Enterotoxigenic Escherichia coli and diffusely adherent E coli as likely causes of a proportion of pathogen-negative travelers' diarrhea - A PCR-based study

Background. Enteropathogens cannot be identified in 40% to 50% of subjects with travelers' diarrhea (TD). Methods. We used polymerase chain reaction (PCR) methods to look for the presence of two bacterial causes of diarrhea in a large group of international travelers after failing to detect a pathogen by conventional tests. DNA was isolated from the diarrheal stool and subjected to PCR from 162 subjects from whom we earlier failed to identify a pathogen in a previous study and included 54 from Antigua, Guatemala, 39 from Guadalajara, Mexico, 29 from Kolkata, India, and 40 from Goa, India. Gene products for enterotoxigenic Escherichia coli (ETEC) - LT (heat-labile enterotoxin) and ST (heat-stable enterotoxin) - and diffusely adherent E coli (DAEC), afa/dr (Afa fimbrial and Dr nonfimbrial family of adhesins), were used. Results. At least one gene product was identified in diarrhea stool samples of 47 of 162 (29%) subjects. ETEC virulence genes (LT, ST) were found in 34 (21%) samples studied, with rates of occurrence ranging from 8% in Goa to 39% for the samples from Guatemala (p = 0.0006). A large number of ST-only strains explained the high ETEC rate in Guatemala. DAEC afa/dr family of adhesions was identified in between 8 and 14% of the samples. Conclusions. ETEC and DAEC were implicated in nearly one-third of the subjects initially diagnosed as pathogen negative. Direct PCR results from stools are consistent with the previous assumption that most undiagnosed TD is bacterial in nature and also highlights the potential value that PCR can add to studies designed to evaluate treatment and preventive interventions for TD, including vaccines.