Bitten

There’s no ignoring it. Some of the students enrolled in this medical entomology class are far more attractive than others. They know it, their classmates know it, and so does Susan Paskewitz, professor and chair of the Department of Entomology.

Paskewitz describes herself as “relatively unattractive,” and she proceeds to prove it using the same test her students have just performed. She fills a small vial with warm water, rubs it between her palms to coat it with volatile compounds from her skin, then places the vial on top of a thin membrane stretched over the top of a plastic container akin to an economy-sized ice cream tub. She invites a visitor to do the same.

Waiting on the other side of that membrane are 20 blood-starved specimens of Aedes aegypti, commonly known as the yellow fever mosquito. Hungry as they are, the insects don’t show a lot of interest in Paskewitz’s vial. They hover near where it touches the membrane, but only two or three land. The visitor’s vial, on the other hand, is a busy spot. At least a dozen have landed and are testing the surface with their needle-like proboscises.

“Wow,” says Paskewitz. “You’re really attractive!”

In another context, those three words could make your day. But not here. Nobody wants this kind of animal magnetism. Nobody wants to be the person who’s cursing and slapping and reaching for the DEET while others are calmly eating their brats and potato salad.

If you’re that person, take heart. Paskewitz can tell you a little bit about why you might have more than your share of interspecies charisma and offer some suggestions on how to scale it back. But first, let’s talk about why this matters.

An average American adult outweighs an average-size mosquito by about 30 million to one. Ounce for ounce, that’s like the USS Nimitz vis-a-vis a good-size duck. But while it’s a safe bet that a 100,000-ton aircraft carrier won’t change course to avoid a six-pound mallard, it’s almost certain that, on a regular basis, you change your behavior to avoid being bitten by a 2.5-milligram mosquito.

Mosquitoes cause us to do things we’d rather not, like dosing ourselves with a repellent that’s sticky and smelly and comes with a sobering warning label (you can apply it to your kids’ skin, but keep the bottle out of their reach), or pulling on long pants, long sleeves, a hat and maybe a head net on a sweltering midsummer day.

Mosquitoes keep us inside when we’d much prefer to go out. In the summer of 2009, Paskewitz and environmental economist Katherine Dickinson, of the Colorado-based National Center for Atmospheric Research, asked a sample of Madison residents how they coped when mosquitoes got fierce.

The second-most-common answer (right after applying repellent) was to stay indoors. About two-thirds of the respondents said they had curtailed outdoor household activities—gardening, yard work, sitting on the deck—in the past month because of mosquitoes, especially in the evening hours, which, for working people, may be the only time available to get a little fresh air. About a third said they had avoided outings, and a similar share said they had avoided outdoor exercise.

Nobody wants to be outside more than John Bates, of Manitowish. An author of seven books about Wisconsin’s north woods and a naturalist by trade, Bates leads interpretive hikes year-round—except in June: “We just kind of throw the month out. The mosquitoes cause too much discomfort for people to listen to interpretation. All we can do is keep walking. People hire me because they want to learn more about the place than they knew before they came. If they can’t stop to listen, what’s the point?”

If we do venture out when mosquitoes are massing, we may not get the experience we were hoping for. Andrew Teichmiller, an outfitter of bikes and paddling gear in Minoqua, recalls mountain biking in 2014, arguably the area’s worst mosquito year ever. “You had to ride the complete trail without stopping, all the way back to the parking lot, and jump in the car, quick, because if you stopped there were 15 or 20 mosquitoes on you immediately.” As for camping: “It’s a different type of experience when you can’t sit by the fire at night and tell stories. You’re forced to run for your tent. It definitely affects the feel of the trip.”

But let’s be clear: A ruined camping trip is far from the worst possible consequence of a mosquito bite.

Mosquitoes transmit diseases that kill nearly a million people every year and sicken hundreds of millions. Tropical and subtropical areas bear the brunt of this, but no place is immune, including Wisconsin. Malaria plagued the immigrants who settled in Wisconsin in the 1800s, and various types of encephalitis are diagnosed on a regular basis.

But today the biggest concern is West Nile virus (WNV). Wisconsin has been relatively lucky since the first case arrived here in 2002, with a total of 230 cases reported through 2014. But all four adjacent states have had bigger outbreaks—notably Illinois, with 2,093 cases total and 884 in its worst year, most of them just across the border in the Chicago area. Wisconsin’s worst year brought 57 cases.

