Reproduction

Worm Study Raises Concern About DEET’s Effect on Reproduction

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Findings suggest need to consider reproductive health alongside prevention of insect-borne diseases

white woman wearing a backpack, viewed from behind, spraying insect repellant onto her bare forearm in a field next to some woods


Researchers have uncovered evidence hinting that the most common bug spray ingredient, DEET, might cause reproductive problems by affecting the formation of egg cells during pregnancy.

The findings come from a study in C. elegans — worms that don’t look like they have much in common with humans yet serve as surprisingly usefulbellwethers of how toxins in the environment affect human reproduction.

The research, published Jan. 4 in iScience, raises difficult questions. Chief among them is how to balance the possible reproductive harms of DEET-containing products in people — including infertility, miscarriage, or birth defects — with the need to ensure that people remain protected from diseases transmitted through insect bites, such as malaria, Lyme disease, West Nile virus, and Zika virus disease.

Whether and how much DEET use causes reproductive problems in humans will need to be confirmed in future studies. Such studies have been scarce to date in humans because of the ethical concerns involved.

Harvard Medicine News spoke with study senior author Monica Colaiácovo, professor of genetics in the Blavatnik Institute at Harvard Medical School, about what her team found and what it means.

Harvard Medicine News: Why did you conduct this study?

Monica Colaiácovo: The biggest motivator was how high DEET scored in our initial screens of how various chemicals in our environment affect meiosis, using the model organism C. elegans. Meiosis is the type of cell division that creates eggs and sperm. DEET was one of our top hits in terms of chromosomes not separating properly, so eggs end up with abnormal numbers of chromosomes. In humans, this can cause miscarriage, stillbirth, infertility, and genetic conditions such as Down syndrome. We knew we had to look at this carefully.

Screen shot of a woman in an office (Monica Colaiácovo) mid-explanation. She has shoulder-length brown hair and is wearing a blazer

Learn about the worm-screening method the Colaiácovo lab developed to quickly identify chemicals that affect meiosis.

Research has shown that DEET products can have neurologic effects on people who use them, but no one had really looked at what DEET is doing in meiosis. We wanted to understand whether it would cause a problem. 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.

HMNews: What did you find?

Colaiácovo: Using our worm model, we showed for the first time that DEET can have effects on meiosis. Then we showed why.

We saw that DEET has a significant impact on gene expression — the pattern of genes that are active or inactive in a cell. We found that the change resulted in oxidative stress (an imbalance between free radicals and antioxidants) and abnormal structure of the materials that form chromosomes, which compromised the ability of chromosomes to separate properly as the cell divided. Egg cells and the worm embryos they gave rise to were less healthy.

Five side-by-side microscopy images with microtubules (stained green) pulling apart chromosomes (stained red) during egg cell division. The left two separate normally. The right three show disorganization
Left two images: Normal chromosome separation during meiosis in worms. Right three images: DEET exposure caused a variety of disruptions. Images: Nara Shin


HMNews: How applicable are the worm findings to humans?

Colaiácovo: That is always the big question. Many human genes have equivalents in C. elegans, and worms are a powerful model for looking at effects on reproduction. C. elegans have been instrumental in uncovering how environmental toxicants such as the plastics chemical BPA can harm reproductive health.

We observed the changes in meiosis when the levels of DEET inside the worms were the same as, and in some cases lower than, what you find in blood or urine samples from the regular human population. That said, the paradigm for exposure wasn’t the same as it is for most people. The worms were exposed to DEET for 24 hours at a time, which may not apply to anyone, or apply to only certain groups, such as agricultural workers. And there are some metabolic and physiological differences between worms and humans.

A very important thing from our studies is that we’re providing reproducible, well-controlled, substantial data. 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.

Four side-by-side microscopy images show individual cell nuclei in C. elegans worms. The left one is normal. The right three show gaps, aggregates, and other abnormalities
DEET exposure led to missing, aggregated, and abnormal eggs in the worms. Images: Nara Shin


HMNews: At this stage, how would you frame recommendations for people? How do you balance reproductive health with prevention of diseases from mosquitoes and ticks?

