On Tuesday, April 21, Shanna H. Swan, PhD, a leading research professor from the University of Missouri School of Medicine, presented findings from her study on the impact exposure to environmental chemicals has on reproductive health. The seminar was sponsored by the Maryland Mothers and Babies Study (MMBS) at the Johns Hopkins Bloomberg School of Public Health. MMBS investigates the special vulnerability of children to the broad array of environmental exposures and promotes healthy environments for children and families. Dr. Swan discussed how low-level environmental exposures to some chemicals, such as some pesticides, are posing challenges to classic epidemiology. In the past, well defined patterns of human disease initiated epidemiological studies. Scientists are now finding that low-level ubiquitous exposures to chemicals and other contaminants do not always manifest as disease clusters, making recognition of the health impact more difficult. Furthermore, identifying individuals to study is challenging because potential study subjects are frequently unaware of their contact with these health risks. Exposures are often detectable only through biomonitoring. Such is the case for Dr. Swan, who is researching the underlying cause of low sperm counts occurring globally. “Pesticides used in agriculture may be contributing to poorer semen quality,” concluded Dr. Swan. “This study was not triggered by disease clusters but rather from a 1992 study showing decreases in sperm motility in the U.S. and Europe, but not in non-western countries,” explained Swan. After comparing sperm concentration, volume, motility and shape, Dr. Swan found significant geographical differences among four U.S. cities. “We were surprised and concerned to discover men in Missouri had only 56 percent as many moving sperm as men in urban Minneapolis,” she remarked. To explain the difference, researchers first looked at variables such as selection bias, confounding and differences in analysis methods. Once those were ruled out, researchers began considering what environmental factors might explain these variations in semen quality. The most obvious difference between the two locations was agricultural versus urban environments. This led Swan to hypothesis that one or more agricultural pesticides contributed to the difference in semen quality. To examine this hypothesis, researchers compared pesticide metabolites in the men’s urine, collected on the same day as the semen. Researchers detected several pesticides, primarily herbicides, more often in cases than controls. Men with higher concentration of the pesticide alachlor were 30 times more likely to be a case. Dr. Swan is currently replicating the study in Iowa City, Iowa, and based on preliminary results, semen quality in Iowa City appears to be similar to that in mid-Missouri. Dr. Swan hopes to strengthen these studies by increasing sample size, and examining effects of pesticide mixtures, their impact on multiple endpoints and on population -level outcomes such as sex ratio in children born to pesticide exposed men.—Brian Fitzek Public Affairs Media Contacts for the Johns Hopkins Bloomberg School of Public Health: Tim Parsons or Kenna Brigham at 410-955-6878 or paffairs@jhsph.edu.
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