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Tracking mosquitoes for the sake of public health

Under a cloud of mosquitoes on a muggy June morning, Susan Paskewitz sits down in the grass, rolls up her pant leg and extends her calf as bait. Within moments a mosquito starts to land, and yet the University of Wisconsin-Madison entomology professor steadfastly refuses to swat. Instead, she gently vacuums it up with a special sampling wand and then watches for the next. She doesn”t wait long. A “human landing catch” survey like this typically nets five to 10 mosquitoes in 10 minutes, or more than 100 of the insects on the worst days.

As the final step, Paskewitz will usually count and identify the trapped mosquitoes in the lab after they”ve dried up and died. Today, though, she can already tell what”s biting: Aedes stimulans, a floodwater species. What gives it away is its form (“large and slow,” she says) and the time of year. Floodwater mosquitoes appear annually after the spring”s drenching rains.

Although who”s biting may seem immaterial to those who care mainly about not getting bitten, to public health officials the information is essential. When the first human cases of West Nile virus (WNV) emerged in Wisconsin in 2002, the state faced a daunting set of questions: Among our millions of mosquitoes, how many were Culex, the genus that carries the virus and transmits it to humans? In which of Wisconsin”s countless water bodies was Culex breeding? And just how often was the WNV carrier biting people?

In the years since, Paskewitz and her graduate student, Patrick Irwin, have helped the Department of Public Health for Madison and Dane County answer all those questions. And while WNV has yet to pose the threat here that it has in other states, their expertise has proven invaluable, says John Hausbeck, the department”s environmental health services supervisor.

For example, a week before this year”s annual fireworks extravaganza, Rhythm and Booms, Irwin made a startling discovery: At the event”s Madison location, surveillance traps that normally caught 200 to 300 mosquitoes in 12 hours were stuffed with up to 20,000. He immediately called Hausbeck, who then issued a press release alerting people to the possibility for thousands of bloodthirsty party-crashers.

Yet, while he urged people to protect themselves, Hausbeck was also able to reassure them that the chance of contracting WNV was low.

“Because Susan and Patrick are doing the human landing catches, we can be very confident in saying that it”s the floodwater mosquitoes, not Culex, which are primarily biting people,” he says. “And that builds into a risk message that”s very important for us.”

Although common in Africa, western Asia, and the Middle East, WNV was never seen in North America until 1999, when an outbreak in New York City hospitalized more than 60 people and caused seven deaths. Carried presumably by infected birds, the virus spread from there down the Atlantic coast and then began moving west, says Paskewitz. While it mainly cycles between birds and mosquitoes, it has now sickened people and domestic animals from coast to coast.

Wisconsin alone saw three deaths and more than 50 cases in 2002, sending public health officials scrambling to set up Culex monitoring programs. One of them was Hausbeck, who began to call on UW-Madison experts, including Paskewitz, for help.

It didn”t take much to pique her interest. Like many a youngster, Paskewitz came to love insects as a teenager for their strange beauty, but what has fueled her career is a commitment to public health. By the time WNV hit, she had already studied the mosquitoes that carry and transmit malaria for more than 15 years, and the tick-borne illness, Lyme disease, for 10. Now, she saw not only another chance to help address a public health concern, but an area ripe for scientific investigation, as well.

“When we first got West Nile virus here, we didn”t really know what to expect. Were we going to see hundreds or thousands of cases, or just a few?” she says. “Plus, we didn”t know what the dynamic would be in our local mosquito community.”

What really sealed her interest, though, was the day she spent with Irwin in the field. Then a UW-Madison master”s student in population health, Irwin was hired by Hausbeck in the summer of 2004 to locate Madison”s prime Culex breeding spots. And quite a job it was. A preliminary map revealed more than 600 potential sites – including marshes, ponds and stormwater ditches – every one of which Irwin was diligently trying to sample for Culex”s aquatic larvae. “I was driving all over Madison,” he marvels.

By the time Paskewitz joined him, however, he”d narrowed the possibilities to a dozen breeding hot spots and the professor was impressed. “She said, ”Wow, you kind of know what you”re doing,”” laughs Irwin. Their collaboration with Hausbeck began soon afterward, and Irwin later became Paskewitz”s doctoral student.

They”ve since learned that unlike most mosquitoes, Culex prefers to deposit its eggs in the most stinking, fetid water imaginable. In fact, when they want to attract Culex females that are ready to lay, the scientists make a mix of chicken poop, brewer”s yeast and grass clippings, and then “brew it up on the back deck until it smells really awful,” laughs Paskewitz.

And where in Madison do such rank conditions exist? Mostly within stormwater ditches, say the researchers, although not all the time. When ditches are streaming with runoff from city streets, there”s no problem. But as soon as flows stop and the water becomes stagnant, they can become “superproducers” of Culex.

To control the mosquito”s numbers, Hausbeck”s department began applying a mosquito larvacide to the 12 most productive sites shortly after Irwin identified them. Similar to Bt – the natural, bacterial insecticide the city uses to combat gypsy moth – the larvacide is environmentally benign and highly specific to mosquitoes. Still, its use has raised concerns, the main one being the development of resistance, says Hausbeck.

“The more we use these larvacides, the more we have to worry about mosquitoes becoming tolerant and them becoming less effective,” he says.

That”s why the trio has launched an experiment this summer to see if supplementing the larvacide with an inexpensive native fish, the fathead minnow, can reduce Culex numbers even further. The idea is that if the minnows eat large numbers of larvae, the city could add larvacides less often or perhaps stop using them altogether.

In the meantime, Paskewitz and Irwin continue to monitor Madison”s overall community of adult mosquitoes, as well as those that target people. So far, the news is good. In human landing catch studies they”ve conducted during the prime mosquito-feeding hours of 5 and 10 p.m., the pair has bagged thousands of mosquitoes, but only a handful of Culex: just 13 last year, and about six the year before, says Paskewitz.

What”s more, she and Irwin began checking mosquitoes for WNV last year, and so far the highest number of infected Culex they”ve seen is five out of 1,000 sampled.

“That”s really pretty low,” says Paskewitz. “And it helps explain, I think, why we saw only 13 (human) cases of West Nile infection last year.”

Still, Hausbeck doesn”t want people to become complacent.

“The point of all this work is for people to take actions to protect themselves,” he says. “The frustrating thing is that the risk (of WNV infection) doesn”t really go down when the biting pressure goes down. It”s going to be that lonely mosquito – which may show up when everyone else is biting or when nobody else is biting – that just happens to be the one that gets you.”