date, there is no evidence that the etiologies differ, although no studies have addressed this specific issue.
The committee identified five case-control studies that examined the association between ALS and exposure to agricultural chemicals. Four did not specifically address exposure to insecticides but used broader categories, such as “pesticides” or “agricultural chemicals,” in their characterization of exposure; and they did not report positive associations (Chancellor et al., 1993; Deapen and Henderson, 1986; Granieri et al., 1981; Savettieri et al., 1991).
The fifth study did specifically address exposures to insecticides. McGuire and colleagues (1997) identified all newly diagnosed ALS patients in a three-county region in western Washington State using a surveillance system. Two controls were matched to each case with one of two techniques: random-digit dialing for cases under 65 years old and Medicare eligibility lists for older cases. The participants were 174 patients with ALS and 348 controls. Exposure information was obtained with a detailed interview that gathered information on all jobs held for at least a year since the age of 15 years. Job information included detailed descriptions of tasks performed and hours worked per week. Subjects also reported on exposures to 28 specific chemical agents, use of protective equipment, and exposure to any accident, spill, or explosion. Information about home activities and hobbies was also gathered. A panel of four industrial hygienists, blinded to the patient’s disease status and self-reported assessment of exposure, rated workplace exposure. ALS was found to be moderately associated with the hygienist panel’s assessment of insecticide exposure (OR=2.1, 95% CI=1.1–4.1) but not with the patients’ self-reports (OR=1.0, 95% CI=0.5–1.8). A dose-response gradient with insecticide exposure was found only for men. In a conditional logistic-regression analysis adjusting for age and education and using the hygienists’ assessments of exposure, the OR for low exposure was 2.0 (95% CI=0.5–7.7), and it rose to 2.8 with high exposure (95% CI=1.1–6.8). The study, however, had several limitations. Cases were less educated than controls, possibly creating a disparity that may have led to a selection bias in the direction of overestimating an effect. There was a higher refusal rate in controls than in cases, which might have resulted in a selection bias of unknown direction. The authors themselves comment that their findings are exploratory.
None of the reports about Gulf War veterans published in peer-reviewed journals specifically addressed ALS. However, a large government-funded epidemiologic study provides preliminary evidence that veterans who served in the Gulf War are nearly twice as likely as their nondeployed counterparts to develop ALS (Feussner, 2002). The committee was unable to obtain preliminary copies of the report for review and therefore could not evaluate its findings in reaching conclusions about insecticide exposure and ALS.
Only one peer-reviewed study (McGuire et al., 1997) expressly examined the relationship between insecticides and ALS. That study provided evidence of a relationship, but the possibility of selection bias resulting in an overestimation of effect cannot be excluded. There were no other studies with which to compare results.
The committee concludes, from its assessment of the epidemiologic literature, that there is inadequate/insufficient evidence to determine whether an association exists between exposure to the insecticides under review and Amyotrophic Lateral Sclerosis.