Meet the mosquito: Annoying, deadly

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Meet the mosquito: Annoying, deadly
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Photo of two feet, covered with mosquitoes.

Does this make you itch or what? But mosquitoes aren’t just a bother; they can carry a bunch of deadly infections, too.
Photo: Freebird

What are mosquitoes? What’s up with their nasty attitude? Can we make them change their evil ways?

Van Nostrand’s Scientific Encyclopedia describes mosquito’s wicked proboscis: “A small two-winged fly with slender body, long legs, and narrow wings bearing scales along the veins.”

Sounds innocuous, right? Not a word about the mosquito’s proboscis, which can furtively slip through your flesh. Nothing about the itch you feel as your body tries to decompose chemicals the mosquito injected during the bite. Not a word about the grave diseases mosquitoes transmit, like West Nile, encephalitis, yellow fever, dengue and malaria.

Not a word about the anti-coagulants that keep your blood flowing.

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Close-up photo of opening in pitcher plant filled with fluid, and many larvae.

Some mosquitoes lay eggs in water pools on certain plants. Mosquito larvae are growing in the pitcher plant Nepenthes attenboroughii.

Not a mention of the need that females (of many species) have to suck blood so they can reproduce.
Blood — supplied by an unwilling coyote, a unwitting raccoon, or an oblivious three-year-old scampering in the backyard — supplies protein for the skeeter’s eggs. (This should not sound strange — maybe you eat eggs to get protein; skeeters just work backward.)

Despite an occasional case of malaria in extra-tropical locations like New York City, and the spread of West Nile virus across parts of the United States, most people in temperate climates view mosquitoes as just a nuisance. And although many species do not carry a known disease, others transmit dangerous human or animal pathogens.

Still, mosquitoes are good for something — like feeding swallows, purple martins, bats and some predatory insects. That means they do good by dying.

Whining and dining! The biology of the mosquito

Mosquito eggs hatch in standing water, and various species require fresh, brackish or salt water. The eggs develop, singly or in groups, into larvae, which then progress into pupae, and finally adults, during a temperature-dependent cycle that can last one to several weeks.

Most mosquitoes fly less than a mile during their lifetimes, but the ferocious salt-marsh mosquito, found in the Everglades and other coastal locations, can migrate 75 to 100 miles.

Mosquitoes live from sea level to altitudes of 10,800 feet (3,600 meters). Humorists from many states, including Wisconsin, native Why Files habitat, proclaim them the state bird.

The air-mobile, bloodsucking lifestyle is successful, and mosquitoes have evolved into more than 3,500 species. Some require specialized habitat, others are generalists. Only females — and not among every species — suck blood to nourish their eggs. Some feed in the day, others at dawn, dusk or night.

Keeping an eye on you!

To bite you, mosquitoes must first find you. To do this, they can detect:

Exhaled carbon dioxide and lactic acid (a by-product of muscle metabolism)

infra-red light from body heat

some of the 300 chemicals released from the skin

To prove that skin chemicals attract skeeters, wipe a piece of glass on the skin and count skeeters that “visit” the glass. Because much of the response to humans is based on individual chemistry, it’s true that some people are mosquito magnets, says Don Barnard, research entomologist at the Agricultural Research Service’s Mosquito and Fly Research Unit.

Carbon dioxide tends to attract mosquitoes and other blood-sucking arthropods like ticks from a distance, Barnard says, “then they use other stimuli from the host to locate their prey.”

Day flying mosquitoes often use vision, movement and changes in contrast, while night fliers may use at least half a dozen chemical cues.

Why learn to attract mosquitoes? Because data from present mosquito captures are difficult to interpret, so good attractants could improve monitoring of skeeter populations.

“It’s like going fishing,” Barnard says. “People can tell you about what they catch, but can’t tell you as much about the character of the fish population in general. If sampling techniques get better, that could help in efforts to control mosquito populations.”

Barnard does not anticipate finding an attractant to bait skeeter death-traps. In other cases, such as the cotton boll weevil, traps baited with chemicals called pheromones are used to trap males, and therefore block mating. Unfortunately, pheromones are usually specific to one species, and so, “There is not anything like a general mosquito attractant for interrupting mating,” Barnard says.

Drawing shows process described; insects emerge from the water at end.

A female mosquito lays eggs in water (sometimes singly, sometimes in “rafts,” as here). The eggs develop into several stages of larvae, and then the pupae, which emerge to form the adult insect.

