What's The Best Way To Keep Mosquitoes From Biting?

Don't get bitten by mosquitoes.

That's the advice offered to the public in virtually every article on the rapidly-spreading, mosquito-borne Zika virus.

There's no arguing with the advice. Zika, once considered a relatively mild flu-like illness, has now been linked to a surge in severe birth defects in Brazil and possibly to cases of paralysis.

But anyone who is a mosquito-magnet must be asking: Can humans really keep the blood-sucking bugs at bay?

To find out how people can best protect themselves. NPR talked with researchers, many of who spend lots of time in mosquito-infested jungles, marshes and tropical areas.

Which repellents work best to stop mosquitoes from biting?

"DEET" is the immediate one-word answer from Dr. William Reisen, professor emeritus at the School of Veterinary Medicine at U.C. Davis and editor of the Journal of Medical Entomology.

"DEET is the standard," agrees Dr. Mustapha Debboun, director of the mosquito control division of Harris County Public Health and Environmental Services in Houston. "All the repellents being tested are tested to see if they beat DEET."

DEET is shorthand for the chemical name N,N-diethyl-meta-toluamide. It's the active ingredient in many insect repellents, which don't kill mosquitoes but keep them away.

Dr. Dan Strickman agrees that DEET is tried and true. Strickman is with the Global Health Program at the Bill and Melinda Gates Foundation (which is a funder of NPR) and author of Prevention of Bug Bites, Stings, and Disease.

DEET appeared on store shelves in 1957. There was some early concern about its safety — speculation that it was linked to neurological problems. But recent reviews, for example a study published in June 2014 in the journal Parasites and Vectors, says, "Animal testing, observational studies and intervention trials have found no evidence of severe adverse events associated with recommended DEET use."

Other repellents work to prevent mosquitoes from biting as well.

But DEET isn't the only weapon. Products containing the active ingredients picaridin and IR 3535 are as effective, says Strickman. And repellents with any of those active ingredients are recommended as safe and effective by the Centers for Disease Control and Prevention. They are widely available around the world.

Actually, Strickman gives the edge to picardin.

"Picaridin is a little more effective than DEET and seems to keep mosquitoes at a greater distance," he says. When people use DEET, mosquitoes may land on them but not bite. When they use a product containing picaridin, mosquitoes are less likely to even land. Repellents with IR 3535 are slightly less effective, Strickman says, but they don't have the strong smell of other products.

Then there is a non-chemical agent, also recommended by the CDC, called PMD. It's the ingredient in lemon eucalyptus oil that makes it repellent to insects. When researchers from New Mexico State University tested a variety of commercial products for their ability to repel mosquitoes, they found that a product containing lemon eucalyptus oil was about as effective and as long lasting as products containing DEET. "For some people, there's a stigma to using chemicals on their skin. They prefer a more natural product," says Stacey Rodriguez, an author of the study published on October 5, 2015 in the Journal of Insect Science.

Not all products deliver what they promise. "We tested a Vitamin B1 skin patch," says Dr. Immo Hansen, professor at the Institute of Applied Biosciences at New Mexico State University and also an author of the study comparing repellents. "We didn't find any evidence that it has any effect on mosquitoes."

One surprising finding was that a perfume, Victoria's Secret Bombshell, was a pretty good repellent. Hansen and Rodriguez said they added it to the products they tested as a positive control, believing its floral scent would attract mosquitoes. It turns out, bugs hated the smell. But in addition to the problem that few people would want to douse all their exposed skin in perfume, there is another impediment to researching many cosmetics. The ingredients are secret. "It's probably composed of dozens of secret ingredients, and maybe one or two of them are repellents," says Rodriguez. "We don't know what the active agent is."

How often should you re-apply a repellent?

Generally, it's a good bet to follow the manufacturer's instructions, experts said. People who will be outside for an hour or two hour should be protected with, say, a product that contains a lower concentration of DEET (about 10 percent — identified on the label). Those who will be out in the woods, or jungle or marshland, should use a higher concentration of 20 to 25 percent, and refresh every four hours or so, says Dr. Jorge Rey, interim director of the Florida Medical Entomology Laboratory in Vero Beach. "The higher the concentration, the longer it lasts," says Rey.

And again, follow manufacturer's directions on the amount used. "A lot of people think that if a little is good, a lot is better," says Reisen. "You don't have to take a bath in the stuff."

What kind of clothing helps protect against bites?

When Rey goes on research trips to highly infested areas, like the Florida Everglades, he suits up. "We wear long pants and long-sleeved shirts," he says. "If it's particularly bad, we use hats with nets coming down over the face. And we depend on repellent on exposed areas." That could mean hands, necks and faces. But don't spray the face, experts say. To avoid irritating the eyes, put the repellent on hands and rub it on the face.

And don't forget the feet. Mosquitoes have quirky olfactory preferences. Many of them, especially the Aedes variety that transmits the Zika virus, love the smell of feet.

"Wearing sandals isn't a good idea," says Rodriguez. Shoes and socks are called for, and tucking pants into socks or shoes helps keep mosquitoes from getting inside clothing. She wears long pants when outdoors in mosquito territory — and definitely not yoga pants. "Spandex is very mosquito friendly. They bite through it. I wear baggier pants and long sleeved shirts, doused in DEET."

Reisen adds high-topped boots to the mosquito prevention outfit and often work gloves. "Since I'm bald as a cucumber, I also wear a hat. I wear glasses, so more and more of me is getting covered."

Strickman lived in Thailand for a while, and he'd start his day armed with a spray bottle of repellent. "I'd spray my socks, the lower part of my trousers and the upper part of my shoes," he says. "The mosquito that transmits Zika has a strong tendency to bite parts of the body that are near the ground."

What else can reduce the risk of mosquito bites?

Mosquitoes can bite at any time of day, but the one that transmits Zika prefers mid-morning and early evening, says Strickman. If possible, stay indoors in screened-in or air-conditioned buildings during those times.

Since these particular mosquitoes breed in standing water in containers like plant pots, old tires, buckets and trash cans, people should rid their immediate area of things that can collect water. "Swimming pools, unless they're abandoned, are okay," says Rey. The chemicals used to keep pools safe for swimming also keep mosquitoes away. It takes some close looking to find every possible breeding ground for mosquitoes. "I've seen some developing in a film of water next to a sink, or in the bottom of a glass people use to brush their teeth," says Strickman. Cleaning up all those areas of standing water can greatly reduce the number of mosquitoes. "It's up to individuals to make their own backyards safe," says Rey. And their front yards and as much of their surrounding environment as possible.

The more people do that kind of basic clean-up, the fewer mosquitoes there will be. "It may not be perfect, but you'll lower the number of mosquitoes tremendously," says Strickman.

Can you get to zero bites?

"There's no way you're going to prevent all the mosquitoes from biting, but you can reduce your chances of getting bitten," says Rodriguez.

And Rey is deeply concerned about Zika because of all that science doesn't yet know about the virus. So he stresses how important it is to use the preventive efforts we have available.

"Your chances of getting infected with some mosquito-borne illness are never zero," he says. "You don't change your lifestyle. But you take precautions."