STANFORD, Calif. — A study of several hundred chemicals used in commercial pesticides has found only weak evidence that any of them are associated with a common birth defect in male infants.
The study, led by epidemiologists at the Stanford University School of Medicine, analyzed thousands of birth records and commercial pesticide application records for eight counties in California's heavily agricultural Central Valley. The researchers aimed to determine whether children were at increased risk of hypospadias if their mothers had lived in relatively close proximity to where pesticides were used while pregnant. Hypospadias is a genital malformation in which the urethral opening is on the underside of the penis rather than on the tip.
In the most detailed study of the largest data sets done to date, 292 individual chemicals and 57 groups of structurally similar chemicals were analyzed. Of those, the study identified 15 that had possible associations with hypospadias. But the researchers say further studies need to be done.
"We didn't see many chemicals that suggested an increased risk, and of those that did, most of them were infrequently used," said Suzan Carmichael, PhD, associate professor of pediatrics and lead author of the study to be published Oct. 28 in Pediatrics. "It is good news that such exposures are rare, but at the same time, when exposures are rare, it makes studies harder to do."
Approximately five of every 1,000 male infants are born with hypospadias, but the cause is usually unknown.
Most previous studies of pesticides and hypospadias focused on risks associated with occupations that involve the use of pesticides. Some studies have suggested slightly increased risks for infants whose mothers or fathers work around pesticides, but many studies suggest no association.
The researchers worked with data on births in the counties of Fresno, Kern, Kings, Madera, Merced, San Joaquin, Stanislaus and Tulare. The Central Valley, composed of the San Joaquin and Sacramento valleys, has one of the highest rates of pesticide usage in the nation.
The study population included all male infants born from 1991 to 2004 to mothers residing in any of the eight counties at the time of birth. The study sample comprised 690 cases of hypospadias, as well as 2,195 controls randomly selected for comparison.
The researchers considered pesticides used within 500 meters of the mother's residence during weeks one to 14 of each pregnancy. Urethral development typically occurs between weeks four and 14.
Hypospadias has a significant impact on public health, as it often requires surgical correction. Approximately 600,000 to 900,000 American males alive today were born with some degree of hypospadias. Even after correction, individuals may have impaired sexual function and emotional and social difficulties stemming from the condition.
"Any birth defect is concerning to parents, and a defect in the genital structure often causes special concern," said William Kennedy, MD, associate professor of urology at Stanford and associate chief of pediatric urology at Lucile Packard Children's Hospital.
"Parents are often reluctant to talk to anyone — even medical professionals — about the baby's condition," Kennedy added. "Fortunately, most corrective surgeries have positive outcomes."
In addition to exposures to individual chemicals and compounds, the researchers looked at exposure to multiple chemicals, but found no evidence to suggest that mothers' exposures to multiple pesticides put their babies at an increased risk of hypospadias.
"These results extend what we know, but at the end of the day they need to be replicated before we can really be sure whether there is, or is not, a real risk associated with these chemicals," said Gary Shaw, DrPH, professor of pediatrics at Stanford and a co-author of the study. Shaw, Kennedy and Carmichael are also members of the Child Health Research Institute at Stanford.
Another Stanford co-author was Wei Yang, MS, research associate in pediatrics.
This work was done in partnership with colleagues at the California Environmental Health Tracking Program. The study was partially supported by grants from the National Institutes of Health (RO1 ES017060) and Centers for Disease Control Prevention.
The Stanford University School of Medicine consistently ranks among the nation's top medical schools, integrating research, medical education, patient care and community service. For more news about the school, please visit http://mednews.stanford.edu. The medical school is part of Stanford Medicine, which includes Stanford Hospital & Clinics and Lucile Packard Children's Hospital. For information about all three, please visit http://stanfordmedicine.org/about/news.html.
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