Asymptomatic Shigella infections in a cohort of Mexican children younger than two years of age.

ABSTRACT

PIP: During January 1990-December 1991, each week, field workers visited the home of 367 children aged 0-24 months from a periurban area southwest of Mexico City (San Pedro Martir and San Andres Totoltepec, Tlalpan) and collected stool specimens from them to determine whether Shigella infections are often asymptomatic. The crude incidence rate of diarrhea, regardless of etiology, was 29 episodes/100 child months during the warmer and rainy months (May-September), while it was 21 episodes/100 child months for the rest of the year (October-April) (relative risk [RR] =1.38). 53 of all children (l4%) had 65 Shigella infections. The overall monthly incidence of symptomatic and asymptomatic Shigella infection was higher during May-September than October-April (3.13 vs. 0.97 episodes/100 children; RR = 3.22). 55% of all Shigella infections (36) were asymptomatic. 32% developed secretory-type diarrhea and 13% had blood present in the stool. The incidence of Shigella infections grew as did the age (0.4-8.2 episodes/100 child months for 0-6 month olds to 18-24 month olds; p 0.0001). The proportion of asymptomatic Shigella infections also increased with age (33% for 0-6 month olds, 40% for 7-12 month olds, 46% for 13-18 month olds, and 78% for 18-24 month olds; p 0.01). Shigella sonnei, S. flexneri, and S. boydii were the only species detected. The 120-140 megadalton virulence plasmid was present in all isolates from asymptomatic and symptomatic children. Mixed infections were rather common in both asymptomatic (47%) and symptomatic (45%) children. Among infants aged less than 12 year months, breast feeding infants were less likely to be infected with Shigella than nonbreast feeding infants (RR = 2.41). On the other hand, among children aged 12-24 months, nonbreast feeding was associated with a lower risk of Shigella infection (RR = 0.69). These findings show that Shigella infections in Mexican children aged 0-24 months range from asymptomatic infections to secretory diarrhea to bloody diarrhea.