Zika Prevention: Can Pregnant Women Safely Use Mosquito Repellants?

The possible connection between Zika virus and microcephaly, a potentially fatal condition in infants, is a serious concern for pregnant women and women trying to become pregnant. Women are being told to take all possible measures to prevent mosquito bites, including using some pretty heavy-duty insect repellants.

But generally, pregnant women are bombarded by advice about avoiding chemicals. And some insect repellants – DEET, for example — come with a long list of warnings. Should women really slather on these repellants frequently during pregnancy?

Experts say yes.

If you are pregnant, and you are in an area with serious mosquito-borne diseases, use repellant with high amounts of DEET, and reapply it as often as necessary, said Dana Boyd Barr, professor of exposure science at Emory University.

Zika poses a potentially dire risk to unborn children, and studies, including Barr's own research, have shown that DEET is a fairly harmless chemical, she said. "Really, there's not any strength of evidence indicating that DEET is harmful to a fetus or harmful to a person," Barr said.

There is general consensus that we don't know much about the long-term effects of DEET, Barr acknowledged. And there are limited studies looking specifically at pregnant women. But when facing the possibility of microcephaly, women would risk far more by not using the repellant than by using it. [Zika Virus FAQs: Top Questions Answered]

For women in regions where the virus is spreading, experts are recommending using many precautions in addition to repellants to prevent mosquito bites, including using mosquito netting, staying indoors if possible, fixing torn screens and covering exposed skin with long sleeves and pants.

For most people, Zika virus isn't a serious infection. Only 1 in 5 people show any symptoms and those are usually mild, including skin rash, fever, headache and joint pain. Zika is spread by mosquitoes (specifically, the Aedes species).

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There are no current treatments or vaccines for this virus. Preventing bites remains key.

In one study on DEET, published in 2014 in the journal PLOS One, researchers looked at how DEET may affect mammals. Specifically, the researchers wanted to know whether DEET inhibited the activity of an enzyme called acetylcholinesterase, which is important in the functioning of the nervous system. They found DEET likely slowed down the uptake of one chemical within the nervous system of the insects, but was not a potent acetylcholinesterase inhibitor, and was not toxic to mammals.

In other work, researchers reviewed many studies of DEET safety, according to the findings published in the journal Parasites & Vectorsin 2014. They found only one study, from 2001, in which researchers looked at DEET use during pregnancy.

The women in this study were in Thailand, and already past their first trimester when they took part in a randomized trial of DEET, which was used to prevent malaria. Some of the women used DEET, while others in a control group did not. Their babies were monitored up to six months after birth and the researchers found that DEET made no difference in the babies' weight, height, head or arm size, or neurological performance.

That study and one other study included in the review did show that DEET can cross the placenta. This other study also showed a borderline association between higher levels of DEET in cord blood and larger abdominal circumference at birth, but the researchers noted no differences in the health of the infants.

There are plant-based mosquito repellants, such as oil of lemon eucalyptus (OLE) and citronella, but these tend to have limited effectiveness because they evaporate quickly. Repellent products containing OLE and its synthetic version, called PMD, are recommended by the CDC, but pure OLE is not because it has not undergone the same rigorous testing. [10 Deadly Diseases That Hopped Across Species]

The CDC also recommends repellants that contain chemicals called picaridin and IR3535. These repellents are newer than DEET and have not been as extensively studied. A 2013 study in the journal Travel Medicine and Infectious Diseasecompared studies that looked at the effectiveness of picaridin, IR3535, DEET and plant-based repellent products against the Aedes species of mosquito and found that products with 20 percent or more DEET provided the best protection.

There is no definitive evidence linking Zika virus to microcephaly. But in Brazil, officials are reporting a dramatic rise in cases of microcephaly in tandem with a Zika virus outbreak. According to Brazil's Ministry of Health, 4,180 cases of suspected microcephaly have been reported in the country since October 2015; in previous years, the country recorded around 150 cases yearly.

"There are some weeks recently where there are more cases of microcephaly in Brazil being reported than in the entire previous year," said Dr. Daniel Lucey, an adjunct professor with Georgetown University Medical Center who specializes in infectious diseases.

Much more research is needed to establish whether the virus actually causes microcephaly, but in the meantime, because the condition is so devastating and brings lifelong effects such as severe cognitive impairment for babies, researchers are advising women to consider putting off becoming pregnant, and avoiding mosquito bites if they do become pregnant.

The Centers for Disease Control and Prevention (CDC) has advised pregnant women to avoid travel to Brazil and about two-dozen other countries where the virus is spreading. Lucey said he thought these advisories are very appropriate and reasonable, given what we know and all the questions remaining about Zika.

The CDC is also currently recommending that pregnant women who have traveled to countries with Zika and have Zika-like symptoms within two weeks of their travel get tested for the virus.There are no treatments for Zika, but doctors may want to monitor fetal growth during the pregnancy if evidence of Zika is found. Testing is not done more widely because of the potential for false positives — the antibodies the immune system makes in response to Zika are similar to those made for other related viruses.

Difficulties with testing also mean that no one knows how many pregnant women have been infected. Therefore, even if researchers were sure that the virus can cause microcephaly in some cases, they still wouldn't know the risk of the condition — in other words, how often a Zika infection actually leads to microcephaly.

The only advice right now is to avoid areas where Zika is being transmitted or, if that's not possible, protect yourself as best you can against bites, including using repellants with DEET.

Women who are pregnant and believe they have Zika virus should talk with their doctor.

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