Is there an association between shigellosis incidence and socioeconomic status in metropolitan Haifa?

Background: Shigellosis incidence rates in Israel have declined continuously over the past 50 years, but they remain 20 times greater than those in the United States. Socioeconomic factors may influence shigellosis morbidity, but this may be difficult to demonstrate in the absence of data for individual patients and when using composite rates for large geographic areas. Use of census tract data for small, relatively homogeneous geographic areas may lessen the effects of the "ecological fallacy." The present study analyzes the effect of socioeconomic status (SES) on shigellosis morbidity in the Haifa metropolitan region.

Methods: The study population consisted of the 7 cities in the Haifa subdistrict that constitute the greater metropolitan region. Cases of shigellosis reported during the years 2000 and 2001 were mapped, and age-standardized rates were calculated for the census tract areas. The incidence rates were then compared with the SES category of the census tract using the Kruskal-Wallis test.

Results: No association was found between incidence rates of shigellosis and SES category of the census tract areas in the Haifa metropolitan area for the years 2000 and 2001 (Kruskal-Wallis chi(2)=0.440; P=.803).

Conclusion: We found no association between shigellosis morbidity and socioeconomic status. This finding is probably real and not the result of reporting bias. Analysis of morbidity using small geographical units such as census tracts is more accurate than analysis using large geographical areas such as cities.