Colaiácovo: My family is from South America, where Zika and dengue, for example, are common, and I want to make sure people are not scared away from being careful. So-called tropical diseases transmitted by insects are moving into new regions of the world as the climate changes, putting more and more people at risk. The consequences of stopping the use of insect repellents can be very serious

So, we want prevention. We want repellents. And DEET is a very effective option we have right now. 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. Our work suggests this is very important for pregnant women because female meiosis begins in the developing fetus in the womb.

I would love to see research give rise to best practices for applying DEET products during pregnancy, when there’s often so much confusion and anxiety about what to do or not do.

I also hope our work helps drive the development of DEET alternatives 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.

Research Proves ‘Gender-Bending’ Chemicals Affect Reproduction

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New research proves that ‘gender-bending’ chemicals, which leach from human products into rivers and oceans, can have a significant impact on the ability of fish to breed. The study has important implications for understanding the impacts of these chemicals.

Endocrine-disrupting chemicals (EDCs) affect hormones. They are found in products ranging from contraceptive drugs to liquid detergents.

They have been seeping into rivers through the sewage system for decades and alter the biology of fish, changing male fish to make them more female.

gender-bending-chemical-11-18

Eurekalert reports:

“Until now, there has been no solid evidence to show the long-term impact of this effect on fish in the wild — but the new research focusing on wild roach in two UK rivers (Bourne and Arun) has provided new evidence

… It was found that intersex fish — those that had their sexuality compromised by EDCs and which contain both male (sperm) and female (eggs) sex cells — had their reproductive performance reduced by up to 76 percent.”

Dr. Mercola’s Comments:

Intersex fish contain both sperm and eggs, and this new study found that their reproductive abilities may be reduced by up to 76 percent. How did these fish end up “intersex”?

Through exposure to endocrine-disrupting chemicals (EDCs) that are polluting the environment — a finding that raises serious implications for humans as well because all vertebrates have similar sex hormone receptors.

Already these chemicals, nicknamed “gender-benders,” are causing the males in many species to become feminized. In some rivers in Britain, up to 50 percent of male fish are now growing eggs along with sperm in their testes. Hermaphrodite polar bears have also been found.

What are Some of the Most Common Gender-Bending Chemicals?

These chemicals are virtually everywhere in the modern world, lurking in personal care products, food containers, medical tubing, toys and more. Some of the top offenders you should be aware of, and watch out for, include:

  • Phthalates — Exposure to phthalates can lead to incomplete testicular descent in fetuses, reduced sperm counts, testicular atrophy or structural abnormality and inflammation in newborns.Phthalates are found in vinyl flooring, detergents, automotive plastics, soap, shampoo, deodorants, fragrances, hair spray, nail polish, plastic bags, food packaging, garden hoses, inflatable toys, blood-storage bags, and intravenous medical tubing.
  • Bisphenol A (BPA) — A common ingredient in many plastics, including those in reusable water bottles and resins lining some food cans and dental sealants, can change the course of fetal development in a way that increases your risk of breast cancer. Polycarbonate plastics contain BPA.BPA has been detected in the umbilical cord blood of 90 percent of newborn infants tested.
  • Perfluorooctanoic acid (PFOA) — Found in grease- and water-resistant coatings like Teflon and Gore-Tex, is a likely carcinogen.
  • Methoxychlor and Vinclozin— An insecticide and a fungicide respectively, have been found to cause changes to male mice born for as many as four subsequent generations after the initial exposure.
  • Nonylphenol ethoxylates (NPEs) — Known to be potent endocrine disrupters, these chemicals affect gene expression by turning on or off certain genes, and interfere with the way your glandular system works.They mimic the female hormone estrogen, and have been implicated as one reason behind some marine species switching from male to female.
  • Bovine growth hormones commonly added to commercial dairy have been implicated as a contributor to premature adolescence.
  • Non-fermented soy products, which are loaded with hormone-like substances.
  • MSG — A food additive that’s been linked to reduced fertility.
  • Fluoride — This chemical in the U.S. water supply has been linked to lower fertility rates, hormone disruption and low sperm counts.

