BU Today

Science & Tech

The Mosquito Hunter

BU researcher: they’re efficient, enigmatic, and yes, sometimes deadly

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When Richard Pollack vacations with his wife, she is drawn to architecture and history. He gazes into puddles. A public health entomologist and a College of Arts & Sciences biology department visiting researcher, Pollack embodies an encyclopedic knowledge of, and unquenchable curiosity about, what are arguably nature’s most unloved creatures, mosquitoes and ticks. There’s much to grudgingly admire about the evolutionary insistence of blood-feeding arthropods, but it’s the Massachusetts public’s fears of encephalitis, West Nile virus, and Lyme disease that have made Pollack a fixture on radio and local television.

In late July a Middlesex County man in his 60s became ill with the mosquito-borne West Nile virus. He reportedly remains hospitalized. Around the country the virus has killed 41 people and sickened 1,118 others, according to the federal Centers for Disease Control and Prevention (CDC), making it the worst outbreak in the United States for this time of year since the disease was discovered here in 1999. Approximately 75 percent of the cases have been reported from five states (Texas, Mississippi, Louisiana, South Dakota, and Oklahoma) and almost half of all cases have been reported from Texas, the CDC reports in its West Nile virus update.

When local health departments want the lowdown on mosquito-borne illness risk, they often turn to Pollack. Pollack has a bit of the mad scientist about him. He habitually stalks putrid swamps, uses his English setter to “collect” tick samples, and runs a tick-testing business out of his basement. (Occasionally he must inform concerned clients that the “deer ticks” they sent him were in fact specks of belly button lint.) When headlines around southeastern Massachusetts this summer were dominated by reports of a high risk of the potentially deadly mosquito-borne Eastern equine encephalitis (EEE) virus and West Nile virus, Pollack appeared on NECN to put matters into perspective.

West Nile virus is carried by the northern house mosquito, Culex pipiens, which thrives in the catch basins along city and suburban streets, says Pollack. In Texas, where the most cases have been reported, a drought has kept water levels stagnant and ideal for mosquito breeding. “This mosquito flies mainly at night and feeds preferentially on birds in the canopy of trees,” says Pollack, “but it will feed on people and other animals if given the opportunity.” The female mosquito acquires West Nile from an infected bird during blood feeding. While EEE infection will likely result in death or a lifelong neurological deficit, most people bitten by a West Nile virus–infected mosquito won’t suffer serious illness, he says, and although most mosquitoes will be free of virus, “you should be distrustful of any mosquito that comes your way. Slap first, ask questions later.”

The EEE virus infects birds living in or around freshwater swamps and spreads from bird to bird via infected mosquitoes, which sometimes bite humans or horses, putting them at risk of contracting the disease. While mosquitoes in many towns have tested positive for the virus, the first time EEE turned up in samples of mosquitoes on Cape Cod was in mid-July. For humans, “the EEE risk is tiny, but not nonexistent,” says Pollack as he bushwhacks on a summer morning into a white cedar swamp in Sudbury, Mass., in search of the potentially EEE virus–carrying Culiseta melanura, which he calls “the most dangerous mosquito.”

In infected humans, symptoms of EEE surface in 4 to 10 days and include fever, headache, irritability, restlessness, drowsiness, anorexia, vomiting, diarrhea, bluish skin, convulsions, and coma. The most serious form of EEE causes brain swelling that results in death in a third of all cases, according to the CDC. Children and the elderly are at the greatest risk of dying. There is no vaccine for the virus.

Richard Pollack, cedar wetlands, Sunbury, MA

Entomologist Richard Pollack displays a modest tool of his trade, a special ladle to scoop mud from swamp “crypts” that house mosquito larvae.

The Massachusetts Executive Office of Health and Human Services offers daily mosquito-borne virus updates with risk maps and tallies of animals and people known to be infected with the EEE virus or the less serious West Nile virus, which affected six people in the state last year, all of whom survived. Last summer there were two reported EEE cases, resulting in the death of one, an 80-year-old Bristol, R.I., man. There have been no cases of EEE this year, but the Massachusetts Department of Public Health commenced aerial spraying for mosquitoes in 21 southeastern Massachusetts communities in July. The pesticide used, Anvil, contains the mosquito killer sumithrin, a chemical relative of a compound produced by chrysanthemums.

Bearing the modest tools of his trade—a scooper resembling an elongated soup ladle, a $4 hand-operated bilge pump, and a turkey baster, Pollack lurches from spongy mound to spongy mound on the hottest day of the summer searching for Culiseta larvae in crypts—ecologically distinct, highly acidic water pockets beneath the cedar trees. Examining samples of soupy swamp water with his naked eye, he can pick out the larvae, and even this crude level of observation gives him an idea of what we’re in for week to week, mosquito-wise.

