(CN) — Researchers from Harvard Medical School say in a study released Thursday that one of the most common pesticide chemicals on the market may cause reproductive issues through malformed eggs during pregnancy.
The study published in iScience explores the reproductive effects of the chemical N,N-diethyl-meta-toluamide or deet, a primary active ingredient for various topical insect repellents and one that’s effective for preventing vector-borne diseases, such as Lyme disease, malaria, West Nile virus or Zika virus disease.
Using the model organism C. elegans — a worm species that shares 60% of its genes with humans — the researchers explored whether deet affected the process of meiosis or cell division after exposure.
The results, they explained, showed that deet caused cell death, altered meiotic recombination, chromosomal abnormalities and the disruption of segregation — when a set of homologous chromosomes split apart. In other words, the worms’ exposure to deet caused their chromosomes to split improperly, leaving their eggs with an abnormal number of chromosomes.
“In humans, this can cause miscarriage, stillbirth, infertility and genetic conditions such as Down syndrome,” said senior study author and Harvard professor of genetics Monica Colaiácovo in an interview with Harvard Medicine News. “We knew we had to look at this carefully.”
For two decades, Colaiácovo’s lab at Harvard’s Blavatnik Institute has worked with the same worm species to investigate the links between environmental chemical exposure and abnormal chromosome numbers. In 2013, her lab developed a screening method to flag dangerous chemicals for pregnant women. Six years later, Colaiácovo applied this process for another study to test 46 chemicals, including pesticides, fracking byproducts and phthalates — a common substance found in plastics and some cosmetics.
The 2019 study found that 41% of tested chemicals could damage egg cell DNA to an equal or greater extent than bisphenol A or BPA, another type of plasticizer associated with health risks for both humans and animals. Among those damaging chemicals was deet, inspiring Colaiácovo and her fellow researchers to look closer at how it may affect human reproduction.
“We wanted to understand whether it would cause a problem,” Colaiácovo said. “Only a few human studies have been done, and practically everyone uses deet, so the possibility that it could affect reproduction felt palpable for people in our lab.”
According to the study, 30% of Americans are estimated to use deet-containing products and approximately 1,800 tons of deet is used annually nationwide. But even those who avoid topical deet can still absorb the chemical through contaminated water, the authors write, adding how trace amounts of deet have been detected in drinking water worldwide.
By analyzing RNA sequences, the researchers linked deet-induced alterations to gene expressions involving redox processes, chromatin structure, reduced mitochondrial function, impaired DNA function and defects during early embryogenesis.
“Our analysis suggests a link between deet-induced alterations in the expression of genes regulating redox processes and chromatin structure to the observed reduction in mitochondrial function and increased frequency of germline defects,” the researchers wrote in the study.
In comparing internal deet levels found in the worms to those detected in several human samples, the researchers believe their findings may be relevant for mammalian reproductive health. Previously, researchers detected over 19 times more deet in children’s urine after eight hours of a single dermal application of an insect repellent with a concentration of 12%. In another study, 150 cesarean patients in New Jersey had slightly less deet than the worms in their maternal and chord serums from the occasional pesticide use while pregnant.
“A very important thing from our studies is that we’re providing reproducible, well-controlled, substantial data,” Colaiácovo said. “Other groups can take up this work in mice or other animal models and further advance our understanding of what deet may be doing in human reproductive systems. We can also build on this evidence to flag chemicals of concern for policymakers.”
But while the effects of deet on worm cells may cause concern, the researchers caution against extrapolating results from worms to humans, especially since the worm’s exposure levels are not the same as the general public and more aligned with exposure in the agricultural industry.
And because disease-transmitting insects are moving to warming climates worldwide, Colaiácovo said the findings should not prevent anyone from using pesticides.
“The consequences of stopping the use of insect repellents can be very serious,” Colaiácovo said, adding that prevention and repellents are necessary, and deet is an effective available option.
“At this moment, I would say we should be aware of the potential reproductive risks of deet-containing products and be sure to follow the application instructions when using them,” Colaiácovo said. “Our work suggests this is very important for pregnant women because female meiosis begins in the developing fetus in the womb.”
Moving forward, Colaiácovo said she hopes to see future research develop best practices for applying deet products during pregnancy. She also hopes that her study will help drive alternatives to deet products that are safe and effective.
“It would be great to have an effective insect repellent that doesn’t make us worry about our health or that of our children,” Colaiácovo said.
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