Most cases of WNV bring no symptoms, according to the Centers for Disease Control, but about one in five can involve a fever, headache, body aches, vomiting and a fatigue that can last for weeks or months. Fewer than 1 percent of WNV victims display severe neurologic symptoms, including disorientation, coma, tremors, seizures or paralysis, and of those, about 1 percent die.

Nevertheless, Wisconsin residents are bothered much more by the nuisance of biting mosquitoes than they are worried about West Nile virus. The Madison residents responding to Katherine Dickinson’s 2009 survey said they’d be willing to pay an average of $149 for a hypothetical program to control nuisance mosquitoes, but wouldn’t pay anything for one targeted at mosquitoes carrying WNV when risks were as low as they were at the time (about one case per year in Madison with a population of 250,000).

It’s not surprising to find that attitude in Wisconsin, where mosquito-borne disease is relatively rare, but Dickinson says that people tend to think the same way in places where mosquito bites are often fatal. She observes that in Tanzania, biting mosquitoes were a major factor motivating people to use bed nets. “It was a similar situation to ours,” she says. “Some mosquitoes are more noticeable and more of a nuisance, but those that transmit malaria are kind of sneaky; people don’t feel them biting as much. In areas where mosquitoes were more of a nuisance, people used the bed nets more.”

Biting-wise, there’s an important distinction between nuisance mosquitoes and the ones that transmit WNV. The former come at us aggressively, in such staggering numbers that they’re impossible to ignore. They remind us to protect ourselves. Culex pipiens, the WNV vectors, are more subtle and harder to notice.

Nuisance mosquitoes and the WNV carriers also show up at different times. The most annoying biters—Aedes vexans in particular—are floodwater species that breed after a stretch of wet weather. Culex breed in water that stagnates during a dry spell.

“When it’s been really dry, the water just sits in the stormwater catch basins that are the biggest sources of the WNV vectors,” says Paskewitz. “There’s not enough rain to flush them. Things get more fetid, stinkier. That’s the year when we see a ton of Culex.”

The take-home message: If you only grab the DEET when the biting is so bad that you can’t stand to be without it, you’re not protecting yourself against West Nile virus.
“You need to protect yourself against bites even if you’re not getting a lot of them,” says John Hausbeck, director of environmental health services for Dane County and the City of Madison. “We’ll see summers where it’s really dry and the floodwater mosquitoes are very limited, but we still have plenty of small pools that the Culex can breed in.”

That “biting pressure” is something that Hausbeck needs to stay on top of, and Paskewitz helps with that. She and former grad student Patrick Irwin PhD’10 were able to characterize the types of sites where Culex are most likely to breed and identified alternatives for treating them—for example, introducing fathead minnows to feed on Culex larvae. She and her students analyze the mosquitoes trapped in the area to see how many are Culex and whether they’re carrying WNV. Their data tell Hausbeck whether he needs to issue a public alert.

It’s important to remain vigilant. “When West Nile first came into the country, people doubted it would make it through the first winter,” Paskewitz says. “Well, it did persist, and in a very short period of time it whipped across the whole country. We’ve had a lot of cases in new places. First it was really bad in North and South Dakota. Then Colorado and Arizona. Then Texas, Illinois. It’s really hard to predict. And given the vagaries of climate, we just don’t know whether the next year it might be Wisconsin.”

Maybe WNV hasn’t changed Wisconsin residents’ ideas about why to guard against mosquito bites, but it certainly has spurred a lot of questions about how. There is a seemingly endless list of products and strategies, that, according to somebody, will eliminate mosquitoes or repel them—and since WNV arrived, Paskewitz has been getting questions about pretty much all of them.

“They call me to ask, ‘Would this work or wouldn’t it?’ There is a lot of misinformation out there and not many good sources of information, so I realized I needed to get a better idea of what the science was behind these things,” Paskewitz says.

As she comes up with answers, she posts summaries online. Her website, http://go.wisc.edu/mosquitoes, gets plenty of visits (55,000 last year) and triggers a lot of calls from media from across the nation.

