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September 16, 2014

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Man vs. mosquito

Rising from marshes and abandoned pools, these disease-carrying blood-suckers are trying to make a meal out of you

Image

Steve Marcus

Environmental specialist Robert Cole examines a pool at a foreclosed home in 2010.

The culex tarsalis has a small white ring around the middle of its proboscis, which is otherwise brown and microscopically serrated for sawing, which is what mosquitoes do — they pierce, then rummage under your skin, looking for what entomologists call a "blood meal."

Hundreds of mosquito traps are hidden in Clark County every year. The traps capture thousands of mosquitoes, which are taken to a lab, frozen, counted, sorted by species and shipped on ice overnight to Reno for further testing.

Click to enlarge photo

A mosquito

Exactly 11,405 mosquitoes were caught and sorted by the Southern Nevada Health District in Clark County last year. Of the 17 species recorded in Southern Nevada, 10 are known carriers of disease, including the tarsalis, with its white-striped needle beak — a plunger loaded with saliva that coagulates blood and, in the infected insect, transfers disease; most worryingly, West Nile Virus.

Health District employees found West Nile in 256 of the mosquitoes they caught last year, while the Center for Disease Control reported 12 human cases of the virus in Nevada. Of course, roughly 80 percent of people infected with West Nile have no idea — it's an illness that can be coma serious or totally imperceptible — so we don't really know how many people are scratching the bad bite.

Meanwhile, it's the middle of mosquito-breeding season. At dusk, male mosquitoes swarm, forming whining knots of bugs sometimes several feet high. Female mosquitoes dive-bomb these swarms, searching for a mate. When two mosquitoes take to each other, they reportedly adjust their wing speeds to a higher frequency — their mating buzz hovers around 1,200 hertz, close to a middle-A note. Once in harmony, they copulate. The process is said to take less than 20 seconds.

Afterwards, the female needs her blood meal. Only the ladies bite — they can't produce viable eggs without the protein and iron. In her typical two-week lifetime, a female mosquito will drink roughly five-millionths of a liter of blood.

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Environmental specialist Robert Cole

And certain blood — certain people — will be more attractive than others.

For some, it's innate. They give off a particular combination of chemical scent markers scientists say is just more inviting.

For others, it's a matter of circumstance. Pregnant women are more attractive to mosquitoes because they exhale more carbon dioxide, a significant lure. People who are active and moving around are also more susceptible — they're breathing heavier (more carbon dioxide), they're moving (motion cues mosquitoes) and physical activity produces lactic acid, another draw. People drinking alcohol are also more attractive to mosquitoes — body temperature rises when you drink, and mosquitoes are attracted to warmth. Scientists have also suggested our scent markers change when we metabolize alcohol, in a way that excites mosquitoes.

The Health District loads its mosquito traps with dry ice — a chunk of frozen carbon dioxide that leaks out like an extended yawn. Environmental Health Supervisor Vivek Raman and colleagues set 509 traps last year, some in suburban neighborhoods, others buried deep in Henderson marshland. Raman keeps a pair of rubber waders in his truck and walks around with what looks like a giant ladle — a dipper he plunges into drainage ditches, wash scum, murky ponds and other sad bodies of desert water, then draws it up close to his face, looking for mosquito larvae, which are also known, with good reason, as "wrigglers."

Raman's biggest haul was a few thousand mosquitoes in one trap — a brown buzzing fog. The coordinates of each trap are plotted into a portable GPS device. When mosquitoes from a particular trap test positive for West Nile, it's noted on the map. Over time, clusters of contagion emerge, each infected trap a little red dot on the map. Several red dots crowd a southeast corner of the Valley, where marshland does double service for mosquitoes: It draws birds (who carry West Nile, and infect mosquitoes that bite them) and provides standing water for lying eggs.

This is the second part of Raman's job: Finding standing water, dipping his ladle in, and deciding which method to fight larvae with. Crouched in a wash by UNLV last week, Raman decides to use a kind of quick-release bacterial larvicide in a small pocket of watery sludge and garbage — he's seen a few wriggling mosquito larvae in the mud, and sprinkles brown pebbles in the area of concern. For larger jobs, he's got a white brick of slow-release larvicide he plunks into water. He's also got a "growth interrupting" powder that actually halts mosquito larvae in development — makes them permanent juveniles. In some cases, he stocks the water with little larvae-eating fish; afterward, he posts a yellow sign in the area that reads "FISH AT WORK" next to a smiling cartoon guppy. For hard-to-reach areas, Raman's got matchbook-size larvicide packets — he chucks them over locked fences and into the green pools of foreclosed homes. Hundreds of them.

"We get crushed with swimming pools," Raman says.

West Nile was first recorded in North America in New York in 1999, according to the CDC. By 2004, the first case of West Nile was recorded in Nevada, and the Health District began testing mosquitoes. By 2006, the number of Nevada cases swelled to 124. Since then, however, the virus has tapered off. The fact that Nevada had only 12 cases last year is partly due, no doubt, to mosquito population-control measures.

West Nile cases have fallen nationally as well, and this drop — paired with the drop in our economy — has prompted many states to cut back mosquito-control programs. While the Health District has yet to trim its eight-person mosquito team, state agriculture officials recently told USA Today that funding cuts in Reno would reduce the number of helicopter larvicide drops by 40 percent, and not without potential consequences.

At the same time cases of West Nile started dropping, the number of foreclosures began to rise — and with them, the number of green pools: ideal mosquito breeding grounds. And so, even though the virus has dwindled, Raman and colleagues are just as busy managing mosquito growth. Last year, the Health District treated 2,547 stagnant pools. This year, about 1,100. These green pools could explain the red dots on Raman's GPS map, little welts that pop up in the middle of residential stretches. If monitoring programs were cut back, some worry, green pools could become ground zero for a West Nile resurgence.

For now, Raman still spends his days slogging around drainage ditches, washes, pools and puddles — setting traps, then checking traps, then setting more.

— Originally published in Las Vegas Weekly

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