetics. Norovirus had the lowest viral load on Day 0 among the three viruses, but it shed longer and at a higher viral load compared to rotavirus and adenovirus on Days 5 and 28. Among the children recruited with acute diarrhea who tested positive for one of the three viruses, norovirus was detected in 46%, followed by rotavirus (30%) and adenovirus (24%). The peak shedding of norovirus has been reported to occur between symptom onset to 5 days after infection [25], with over \(10^{9}\) copies of viral particles shed in one gram of feces [6]. [PERSON] et al. reported that among hospitalized pediatric patients, norovirus viral load increased between days 2 and 9 following illness onset, and most norovirus shedding in feces ceased by day 15, with an average viral load of 7.25 log genome copies/mL for all samples [26]. In our study, we did not observe an increase in viral load after illness onset, but the median viral load of norovirus on Day 0 was 9.9 log GE copies/g stool, and it remained persistently high on Day 5 (9.7 log GE copies/g stool). On Day 28, 70% of children still tested positive for norovirus, with a median viral load of 5.1 log GE copies/g stool. This is consistent with previous reports that younger children shed norovirus in stool for a longer period, especially infants less than 6 months of age, who have been reported to shed detectable virus for a median of 42 days compared to just 10 days among patients older than 1 year of age [27]. Human challenge studies also report that norovirus shedding can continue for up to 60 days after symptoms disappear [6; 28; 29]. Norovirus reinfection is often defined as a positive result 14 days after a previous positive test. Our results support the notion that use of a 14-day interval to define reinfection is inadequate given the longer duration of shedding in young infants. Further studies of norovirus shedding with a larger sample size and among different age groups are needed to characterize the optimal time frame to define incidence of norovirus infection. In recent years, RT-qPCR has become the gold standard for rapid, sensitive, and specific detection of norovirus in clinical samples [30]. However, a limitation of molecular \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Virus**} & \multicolumn{8}{c}{**Viral Load (Log GE Copies/g Stool)**} \\ \cline{2-10}  & \multicolumn{4}{c}{**Adenovirus**} & \multicolumn{4}{c}{**Rotavirus**} & \multicolumn{4}{c}{**Norovirus**} \\ \cline{2-10}  & **Day 0** & **Day 5** & **Day 28** & **Day 0** & **Day 5** & **Day 28** & **Day 0** & **Day 5** & **Day 28** \\ \hline Patient 1 & 13.2 & 11.5 & - & 6.6 & 0 & - & - & - & - \\ \hline Patient 2 & 13.3 & 10.4 & - & 10.3 & 13 & - & - & - & - \\ \hline Patient 3 & 5.6 & 6.1 & 0 & 12.9 & 12.5 & 4.4 & - & - & - \\ \hline Patient 4 & 6.2 & 5.0 & 0 & 12.3 & 11.1 & 0 & - & - & - \\ \hline Patient 5 & - & - & - & 13.1 & 11.4 & - & 5.5 & 0 & - \\ \hline Patient 6 & - & - & - & 12.7 & 6.4 & - & 6.5 & 0 & - \\ \hline Patient 7 & - & - & - & 12.9 & 7.9 & 4.1 & 6.6 & 6.3 & 0 \\ \hline Patient 8 & - & - & - & 0 & 11.8 & 5.6 & 10.3 & 0 & 0 \\ \hline Patient 9 & 4.3 & 5.2 & - & - & - & - & 10.8 & 9.1 & - \\ \hline Patient 10 & 5.6 & 0 & - & - & - & - & 9.5 & 10.2 & - \\ \hline \hline \end{tabular} “-” = either no sample collected or no virus detected.  
\end{table} 
Table 4: The viral load of stool samples in each patient with co-infection of mixed viruses.