Supplementary Material for: Farming History, Insecticide Exposure and the Risk of ALS Mortality

Introduction: ALS is a neurodegenerative disorder with an unknown etiology for 90%-95% of cases. Several environmental and occupational exposures have been investigated although a prospective study examining the association of “ever living/working on a farm” (farming exposure) and insecticide exposure with risk of ALS mortality in women is lacking.

Methods: Within the Women's Health Initiative (WHI), a nested case-control study was conducted among 93,676 women (n=151 ALS death cases, n=1496 matched controls) from the WHI Observational Study. This included a questionnaire about farming and insecticide exposure history one year after baseline. Conditional logistic regression models adjusted for education, smoking, and physical activity estimated risk of ALS death.

Results: One-third of cases and 21% of controls reported farming exposure. Cases had higher odds of farming exposure than controls (adjusted odds ratio [aOR]=1.59, 95% CI: 1.09-2.31) and was highest at durations of 15-19 (aOR=1.81, 95% CI: 1.02-3.21) and ≥20 years (aOR=2.32, 95% CI: 1.10-4.87) compared to no exposure. Tests for interaction revealed that women with farming and smoking exposure had higher odds of ALS death (aOR=2.10, 95% CI: 1.26-3.51, p=0.0045). Little evidence was noted for increased insecticide exposure and ALS mortality risk, although power was limited.

Conclusion: Among post-menopausal women with 25+ years follow-up, a significant association was noted between farming exposure and risk of ALS death. This increased risk was highest among those who ever smoked. Future studies should include biomarkers of exposure and large cohorts of men and women with occupational and residential histories.