Disparities of Shigellosis Rates Among California Children by Census Tract Poverty Level and by Race/Ethnicity, 2000-2010

BACKGROUND: Linking surveillance data with census-level variables can help clarify the role of neighborhood socioeconomic status on health.  The percent of the population living below the poverty line in a census tract (CT) strongly correlates with many adverse health outcomes, but little research has focused on acute infectious diseases.  Shigellosis is a common enteric bacterial disease, restricted to humans, chiefly children.  It is transmitted person-to-person through fecal-oral contact; among children this is through environmental pathways such as contaminated food and water used for drinking or recreation.  Our aim was to determine whether socioeconomic disparities exist in shigellosis rates among children in California.

METHODS: Addresses of 9,740 children aged 14 and younger from 2000-2010 California shigellosis surveillance data were obtained.  Ninety-four percent were successfully geocoded to the CT level by the California Environmental Health Tracking Geocoding Service with verified high accuracy (94%).  Cases were aggregated by CT and stratified into three age categories (0-4, 5-9, 10-14) and five race/ethnicity categories.  The 2000 Census was used for population denominators, and to calculate the percent of the population below the poverty line, categorized into six a priori categories (0-<5%, 5-<10%, 10-<20%, 20-<30%, 30-<40%, > 40%).  Age-standardized incidence rates and incidence rate ratios (IRRs) were used to compare children in the most impoverished CTs with those in the least impoverished CTs. The contribution of socioeconomic inequalities to age-adjusted racial and ethnic disparities was explored using Poisson regression.

RESULTS: Mean age was 5.2 years; 51% of the children were girls and 82% were Hispanic. CTs had from 0 to 14 cases of shigellosis during the study period. The age-standardized incidence rate of shigellosis was 3.8/100,000 person-years in CTs with less than 5% of the population below the poverty line and 21.3/100,000 person-years in CTs with more than 40% of the population below the poverty line, an IRR of 5.6 (95% Confidence Interval (CI): 5.0 – 6.2). After adjusting for race, poverty still showed a strong relationship with shigellosis rates; the IRR comparing children in CTs with the greatest versus the least poverty was 3.4 (CI: 3.0 – 3.8). After adjusting for age and poverty, the IRR comparing Hispanics to whites was 4.8 (CI:  4.4 – 5.3).

CONCLUSIONS: Rates of shigellosis among California children increased with CT poverty.  Differences by CT poverty level were smaller, but still apparent, after adjustment for race. Hispanics had the highest rates of shigellosis, even after adjusting for CT poverty level and age.