Risk factors for sporadic shigellosis, FoodNet 2005

An estimated 450,000 cases of shigellosis occur annually in the United States. Outbreaks have been associated with food, water, child daycare centers, and men who have sex with men. However, for sporadic infections, which account for the majority of cases, risk exposures are poorly characterized.

Foodborne Diseases Active Surveillance Network (FoodNet) conducts active, laboratory-based shigellosis surveillance in 10 US sites. We interviewed cases with illness onset during 2005 about exposures during the week before symptom onset using a standardized questionnaire. The proportion of patients who denied nonfood risks was used to estimate the burden attributable to foodborne transmission.

Overall, 1494 cases were identified. The approximate incidence was 3.9/100,000, with the highest rates among children aged 1-4 years (16.4) and Hispanics (8.4). Of the 929 cases interviewed, 223 (24%) reported international travel in the week before symptom onset. Of the 626 nontraveling cases with complete risk factor information, 298 (48%) reported exposure to daycare or a household member with diarrhea; 99 (16%) reported drinking untreated water or recreational exposure to water; and 16 (3%) reported sexual contact with a person with diarrhea. Two hundred and fifty-nine (41%) denied all nonfood exposures examined.

Sporadic shigellosis is most common among young children and Hispanics. Common exposures include international travel and contact with ill persons or daycare. However, more than one-third of US shigellosis cases annually might be due to food consumed in the United States.