What You Need to Know About Zika and Permethrin Resistance
In the battle against Zika, the Centers for Disease Control and Prevention is in a bind.
On the one hand, it recommends wearing clothing treated with permethrin as a way to prevent mosquitoes from biting and potentially transmitting the virus. On the other hand, the agency has said that mosquitoes are developing resistance to this chemical and its cousins, which are also used as community-wide insecticide sprays.
The problem has become serious in parts of Mexico, across Puerto Rico, and in Miami-Dade County—the site of the only Zika outbreak so far in the mainland U.S. So for aerial spraying, officials have switched from permethrin to naled, a chemical that mosquitoes have yet to exhibit any resistance to but that some scientists say poses greater safety risks. It also can't be applied to clothing.
So here’s the question: If the CDC is moving away from permethrin and its cousins in community-wide spraying, should consumers follow suit when it comes to permethrin-treated clothing? Here’s what you need to know:
What Is Permethrin?
Permethrin is a synthetic chemical that mimics a compound found in the chrysanthemum flower. It is one of several such chemicals—known collectively as pyrethroids—that are used for insect control. The chemicals are neurotoxins: They work by disrupting the nervous system.
Permethrin has been registered by the Environmental Protection Agency since 1979 and is used in more than 2,000 products, including the special soaps used to treat head lice, professionally deployed insecticides, and insect-repellent clothing and clothing sprays. Permethrin is currently the only chemical EPA-registered for use in factory-treated repellent clothing.
Permethrin can be extremely toxic to cats and to fish and other aquatic life because those creatures have a hard time breaking the chemical down in their bodies. The EPA has classified permethrin as a “likely human carcinogen” when ingested. But the agency says that humans can break down small quantities of the chemical fairly quickly and that the amount of permethrin allowed in clothing and other consumer products is too low to pose a risk.
How Widespread Is Permethrin Resistance?
Insecticide resistance can take many different forms. Insects can learn to change their behavior to avoid a given chemical (for example, in the past mosquitoes have learned to avoid walls inside houses when DDT was sprayed there). They can also develop physiological resistance through random genetic mutations—new enzymes that break the chemical down more quickly, for example, or thicker cuticles that make it harder for the chemical to penetrate.
The more an insect population is exposed to a given chemical, the more likely it is to develop resistance because each subsequent exposure kills off more of the susceptible bugs, which in turn makes it that much easier for the resistant ones to thrive.
When it comes to the Aedes mosquito that carries Zika and other diseases, resistance tends to be localized because these mosquitoes don't travel very far. So permethrin may work very well against the bugs in one neighborhood but be totally ineffective the next neighborhood over. And within a resistant population, some mosquitoes may be more resistant than others.
Some scientists say that overall, permethrin resistance is likely to be more prevalent in certain regions. “It’s a bigger issue in the southern states like Texas and Florida,” says Ron Harrison, an entomologist with pest control company Orkin. “The mosquitoes don’t completely die out at the end of the season, so the ones that develop resistance can persist and multiply in the next season. In places like New York, the cold weather kills them all off anyway, so it’s less of a concern.”
But it’s tough to say for certain how widespread the problem is. Not all mosquito-control districts test their mosquito populations for insecticide resistance; those tests are cumbersome and expensive, and most districts don’t have the resources or the expertise. And the ones that do test don't always make their results public. “In the contiguous U.S. we know very little about resistance,” says Lyle Petersen, Ph.D., director of the CDC’s division of Vector-Borne Infectious Diseases. “We think there’s a fair amount of it, but our knowledge is very, very patchy.”
So Should I Ditch My Permethrin Spray or Treated Clothing?
The CDC still recommends permethrin-treated clothing and sprays on its website, but when asked by Consumer Reports, the agency acknowledged that the effectiveness of the products may depend on where you live.
"In areas with high levels of resistance, use of permethrin and related products is not likely to be effective," an agency spokesperson said by email. "In those areas, residents should contact local authorities or a mosquito control district for more information on pesticides."
Here’s the problem, though: Because many mosquito control districts don’t test for resistance in the first place, they may not have much information to provide. The CDC says it's urging mosquito control districts across the country to improve their resistance testing, and it has been providing some grant money to spur action on this front.
In the meantime, Consumer Reports experts say that the safest approach, regardless of where you live, may be to choose a repellent with an active ingredient that mosquitoes haven't yet developed resistance to, such as the repellents that did well in our tests: Sawyer Picaridin (with 20 percent picaridin), Ben's 30 Percent Deet Tick & Insect Wilderness, or Repel Lemon Eucalyptus (with 30 percent oil of lemon ecualyptus). Apply any of those products to to both your exposed skin and your clothing.
Wearing permethrin-treated clothing or spraying clothes with permethrin may provide additional protection, but you should do so in conjunction with repellents on exposed skin.
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