What Are the Risks for Babies of Pesticides Used to Control Zika-Carrying Mosquitoes?

Intern Peter Kennan contributed research to this post.

The Zika virus and its anticipated spread across North America is alarming for two reasons: first, that it can cause birth defects in babies whose pregnant mothers are bitten by an infected mosquito, and second, that a likely response here in the U.S. is increased “fogging”—spraying pesticides throughout neighborhoods to kill adult mosquitoes in an attempt to control the spread of the disease—though that approach also may have risks for babies.

Brazil has reported more than 4,000 cases of children born with small heads and researchers suspect (but haven’t yet confirmed) that the birth defects are linked to mothers’ exposure to Zika-carrying mosquitoes. It makes sense that governments in affected countries are trying to eliminate the Zika-carrying Aedes aegypti mosquitoes. But this approach has a downside: pesticide exposure in utero is bad for babies.

Spraying for mosquitoes isn’t very well targeted. Pesticide may be sprayed from a truck moving slowly down the street or by individual workers carrying smaller units so that they can get closer to where mosquitoes gather. Spraying, also known as fogging, uses organophosphates, such as malathion, and pyrethroids, such as deltamethrin, to kill adult mosquitoes. The practice is used both abroad and in the U.S., where local governments target mosquitoes that may carry West Nile virus.

A fogging operation against mosquitos in Rio de Janeiro, Brazil, in February 2012. CC BY-NC-ND 2.0 Sanofi Pasteur / Marizilda Cruppe

Fogging is relatively ineffective at controlling mosquitoes—especially the Aedes aegypti mosquitoes that may carry Zika. To start with, it affects only adult mosquitoes, not larvae. Additional steps, such as getting rid of standing water or treating mosquito breeding areas with a different pesticide, are required to target other stages of the mosquito lifecycle.

Fogging’s ability to kill a large percentage of adult mosquitoes requires specific conditions. Mosquitoes have to come into contact with airborne droplets of pesticide, and factors that cause the pesticide to fall to the ground or leave the area too soon limit the impact of spraying. Under ideal circumstances, fogging kills up to 90 percent of adult mosquitoes in a treated area, but conditions are less than ideal if there is thick vegetation near homes, high humidity, too much or too little wind, or neighborhood blocks are too big. 

Third, the mosquitoes that carry the Zika virus can live indoors, further curtailing the usefulness of outdoor fogging. Jamaican officials, who have increased fogging in response to Zika fears, admit that only half of mosquitoes in a treated area will be killed because so many live indoors. Though Brazilian authorities spray for mosquitoes inside houses, no anti-mosquito pesticides have been approved for indoor use in the U.S.

A fogging operation against mosquitos in a house in Niterói, Rio de Janeiro, Brazil - March 2012. CC BY-NC-ND 2.0 Sanofi Pasteur / Marizilda Cruppe

Fogging’s limited benefits would be acceptable if it had no ill effects. However, the pesticides used in fogging have the potential to cause harm.

The chemicals used in fogging can kill butterflies and bees by interfering with their nervous system. Populations of pollinators are already on the decline, and increased spraying for mosquitoes could further stress those populations.

Given that spraying for mosquitoes happens more in urban areas than in major agricultural areas home to pollinators, the health impacts of pesticides on human health are a bigger concern. The problem is that we don’t know much about the impact of these chemicals on our health. For example, the CDC says that “Human health effects from malathion at low environmental doses or at biomonitored levels from low environmental exposures are unknown.” That lack of information isn’t unusual. The CDC makes the same statement about synthetic pyrethroids.

There’s mounting evidence suggesting that a mother’s exposure to common pesticides can have health consequences for her child’s nervous system. A recent study by researchers at the University of California, Davis found an increased risk of autism-spectrum disorders and developmental delays in children whose mothers lived closer to areas treated with agricultural pesticides, including organophosphates and pyrethroids, before or during pregnancy. Another study finds that children exposed to synthetic pyrethroids during gestation had slower neural and mental development at age one. The researchers conclude “direct or indirect exposure to synthetic pyrethroid pesticides should be avoided during pregnancy.”

Is the amount of pyrethroid sprayed during fogging enough to trigger problems for babies exposed in utero? I don’t know. Certainly the effects aren’t as immediately obvious as microcephaly, and that makes research and debate much more difficult.

The Zika virus could have life-changing consequences for a baby whose mother is exposed, but so too might pesticide exposure during the wrong developmental window in pregnancy. Given that fogging has limited effectiveness at controlling mosquitoes, increased, widespread use of pesticides may not be worth the risk. As Zika-bearing mosquitoes become more common in the U.S. and we begin to consider how to respond, we should consider the benefits and risks of fogging before embracing that strategy.