A few of her findings:

• Repellents can be very effective, but comparing them is tricky. There are lots of products with varying active ingredients offered in different concentrations and combinations. Generally speaking, DEET, Picaridin, IR3535, and oil of lemon eucalyptus have good track records. There are also a number of other plant-based compounds—garlic, catnip oil, vanilla and oil of cloves, for example—for which there’s less research and conflicting results. The website sums all this up and gives links to more information.
Yard traps get a thumbs-down. “We tested those and didn’t get any positive outcome,” Paskewitz says. Yard traps lure mosquitoes by releasing C02, light or octenol, a compound contained in our breath and sweat. Sure, they can catch mosquitoes by the hundreds, Paskewitz says. But does this significantly reduce the numbers that bite you? Properly controlled studies say “no.”

• “Sonic” devices—wristbands, smartphone apps, etc.—do better at extracting your money than keeping mosquitoes off your deck. “You can test them yourself,” Paskewitz says. “Sit at the picnic table and count how many mosquitoes land on you, then turn on the device and count again. Or you can trust the research and save your money.”

• Bats are busted. The idea that a colony of bats can consume millions of mosquitoes per night came from a study in which someone put a bat in a room full of mosquitoes and estimated how many it ate. The question is, given the choice, is that what bats eat in the wild? Researchers who examined the stomach contents and fecal pellets of bats have found bigger insects, like butterflies, moths and beetles, but very few mosquitoes. “Bat houses are great for conserving bats,” Paskewitz says, “but not for mosquito control.”

• Avoiding bananas—When she first heard the idea that eating bananas makes you more attractive to mosquitoes, Paskewitz raised her eyebrows. “I thought, okay, we’ll debunk that,” she says. She was teaching medical entomology at the time with 24 students—enough for a robust sample—so she made it a class project. For several weeks, each student ate a banana and then performed an attractiveness assay at prescribed intervals. “We were really intrigued. It did look like we were getting an increase a couple hours after eating the bananas.”

Paskewitz repeated the trial the next two times the course was offered, with a few tweaks to the methodology: Half the students ate bananas, the other half grapes. “The third trial was the best of all—the strongest statistical evidence and the most repeatable,” Paskewitz says. “We did it three times and saw a strong difference between the groups. Grapes didn’t matter, bananas did. At that point I was convinced. I think it’s real,” she says. Does that mean you if you leave bananas out of your picnic fruit salad, you can skip the bug spray? Probably not, Paskewitz says.

Because “less attractive” is not the same as mosquito-proof, Paskewitz gets plenty of mosquito bites, probably more than her share, because she spends a lot of time around mosquitoes—in the woods doing field research, in her garden, and in her lab. When you’re a mosquito researcher, getting bitten comes with the job.

What Makes You Attractive?

It sounds like the topic of an article in Seventeen magazine—and, interestingly, some of the same general categories apply whether you’re talking about your appeal to a mosquito or to a certain someone of your own species.

Your breath. If you breathe, you’re mosquito bait. Every breath adds to a plume of carbon dioxide (CO2 levels in your breath are 100 times that of the atmosphere) emanating from where you stand. “That’s the big signal,” says entomology professor Susan Paskewitz. “Insects are very sensitive to chemical cues. They’ll zigzag to pick up the chemical as it gets stronger and stronger, circling to narrow in on you.”

Your aroma. Once they find you, mosquitoes use chemical cues to decide whether to land and dig in. They have a lot to sort through: You emit roughly 400 different compounds from your skin and 200 in your breath. Many mosquito species won’t land on humans, even if they’re starved for blood. Others will bite us in a pinch but prefer other hosts, Paskewitz says.

Your genes. Perhaps you were born to be bitten. A pilot study at the London School of Hygiene & Tropical Medicine found that identical twin sisters were significantly more alike in their attractiveness to mosquitoes than were non-identical twins. Since identical twins are closely matched genetically, this suggests that some of your Culicidae charisma is inherited. Some volatile compounds on our skin are produced by skin cells (others are produced by bacteria), which would be gene-regulated, the study’s authors note.

Your jeans. What color you wear matters. This is based on a series of studies in which researchers draped different colors of cloth on human volunteers or on robots heated to simulate human body temperatures, then counted mosquito landings. For the most part, darker colors were more attractive. White was least attractive, followed by yellow, blue, red and black.

Your smelly feet. “The malaria mosquito is really attracted to the smell of funky feet,” Paskewitz says. “It’s a classic story in medical entomology. The compound that makes feet smell funky and attractive to mosquitoes is the same one that causes Limburger cheese to smell the way it does.” That compound is produced by bacteria that can accumulate in the moist spots between your toes, and are kin to those used to culture Limburger.

