Worm study finds DEET insect repellent may cause reproductive problems

US researchers have published new evidence the common bug spray ingredient DEET may cause reproductive problems.

While the study was undertaken using Caenorhabditis elegans, a species of nematode worm used extensively as model organisms in scientific research, it shows for the first time that DEET can interfere with meiosis – a type of cell division that produces egg and sperm cells.

DEET (diethyltoluamide) has been widely used in insect repellents since the 1950s. Whether it causes reproductive harms in humans, however, must still be confirmed in further studies.

Senior author of the research published in the journal iScience, Monica Colaiácovo, professor of genetics in the Blavatnik Institute at Harvard Medical School in the US, says that DEET has a significant impact on gene expression in C. elegans.

“[Gene expression is] the pattern of genes that are active or inactive in a cell,” she explains.

“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.”

Why does impacts on meiosis matter?

In human meiosis, chromosomes in the parent cell separate during cell division so that each of the daughter cells – egg or sperm cells – have 1 set of 23 chromosomes each.

Later, when the egg and sperm cell combine during reproduction, the resulting embryo has the full set of 23 of maternal and 23 paternal chromosomes.

Microscope images showing green fluorescent chromosomes undergoing meiosis
Left two images: Normal chromosome separation during meiosis in worms. Right three images: DEET exposure caused a variety of disruptions. Credit: Nara Shin

This study found that exposure to DEET led to worm egg cells with abnormal numbers of chromosomes. The resulting embryos were also less healthy, with increased lethality and defects detected during early embryogenesis.

“In humans, [abnormal numbers of chromosomes] can cause miscarriage, stillbirth, infertility, and genetic conditions such as Down syndrome,” says Colaiácovo.

But how applicable are these findings to humans using DEET-containing products?

Co-first author of the study Dr Laura Lascarez-Lagunas, a Research Associate in Colaiácovo’s lab, tells Cosmos that C. elegans is a well-established model for meiosis studies that shares a high degree of gene conservation with humans and is highly predictive of human reproductive toxicity.

“However, the translation of our findings into humans requires caution and further investigation,” warns Lascarez-Lagunas.

“The first aspect to take into consideration is the duration of the exposure. In our study, worms were exposed to DEET for a 24-hour period. Although in some areas where arthropod-transmitted diseases are endemic the application of DEET is continuous, humans are usually exposed to DEET for shorter windows of time.”

To advance our understanding of the potential impact of DEET in human reproductive systems, Lascarez-Lagunas says future studies need to assess the effects of DEET on meiosis using a range of shorter windows of exposure.

“This should be accompanied by measurements of the internal levels at the end of these exposures for comparisons with the levels found in human biological samples such as urine and blood,” says Lascarez-Lagunas.

“Another aspect that requires further studies is the comparison of DEET metabolism between these worms and humans.”

Black and white microscope image of eggs in worms. The image on the left shows normal eggs and the three images on the right show abnormal eggs after deet exposure
DEET exposure led to missing, aggregated, and abnormal eggs in the worms. Credit: Nara Shin

Colaiácovo adds that some animal studies have shown reproductive toxicity from DEET exposure in males.

“Although these studies suggest that DEET impacts spermatogenesis, how DEET affects male meiosis remains unclear. Studies focused on understanding the mechanisms underlying these effects are therefore needed,” she says.

Where female meiosis begins in the developing foetus in the womb, meiosis in human males occurs during spermatogenesis (the production of sperm), which begins after puberty and continues throughout their lifetime.

“We are developing and adapting techniques to better understand the impact of different chemicals in male meiosis. DEET is one of the chemicals we plan to include in our analysis in the near future,” she concludes.

A balancing act

The research highlights the need to balance possible reproductive harms with continuing to protect people from insect-borne diseases, such as malaria and Zika virus, which carry their own risk reproductive risks to infected women.

“I want to make sure people are not scared away from being careful,” stresses Colaiácovo.

“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.

“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 foetus in the womb.”

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