Every summer, West Nile moves across California, spread by mosquitoes. The insects pick up the virus when they feed on infected birds -- then spread West Nile to humans when they munch on us.
Avoiding mosquito bites is ideal for lots of reasons besides West Nile. What repellents are best? And why do mosquitoes seem to prefer some people over others? NPR's Shots blog posed those questions and more to Dr. Roger Nasci, chief of the branch of the Centers for Disease Control and Prevention that tracks insect-borne viruses. Here's the Shots Q&A:
Q: So why is it that backyard mosquitoes swarm and feast on some people, while others seem to be less palatable to them? Is there any truth to the claim that some blood is sweeter?
A: Well, first off, let's remember that here in the U.S. we have 175 species of mosquitoes, and a relatively small number of them feed on people. But they'll feed on anything that has blood.
The mosquitoes that feed on people are attracted to over 300 gases and other compounds emitted by human skin. And what's attractive for one mosquito species might not be attractive to another. The most well-known compound is carbon dioxide. They can detect that fairly far away, from about 35 to 40 feet. They will fly towards carbon dioxide, and so in a lot of cases in the backyard that's you and me.
Other compounds various mosquitoes like are lactic acid and water. So it can be the composition of the sweat you exude that makes you more attractive, but the research doesn't show that hygiene really has anything to do with it. We also know that pregnant women are a little more attractive, as are people drinking alcohol. But it's really a very complex set of cues that mosquitoes use, so it's hard to make gross generalizations.
Q: Sounds like that would make coming up with repellents very complicated.
A: Yes, it does.
Q: Has mosquito repellent been getting stronger over time?
A: No, we have had Jungle Juice – that stuff that's 90 to 100 percent DEET – for a long time.
Q: So what trends are you seeing around repellents?
A: We're seeing a real proliferation of new registered products that have different chemicals, and are giving consumers more choices. We've also seen significant advances in formulation chemistry – meaning how the active ingredients are packaged with other compounds. One innovation is time release, so low concentrations of DEET or other active ingredients that can provide long-lasting protection.
For example, the Department of Defense now sends soldiers out into the field with time-release DEET and uniforms treated with permethrin. There are actually a lot of treated clothing products available these days – the permethrin lasts even after several washings.
Q: Do you have any concerns about the repellents we use today?
A: There are two concerns I have about repellents. One is that there are products coming onto the marketplace that have not been demonstrated as effective. The products we recommend on our website are DEET, picaridin, IR3535 and some oil of lemon eucalyptus and para-menthane-diol products. There is a body of scientific literature showing these products to be effective. And they're registered with the Environmental Protection Agency.
The other trend that concerns me is people don't use repellents – they're underutilized, especially in outbreaks of mosquito-borne illnesses like West Nile virus. Even when these outbreaks are publicized, we have not been successful at getting more people to use repellents effectively.
Q: Is there any evidence of mosquitoes becoming resistant to DEET or other products?
A: No, we're not seeing any evidence that mosquitoes are ignoring the repellents. The reason is that humans don't provide the primary blood source for mosquitoes — they mostly feed on animals, like birds. So we're not selecting for the behavior that would lead to resistance. DEET is still the gold standard.
Q: Does it matter whether your repellent is 5 percent or 30 percent DEET?
A: The amount of DEET can affect how long it lasts. But the research suggests that once you get about 50 percent DEET it's not going to make a lot of difference. The variable is how much you are putting on your skin.
Q: So how do you protect yourself from mosquitoes? Or, given that you're an expert on mosquito-borne diseases, do you just prefer to stay inside?
A: When I'm going camping and hiking in the Colorado mountains — and we actually can have a lot of mosquitoes up there — I am using a topical repellent for my skin and wearing treated clothing. With those two, you get optimum protection.