Your drinking habits. A number of researchers speculate that drinking alcohol makes you more attractive to mosquitoes. A team in Japan put this to the test. They asked some volunteers to drink 350 ml of beer while a control subject did not. The percentage of mosquito landings after alcohol consumption increased substantially. Why this happens is unresolved, although some have speculated that people who have been drinking are easier targets because they move more slowly.

Getting Under Your Skin

Maybe you don’t get more mosquito bites than other people. Maybe your body just makes a bigger deal of it. The swelling, redness and itching are signs of your immune system kicking into gear, explains Apple Bodemer, an assistant professor of dermatology at the UW–Madison School of Medicine and Public Health. And some people’s immune systems kick harder than others.

A mosquito bite involves give and take. Before drawing out up to .001 milliliters of your blood, the mosquito injects a bit of its saliva, which contains anticoagulants to prevent clotting. You can spare the blood, but the saliva is a problem. That’s how disease gets transmitted. And the saliva contains foreign proteins, or antigens, that spur your immune system to create antibodies, Bodemer explains. “When antibodies bind to the antigens, it initiates an inflammatory response, which includes the release of histamine, which causes the blood vessels to dilate, which brings the swelling and redness and the inflammatory mediators that are responsible for the itching.”

This doesn’t happen the first time you’re bitten. It’s the second time, when your body has built up the antibodies, that your immune system engages. If you get bitten enough times by the same strain of mosquito, you may become desensitized and have either a very mild reaction or no reaction at all to the bites. “People often have more vigorous immune responses early in the season and then, as the summer goes on, they don’t have as much swelling and redness and itching,” Bodemer says. “But when you go a winter without any exposure, you often become resensitized.”

For the same reason, younger kids tend to have more aggressive reactions. Once they’ve had several years of mosquito exposure, their response tends to die down, Bodemer says.
As for scratching? Doctor’s orders: Don’t! “Scratching really promotes the full inflammatory reaction. It causes more irritation, causing the blood vessels to be more dilated and further dispersing the inflammatory mediators. It initiates a cycle of swelling, redness and itching. If you can avoid scratching, a lot of times the bumps will disappear.”

Antihistamines can ease the itching, she says, or you can try a home remedy: “I paint a little clear nail polish on the mosquito bite. That will stop the itching to some degree and allow the inflammation to clear up more quickly,” Bodemer says. “Some people cover the bite with Scotch tape for two to four hours. The tape stops you from scratching and when you peel it off, it removes some of the mosquito saliva.”

Wisconsin’s Pestilent Past

Wisconsin’s 19th-century settlers knew that mosquitoes were biting them, and they knew that something was making them sick—but they didn’t put the two together.

Their doctors blamed the ailment on “malarial vapors” emitted by decaying vegetation in the swamps, according to Peter T. Harstad, a UW–Madison educated historian who authored several articles on the health of Midwestern settlers. Harstad used reports by military and civilian doctors as well as immigrants’ diaries and letters to chronicle the devastation caused by what was sometimes called “intermittent fever” because the symptoms—chills, aches and a general fatigue—often recurred over a period of months or years.

“I became sick as soon as I came here and have been sick for eighteen months with malarial fever, which is very severe and painful and sometimes fatal,” reads one letter excerpted by Harstad, written in 1941 by a resident of Muskego. “My wife and I are now somewhat better, but far from being well. This year seventy or eighty Norwegians died here … Many became widows and fatherless this year.” About 13 percent of Muskego’s population died that year, Harstad estimates. The town was hard hit because of an abundance of marshes, a relatively warm climate, and the fact that Norwegian immigrants had no resistance to the disease.

Soldiers also suffered. Harstad cites army reports of malaria outbreaks as far north as Ft. Snelling, near present-day St. Paul. Hardest hit was Ft. Crawford, located amid miles of Mississippi River wetlands at Prairie du Chien. In the fall of 1930, there were about 150 cases reported among the 190 soldiers stationed there. To treat the disease, army surgeons were directed to “extract from twelve to twenty ounces of blood, an operation which it is sometimes required to repeat once or twice.” Wisconsin was mostly malaria-free by the end of the 19th century, as farmers drained wetlands and better housing shut out mosquitoes.