we conducted a prospective , observational study among adults hospitalized with laboratory - confirmed seasonal influenza a infections during 20062009 . briefly , patients were admitted if severe symptoms , respiratory or cardiovascular complications , or exacerbations of underlying conditions developed . when the patients sought care , nasopharyngeal aspirates ( npas ) were collected for diagnosis by using immunofluorescence assay or reverse transcription pcr . patients with confirmed influenza a were recruited if they were > 18 years of age and sought care within 1 week of illness onset . patients with pandemic ( h1n1 ) 2009 virus infections were excluded and reported separately ( 9 ) . after providing written , informed consent , patients were asked to submit 1 stool specimen for viral rna detection during hospitalization , regardless of gastrointestinal symptoms . for comparison , fecal shedding of respiratory syncytial virus ( rsv ) and parainfluenza virus ( piv ) were studied during a 10-month period by using a similar approach . ethical approval for the study was obtained from the institutional review boards of the chinese university of hong kong . all stool specimens were subjected to influenza viral rna detection by using quantitative real - time reverse transcription pcr targeting the matrix gene ( 6 ) . if positive , virus subtyping was performed by using h1- and h3-specific conventional pcrs . freshly collected stool specimens during 1 seasonal peak were simultaneously subjected to viral rna detection and virus isolation by using mdck cells . detailed methods for fecal detection of influenza viruses and rsv and piv are provided in the technical appendix . lectin histochemical analysis and double immunofluorescence staining were used to study the distribution of influenza virus receptors on human small and large intestinal tissues . an in vitro virus binding study on intestinal tissues was also performed by using inactivated human virus isolates of subtypes h1n1 ( a / hongkong / cuhk-13003/2002 ) and h3n2 ( a / hongkong / cuhk-22910/2004 ) ( technical appendix ) . a total of 119 hospitalized adults with seasonal influenza a infections were studied ( table 1 ) . their median age was 71 years ( interquartile range [ iqr ] 5779 years ) , and most ( 66% ) had concurrent conditions ; 5% were profoundly immunosuppressed . vomiting and diarrhea were reported by 15 ( 13% ) and 7 ( 6% ) patients , respectively . influenza a viral rna was detected in 56 of 119 of stool samples , collected at a median interval of 3 days ( iqr 35 days ) from onset ( detection rates by study year and virus subtype are shown in table 2 ) . detection rate by day from onset ranged from 31% to 63% and showed a trend to decrease toward the end of the week ( figure 1 , panel a ) . overall , the mean  sd fecal viral rna concentration was 4.4  0.8 log10 copies / g of feces and the median ( iqr ) was 4.2 ( 3.85.0 ) log10 copies / g of feces ; concentrations tended to decrease with longer time elapsed from onset ( figure 1 , panel b ) . * univariate comparisons of categorical and continuous variables were performed by using fisher exact test and mann - whitney u test or student t test , whenever appropriate . iqr , interquartile range ; icu , intensive care unit ; na , not applicable . defined as congestive heart failure ; cerebrovascular , neoplastic , chronic liver , and renal diseases ; diabetes ; ischemic heart disease ; or use of immunosuppressant drugs ( 8) . approximately 5% of patients were profoundly immunocompromised .   oseltamivir ( standard oral regimen , 75 mg 2/d for 5 d ) . defined as pneumonia , acute bronchitis , acute exacerbation of chronic respiratory conditions ( e.g. , chronic obstructive pulmonary disease , asthma ) ; acute coronary syndrome , decompensated heart failure , arrhythmia , or acute cerebrovascular events ( 8) . * virus subtyping results were unavailable for 9 cases in 2006 , 4 cases in 2007 , and 1 case in 2009 . iqr , interquartile range ; na , not applicable . fecal seasonal influenza a viral rna detection rate and its concentration , by number of days after illness onset , hong kong , 20062009 . fecal viral rna concentration was determined by using quantitative real - time reverse transcription pcr specific for the viral matrix gene and was expressed as log10 rna copies / g of stool . the lower detection limit of the assay was 3.7 log10 rna copies / g of stool . in most ( 77% ) viral rna  positive samples , further h1- or h3-specific pcrs identified 7 cases as h1 and 36 cases as h3 ; unsuccessful amplification was associated with lower viral ( matrix gene ) concentrations ( median [ iqr ] 3.9 [ 3.84.1 ] vs. 4.4 [ 3.85.1 ] log10 copies / g of stool ; p = 0.04 ) . no discrepancy was found between these and the subtyping results of the virus isolates from npas . fecal viral rna detection rate and thirty - eight stool samples from 1 seasonal peak were subjected to virus isolation and viral rna detection . in 10 cases , cytotoxicity occurred ( procedure discontinued ) ; in the remaining 28 cases , 12 were viral rna positive ; only 1 showed virus growth . this sample was from an 82-year - old man with dilated cardiomyopathy hospitalized for seasonal influenza a ( h1n1 ) pneumonia and heart failure ; diarrhea was absent . among 25 confirmed rsv or piv infections ( median [ iqr ] age 71 [ 5579 ] years ) , viral rna was detected in 5 fecal samples ( collected at median [ iqr ] 4 [ 36 ] days after onset ) ; none was culture positive . fecal viral rna positivity was lower compared with that of seasonal influenza viruses ( p = 0.01 ) . patients with positive and negative fecal viral rna detection results were compared ( table 1 ) . positive fecal viral rna detection was associated with younger age , shorter interval from illness onset to sample collection , lymphopenia , and positive virus isolation . multivariate logistic regression showed that lymphopenia ( adjusted odds ratio 2.36 , 95% confidence interval 1.025.47 ; p = 0.045 ) and positive virus isolation in npas ( adjusted odds ratio 3.76 , 95% confidence interval 1.0713.20 ; p = 0.039 ) were significant explanatory variables . fecal viral rna concentrations were also analyzed , and no association with clinical outcomes was found ( data not shown ) , except for a negative correlation with lymphocyte count ( spearman  0.37 , p = 0.047 ) . lectin histochemical analysis showed that sialic acid  2,6-gal ( human - like influenza virus receptor ) was absent from epithelial surface of small and large intestines and was found only in lamina propria cells . - like influenza virus receptor ) was found on large intestinal epithelial cells and in lamina propria cells . virus - binding study showed that neither seasonal influenza a ( h1n1 ) nor a ( h3n2 ) virus bind to small and large intestinal epithelial surface , but they bind to a subset of cd45 + leukocytes ( figure 2 ) . intestinal distribution of influenza virus receptors and in vitro binding of inactivated seasonal influenza a ( h1n1 ) and a ( h3n2 ) viruses to human duodenal and colonic tissues . images in the panels labeled overlay show the green , red , and blue ( nuclei counterstain ) color channels in the same view . a ) double immunofluorescence staining showing that human - like influenza virus receptor sialic acid ( sa )  2,6-gal ; green ) was not found on epithelial surface of small and large intestines but in lamina propria cells . avian - like influenza virus receptor ( sa 2,3-gal ; green ) was found on colonic epithelial surface and in lamina propria cells . part of receptor - positive cells coexpressed cd45 ( leukocyte common antigen ; red ) , representing leukocytes . b ) in vitro virus binding showing that neither seasonal influenza a ( h1n1 ) nor a ( h3n2 ) viruses bind to epithelial surface of small and large intestines but only to a subset of intestinal cd45 + leukocytes interspersed in the lamina propria and submucosa . direct intestinal infection by seasonal influenza viruses seems an unlikely explanation for the frequent fecal detection of viral rna in the patients reported here . no clinical correlation was shown for rna positivity ( but was shown with lymphopenia and positive virus isolation in npa , indicating higher virus load ) , and culture positivity is rare ( 4,5,10,11 ) . human - like influenza virus receptor is not found to express on normal intestinal epithelial cells ( 12 ) . these findings agree with reports which showed that intestinal cells and tissues do not support efficient replication of seasonal viruses ( 12,13 ) , thus their low potential to cause direct intestinal infection . alternatively , swallowing of virus - containing nasopharyngeal secretions ( although it seems inadequate to explain the higher rate of detecting fecal viral rna than rsv or piv ) and hematogenous dissemination to organs through infected lymphocytes or macrophages in severe influenza cases with high virus load ( spillover ) are possible explanations for fecal viral rna detection ( 2,14 ) . our findings on virus receptor distribution and in vitro virus binding to intestinal lamina notably , viral rna positivity in nonpulmonary tissues infiltrating mononuclear cells without detectable viral particles or antigens or tissue damage has been reported ( 15 ) . our study does not reject the possibility of seasonal influenza viruses causing occasional , disseminated infection in profoundly immunosuppressed persons because receptor affinity is not absolute ( 2 ) . conversely , highly pathogenic avian influenza ( h5n1 ) and pandemic ( h1n1 ) 2009 viruses have the ability to bind to avian - like influenza virus receptors on colonic epithelium and to replicate efficiently in intestinal cells and tissues ( 12 ) . their enhanced potential to cause direct intestinal infections and fecal  oral transmission deserve further investigation .
in a cohort of hospitalized adults with seasonal influenza a in hong kong , viral rna was frequently ( 47% ) detected in stool specimens . 
 viable virus was rarely isolated . 
 viral rna positivity had little correlation with gastrointestinal symptoms and outcomes . in vitro studies suggested low potential for seasonal influenza viruses to cause direct intestinal infections .