What each of these substances has in common is that they can affect you and your children’s, endocrine system and reproductive health.

The glands of your endocrine system and the hormones they release influence nearly every cell, organ, and function of your body. They are instrumental in regulating your mood, growth and development, tissue function, metabolism, as well as sexual function and reproductive processes.

Numerous studies over the past decade confirm that males may be particularly at risk to these chemicals. Decreased sperm motility and concentration, as well as genital abnormalities in baby boys have been linked to these chemicals.

Endocrine-Disrupting Chemicals Pose Steep Risks to Baby Boys

The more a pregnant woman is exposed to high levels of one type of endocrine-disrupter known as phthalates, the greater the risk her son will have smaller genitals and incomplete testicular descent, leading to impaired reproductive development. The chemical also appears to make the overall genital tracts of boys slightly more feminine.

It is believed that phthalates have these adverse effects because they reduce testosterone synthesis by interfering with an enzyme needed to produce the male hormone.

In one study, women who had higher concentrations of two types of phthalates (DEHP and DBP) also had boys who appeared more feminized in their personality while playing.

The presence of phthalates in the mothers was not associated with any differences in the girls’ play behavior. However, that is NOT an indication that phthalates are “safe” for women.

They also affect women’s endocrine systems and reproductive health although the effects are different between the genders. For instance, girls are also reaching puberty earlier than ever before, and these chemicals may be culprits.

Bovine growth hormones used in food production, soy foods, and bisphenol-A (BPA), just to name a few, add to the problem as they also mimic estrogen and disrupt your endocrine system.

Other studies have linked the chemicasl to thyroid problems in both women and men, and researchers have also suggested a link between phthalates and illnesses like allergies, asthma, and contact dermatitis, all of which are on the rise in children.

How to Reduce Your Exposure to Gender-Bender Chemicals

Although legislators in Washington, Vermont and California have restricted phthalate use in children’s goods, and several major retailers, including Wal-Mart, Toys-R-Us, Lego, Evenflo and Gerber are phasing out phthalate-laden toys voluntarily, these chemicals are still very widely used.

You can help to cut back on your exposure with the following tips, and though they are important for everyone, pregnant women and women who may become pregnant should pay particular attention to reducing their exposure as much as possible:

  1. As much as possible, buy and eat organic produce and free-range, organic foods to reduce your exposure to pesticides and fertilizers.
  2. Rather than eating conventional or farm-raised fish, which are often heavily contaminated with EDCs, PCBs and mercury, supplement with a high-quality purified fish or krill oil, or eat fish that is wild-caught and lab tested for purity.
  3. Eat mostly raw, fresh foods, steering clear of processed, prepackaged foods of all kinds. This way you automatically avoid artificial food additives of all kinds, including dangerous artificial sweeteners, food coloring and MSG.
  4. Store your food and beverages in glass rather than plastic, and avoid using plastic wrap.
  5. Have your tap water tested and, if contaminants are found, install an appropriate water filter on all your faucets (even those in your shower or bath).
  6. Only use natural cleaning products in your home.
  7. Switch over to natural brands of toiletries such as shampoo, toothpaste, antiperspirants and cosmetics. The Environmental Working Group has a great safety guide to help you find personal care products that are free of phthalates and other potentially dangerous chemicals.I also offer one of the highest quality organic skin care lines, shampoo and conditioner, and body butter that are completely natural and safe.
  8. Avoid using artificial air fresheners, dryer sheets, fabric softeners or other synthetic fragrances.
  9. Replace your Teflon pots and pans with ceramic or glass cookware.
  10. When redoing your home, look for “green,” toxin-free alternatives in lieu of regular paint and vinyl floor coverings.
  11. Replace your vinyl shower curtain with one made of fabric or install glass shower doors.