Skip to Main Content

SAN JUAN, Puerto Rico — The advice is right there, on the website of the Centers for Disease Control and Prevention. To avoid contracting Zika, treat your clothing with an insecticide known as permethrin or buy apparel that is already treated with it.

If you live in or plan to visit Puerto Rico, there’s just one problem with that recommendation. The Aedes mosquitoes that transmit the virus on this Caribbean island have developed resistance to permethrin. The chemical isn’t working here.

And it’s not just Puerto Rico. Other places, including parts of Mexico, are reporting that Aedes mosquitoes are no longer quelled by permethrin, said Roberto Barrera, chief of entomology and ecology activity at the CDC’s dengue branch.


“It’s probably not going to work well in the places where it has been used as the main insecticide for many years,” Barrera told STAT in a recent interview. “You’re not going to be able to kill off most of the mosquitoes.”

The growing resistance to permethrin is a blow for public health authorities hoping to limit the damage Zika may wreak in Puerto Rico, where the virus is expected to spread widely and rapidly.


The first locally acquired Zika case was spotted here in late November. As of late last week, there were 157 confirmed cases — and doubtless many more in people who had no symptoms or were so mildly ill they didn’t seek medical care.

Permethrin is toxic to mosquitoes but safe for use to protect people. While many people feel ambivalent or worse about insect repellants, permethrin has nevertheless gained a high degree of consumer acceptance.

Zika CDC Dengue Branch
Adult mosquitoes feed on animal blood kept in a pouch at the entomology laboratory at the CDC dengue branch. Erika P. Rodriguez for STAT

CDC mosquito experts are in the process of trying to find effective alternatives for use in Puerto Rico.

In a lab in the collection of low-slung buildings that makes up CDC’s San Juan campus, researchers have grown crops of mosquitoes from eggs collected on the island.

Strips of brown, sticky germination paper were placed in locales — flowerpots containing a bit of standing water, for instance — where female Aedes aegypti mosquitoes might lay eggs. When collected later, the paper strips were dotted with what resembled a heavy dusting of cracked peppercorns.

Soaked in trays of water, the tiny black eggs produced tadpole-like larvae that are allowed to grow into adult mosquitoes. These are kept in mesh cages and fed pig’s blood.

Zika CDC Dengue Branch
Vizcaino points to areas on the island where researchers have collected mosquitoes for testing. Erika P. Rodriguez for STAT

The adults are being exposed to a variety of different insecticides to see which ones are still effective against Aedes aegpti.

“If it doesn’t kill them, then we’re looking for another insecticide, another option,” said Lucrecia Vizcaino, a biologist with CDC’s entomology branch.

Barrera said the team should soon be able to offer advice on what insecticides will be effective in Puerto Rico.

In the meantime, health officials are trying to take other measures to protect pregnant women, who face a particularly severe risk, given mounting evidence that Zika infection during pregnancy can lead to birth defects in infants.

Officials have offered to visit their homes and look for mosquito breeding sites, if they would like. There are also plans for targeted spraying of insecticides around these homes.

But Barrera said spraying has to be augmented by a broader mosquito reduction program.

Papers filled with mosquito eggs are dried at the insectary at the entomology laboratory at the CDC dengue branch. Erika P. Rodriguez for STAT

“The control of mosquitoes should not be based exclusively on the use of insecticides,” he said, adding that cleaning of patios and gardens to eliminate breeding sites is key.

With no vaccine to prevent Zika infection, trying to drive down the numbers of mosquitoes that can spread the virus is the best option public health authorities have to respond to this outbreak.

But vector control, as it is called, is a difficult and relentless task, one that has met mixed results when Puerto Rico has tried to battle dengue — carried by the same mosquitoes — in the past.

“I suspect interest in community-based mosquito control will come back up now that there are other [disease] threats,” said Dr. Jose Rigau, a retired physician and epidemiologist in San Juan who has worked on dengue control in the past. “But it is very difficult work.”