Background: Enterotoxigenic E. coli (ETEC) is a major enteropathogen in developing countries.

Objectives: To examine the incidence of symptomatic and asymptomatic ETEC infection among low-income Mexican children <24 months of age and protection by breastfeeding (BF).

Methods: From 4/88 to 12/91, 317 infants residing in the SW outskirts of Mexico City were monitored up to 2 yrs for dietary intake and diarrhea incidence. Stool samples were collected during weekly home visits and whenever a child experienced diarrhea (occurrence of twice the usual stooling pattern or at least 3 loose stools / 24 hrs). Stool specimens were cultured on MacConkey agar within 4 hours of collection. Three lactose positive colonies identified as E. coli were tested for heat-labile toxin (LT) and heat-stable toxin (ST) using GMI-ELISA inhibition methods. Symptom-associated stool samples were also tested for other bacterial, parasitic and viral enteropathogens. Results: During the follow-up period, 525 ETEC infections were detected (1.6 episodes/child-year) of which 26% were symptomatic. The proportion symptomatic did not change by enterotoxin profile, infection history, or season. Over 50% of all isolates were positive for heat stable enterotoxin (ST). Of the 317 study infants, 248 (78%) experienced at least one ETEC infection before two years of age, 154 (62%) experienced a second infection, 73 (29%) had > 3 infections; 107 (34%) experienced at least one symptomatic ETEC infection. 1228 diarrhea episodes were detected(3.8 episodes/child-yr); 138 (11.2%) were ETEC-associated diarrhea (0.4 episodes/child-yr). In this cohort, 92% of mothers initiated BF: less than half still breastfed after 6 months. The age-adjusted incidence of first ETEC infections decreased with each month of BF (RR = 0.63; 95% CI, 0.58-0.70). There was a trend towards decreased risk of symptomatic first ETEC infection associated with the percent of daily feedings that were breastmilk (100% vs 0% RR=0.40, 95% CI, 0.20-1.20, NS). Conclusions: These data suggest that ETEC infection is common and an important cause of infant diarrhea in Mexico and that BF may offer cumulative protection against ETEC infection.