Pollack is endlessly fascinated by the specificity of Culiseta’s swamp habitat. “The Culesita females fly away from the swamp, feed on birds, return to the swamp, and lay their eggs in these subterranean reservoirs,” he explains as he pokes his ladle into the mud. “When the larvae hatch, they float around and feed on bacteria.”

A lot is known about the mosquitoes in our midst. Massachusetts is home to 51 species, and their habitats range from swamps to river flood plains to fleeting woodland pools to the standing water that collects in abandoned tires or kiddie pools. Only female mosquitoes are blood feeders; the males feed on nectar or other carbohydrate sources. Some mammal-favoring species are aggressive, with apt names such as the common summer biter Aedus vexans, the vicious Coquillettidia perturbans, or the tenacious Ochlerotatus excrucians. Other species prefer feeding on birds, frogs, or snakes to sinking their proboscises into humans. Drawn to the carbon dioxide we exhale, or by sight or smell, mosquitoes are often selective about whom they bite, and only three species venture into houses. Although they’re most active in spring and summer, mosquitoes can survive until the first frost. The majority of local species are most active during dusk or dawn, but some feed through the night, and at least one species bites during sunny days.

Pollack apologizes for any sexist overtones when he asserts that when it comes to mosquitoes, it’s the old females that are the most dangerous. “They’ve been around the block, so to speak,” he says. “They’ve had many opportunities to blood-feed and potentially acquire infection, and the older ones have had time to incubate and amplify the virus.” Being bitten by a young mosquito is annoying, but older mosquitoes can deliver deadly pathogens in their saliva, says Pollack, who is working with state health officials to determine the age and gender of mosquito samples to assess numbers and infection rates both before and in the wake of pesticide spraying.

Pollack, who has taught at the BU School of Public Health and at Harvard, describes the relationship between virus and mosquito as “very clever.” The EEE virus–carrying mosquito is “a living test tube,” he says. “It has the ability to acquire, maintain, and then, most importantly, transmit the virus.” Feeding on an infected bird, the mosquito gets a bellyful of virus particles, he says. “If she’s the right kind of mosquito, that virus will attach to cells of her stomach and infect her and start to grow, so that for every virus going in, thousands are produced and will circulate around her blood stream and her salivary glands, and after that, when she bites, she can transfer the virus to another bird—or us.”

Richard Pollack, EEE carrying mosquitoes researcher, Sudbury, MA

To obtain mosquito samples for testing, Richard Pollack uses a standard device called a CDC light trap, which draws mosquitoes with carbon dioxide bait and a small battery-powered lamp.

How can we protect ourselves? Pollack is a big fan of FDA-approved insect repellents containing DEET. “Though other products may smell nice, it’s the only thing that really works,” he says. Beyond covering up as much as summer heat will allow and dousing ourselves with bug spray, there’s a lot that people can do to make their environments unfriendly to mosquitoes. The state health department urges residents to drain standing water from garbage cans, birdbaths, and flowerpot saucers, and repair any holes in window screens. Although worries over mosquito-borne viruses can grow out of proportion (Pollack heard of a woman whose EEE phobia led her to sell her house at a loss and move), he says it’s a good idea to stay indoors between dusk and dawn if possible, when mosquitoes are most active.

Many wince at the notion, but mosquitoes’ sole contribution to the balance of nature may be to deliver the pathogens, which, although Pollack is hesitant to suggest it “in polite conversation,” play a role in population control. “There is no known mosquito-specific predator anywhere in the world,” he says, and while the notion of scientifically expunging mosquitoes from the planet is disturbing to some environmentalists, “it’s a very common misconception that if we had the ability to flip a switch and kill all the mosquitoes instantly, fish or bats would go hungry. The reality is that everything that feeds on mosquitoes will feed on all sorts of things that are small, like flies and moths, and I’d think that if I were a bat, I’d prefer to feed on a big juicy moth rather than a measly mosquito.”

According to some estimates, mosquito-borne diseases—including the ongoing scourges of malaria, dengue fever, and yellow fever—have been responsible for half the human deaths throughout history. Describing man versus mosquito as a great “arms struggle,” Pollack says that mosquitoes “are a very capable opponent. We throw something at them, they adapt.” He cites reports, for example, of mosquitoes in Africa feeding earlier in the day as an adaptive response to Africans’ increased use of protective bed netting at night. And as sophisticated as it gets, genetic modification in the laboratory (one experimental technique is to engineer and release males that produce nonviable offspring) is unlikely to put a significant dent in mosquito species, according to Pollack. “Nature genetically modifies mosquitoes all the time,” he says. “Nature is a wonderful laboratory—much more capable than us, and with much better funding.”

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Susan Seligson, Senior Writer for BU Today and Bostonia
Susan Seligson

Susan Seligson can be reached at sueselig@bu.edu.

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