Maternal Pesticide Exposure and Neural Tube Defects in Mexican Americans
Introduction
Various types of pesticides have been associated with neural tube defects (NTDs) in animal models 1, 2, 3, and findings from epidemiologic studies have suggested that these chemicals might also confer risk for NTDs in humans. Most epidemiologic studies have focused on occupational exposures to pesticides with positive associations noted between maternal employment in agriculture and NTDs in offspring 4, 5, 6, 7, 8.
Fewer studies have examined the relation between household use of pesticides and other environmental exposures and these defects. White et al. (9) noted elevated standardized risk ratios for spina bifida in areas in New Brunswick, Canada with higher agricultural chemical exposure activity. In a case-control study conducted in California (United States), women who reported professional pesticide applications in their homes or living within 0.25 mile of agricultural crops were at increased risk of NTD-affected pregnancies (10). Using the same case-control study population of infants with NTDs and nonmalformed controls, Rull et al. (11) explored the validity of maternal self-reported proximity to agricultural crops by comparing women's responses against historical land-use survey maps. With land-use maps serving as the “gold standard” for proximity, case-women were noted to more accurately recall living near such crops than control women (given that the maps indicated close proximity), thereby leading to a positively biased odds ratio (OR) for self-reported proximity in relation to NTDs in offspring. These investigators then examined the relation between neural tube defects and maternal residential proximity to agricultural pesticide applications by linking historical land-use survey maps and pesticide use reports to mothers' addresses during the periconceptional period (12). Women who lived within 1000 meters of crops that were treated with selected pesticides were more likely to have NTD-affected pregnancies than women who lived farther away from such crops.
In the United States, Mexican Americans have some of the highest rates of NTDs compared with other ethnic/racial groups 13, 14, 15; in Texas, the highest rates of these defects are found among births to Mexican American residents living in counties adjacent to the border with Mexico. In 1991, a cluster of anencephalic births was reported in one of these counties (Cameron County), and the results of a subsequent investigation showed the prevalence of NTDs to be much higher in this county than the United States (Texas Department of Health, unpublished report, 1992; Hendricks et al. [16]). In response to these findings, a surveillance project and case-control study were initiated to identify risk factors for NTDs in this population. Given the heavy agricultural activity in several regions of the Texas-Mexico border area, we explored the relation between environmental pesticide exposure and risk of NTDs among the Texas-Mexico border population. In the assessment of environmental exposures, potential occupational exposures were also taken into account, although the role of occupational exposures in this population has already been reported in a previous publication (17).
Section snippets
Selection of Cases and Controls
This study used data from the Texas Neural Tube Defect Project, which was implemented by the Texas Department of Health in 1993. The surveillance portion of this project involved active surveillance of NTD births from multiple sources, including genetic clinics and ultrasound centers (fetuses diagnosed prenatally), hospitals, birthing centers, abortion centers, prenatal clinics, and lay and certified midwives. A case was defined as a resident of one of the 14 Texas counties along the
Results
Table 1 compares selected demographic and behavioral characteristics between the case and control participants. Among interviewed women, information was missing regarding sources of pesticide exposure for four case- and three control-women. Although case- and control-women had similar age distributions, case-women were more likely to have less education, to ingest less than 400 μg of folic acid/folate per day, and to smoke or be exposed to secondhand smoking.
A total of 56 (30.4%) case-women and
Discussion
Results of this study suggest that maternal exposure to pesticides during the periconceptional period may confer risk for NTDs among offspring in Mexican American women, particularly exposures within the home or those associated with living in close proximity to cultivated fields. Maternal pesticide exposure was more strongly associated with risk for anencephaly than for spina bifida. For both types of defects, however, risk appeared to increase as the sources of exposure opportunities
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