Outbreaks of Cutaneous Anthrax among Humans in Bangladesh: Understanding the Risk Factors from a Case-Control Analysis

Background: Though exposure to infected ruminants has been identified as a major risk for cutaneous anthrax, individual susceptibilities have not been fully investigated. The objective of this study was to determine risk factors that could guide measures to prevent anthrax in humans. Methods During 2012-2013, we investigated 7 anthrax outbreaks in 4 districts. A case was any person with an acute skin lesion developing from a papular through vesicular to central black eschar over 2-6 days and exposed in the previous 2 weeks to raw meat, carcass, body fluid or by-product of sick or dead ruminants with clinical and/or laboratory evidence of anthrax. All persons from outbreak communities who did not develop clinical illness within 2 weeks of exposure were listed as controls. For a case-control study, we recruited one control from each of the four households nearest to an enrolled case. Odds ratios (OR), 95% confidence intervals (CI) and p-values were estimated by logistic regression. Multivariate logistic regression was done using variables significant at P<0.05.

Results: Overall, 207 (15%) cases were identified among 1,359 exposed individuals. Compared to controls on univariate analysis, cases were more likely to have raw meat exposure while slaughtering or butchering (OR=36.0), have observed animal slaughter (OR=5.0), had a cut injury on exposed body parts (OR=26.0), have skipped ≥2 meals per week (OR=5.0), be aged ≥18 years (OR=3.0), and to smoke tobacco (OR=7.0); they were less likely to wash hands after handling raw meat or observing a slaughter (OR=0.5). On multivariate analysis, raw meat exposure (adjusted OR [AOR]=36.7, CI=11.3-118.7), observing animal slaughter (AOR= 4.0, CI=1.3-12.0), skipping meals (AOR=4.0, CI:1.2-10.9), smoking tobacco (AOR=2.0, CI:1.2-3.2) and washing hands (AOR=0.5, CI:0.3-0.8) were independently associated with anthrax infection. Conclusions Exposure to raw meat while slaughtering or butchering was identified as the strongest risk factor, underscoring the need to ensure livestock vaccination. However, this is difficult to accomplish in resource-poor countries like Bangladesh. Meanwhile, implementation of a hand washing intervention could ameliorate risk. Smoking and skipping meals may increase susceptibility by lowering host immunity; these novel risk factors warrant future studies.