ASYMPTOMATIC SHIGELLA INFECTIONS IN A COHORT OF MEXICAN CHILDREN YOUNGER THAN 2 YEARS OF AGE

The proportion of Shigella infections that occur asymptomatically in y oung children has not been established. A community-based cohort study of 367 infants was followed prospectively by weekly home visits from January, 1990, through December, 1991. Stool samples were collected we ekly and when diarrhea occurred and were tested for Shigella and other enteropathogens. There were 2925 child months of observation and 65 e pisodes of Shigella infection. There were 3.1 episodes/100 child month s during the warm season (May through September) and 0.97 episode/100 child months during the cold season. Shigella infections were rare dur ing the first 6 months of life but increased with age (P < 0.0001). Ov erall 55% of detected infections were asymptomatic. The proportion of infections that were asymptomatic increased as age increased (P < 0.01 ). Symptom status was not significantly associated with Shigella speci es or season. All isolates from symptomatic and asymptomatic children had the 120- to 140-megadalton virulence plasmid. We conclude that inf ections with virulent strains of Shigella are commonly asymptomatic in Mexican children during the first 2 years of life.