Not just a natural thing

Mating may be the ultimate natural activity, but the location and intensity of mosquito-borne disease is strongly affected by economics and culture. Decisions on where and how to live can promote the growth and spread of the insects themselves, along with the disease they transmit as vectors. (Vectors are organisms that transmit pathogens; because they penetrate the bloodstream, mosquitoes are highly effective vectors.)

Many human decisions result in more mosquitoes living near more people:

Fast population growth, mostly in developing tropical countries, has created urban sprawl where unscreened housing is set amid multiple breeding places

Jet planes move pathogens almost instantly across the globe

Tires and plastic packaging make ideal habitat for mosquito reproduction (some species can breed in a sodden bottle cap)

Poor people and nations have difficulty affording campaigns against mosquitoes and the diseases they vector

Example #3: Malaria — Setback for an ancient killer

Malaria, long considered the cardinal crime of mosquitoes, is caused by a blood-borne parasite that infects and destroys red blood cells. The red blood cells carry oxygen through the body, so the result is repeated bouts of fever and anemia.

Although statistics on a disease that affects poor people in remote locations are seldom exact, and sometimes debated, just 15 years ago, malaria was killing more than 1 million annually.

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Photo shows Anopheles taking blood meal with bristles on body and compound eyes clearly visible.

The malaria vector Anopheles stephensi drinks a blood meal from a human host through its pointed proboscis. A droplet of the liquid fraction of blood is expelled from the abdomen to make room for solid nutrients gathered from the blood.

Malaria parasites are in the genus Plasmodium, and they have different characteristics and lethality. P. falciparum can infect any red-blood cell, causing severe anemia and kidney failure, or constricting small blood vessels in the brain.

However, according to the World Malaria Report, 2013, P. vivax develops faster in Anopheles mosquitoes, and at wider temperature ranges, “enabling transmission to occur from younger mosquitoes and in a wider variety of geographical conditions.”

Despite progress, malaria remains a major public-health problem, according to the World Health Organization:

207 million cases in 2012 (uncertainty range: 135 to 287 million)

627,000 deaths (uncertainty range: 473,000 to 789,000)

562,000 deaths (90% of all deaths in 2012) were in sub-Saharan Africa

The African death toll has dropped significantly from 802,000 in 2000

Uneven protection in sub-Saharan Africa, 2012

Risk is highest in a band from Ethiopia to South Africa

Protection against malaria with insecticide-treated net or indoor residual spraying has stalled after a dramatic rise.

Because the mosquitoes that are malaria vectors are spreading as the climate warms, and malaria parasites cannot develop below about 16°C, global warming could expand malaria’s range.

The malaria parasite’s life cycle

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Diagram shows the transmission and replication pathways of the malaria parasite through its mosquito vector.

1 A female mosquito carrying the parasite drinks blood, transmit threadlike sporozoites into the new host;

2 Sporozoites travel to the liver, multiply and form a spore called a merozoite;

3 Merozoites penetrate red blood cells and devour hemoglobin, which transports oxygen. The blood cell disintegrates, releasing a new generation of merozoites that infect other blood cells;

4 A few merozoites form a sexual stage, which can be sucked up by another mosquito taking a blood meal;

5 Two sexually active merozoites meet in the mosquito’s gut, starting a new generation of parasites that eventually form new sporozoites. The host mosquito is now capable of further infection.

Diagram: NIAID

A gradual success against malaria

About 50 years ago, malaria was in the grip of a pincer attack. Larvacides and insecticides, primarily DDT, were lambasting mosquitoes before they could transmit the parasite. A cheap, plant-derived medicine called chloroquine could kill parasites before they gained a foothold in their victims.

And so from the end of World War II through roughly 1970, malaria was on the decline. With its status as a global scourge waning, malaria was no place to make a big name in medicine. Then nature took over, with help from modern ecological disturbances and tight government budgets:

Most important malaria vectors evolved resistance to at least one insecticide

Malaria parasites evolved resistance to chloroquine, then to successor drugs

Use of DDT, a mainstay in many anti-mosquito campaigns, declined after the discovery that it decomposes into compounds that it causes birds to lay soft, non-viable eggs

Increases in travel and international trade moved drug-resistant parasites around the globe

Some governments could no longer afford the expanded campaign against malaria

The unfortunate result was a resurgence of malaria during the last decades of the 20th century. Then, around 2000, malaria again began to subside, due to a great deal of international funding for four straightforward techniques, backed with a great deal of education and community involvement:

Bed nets, used to protect sleeping people from bites

Insecticide spraying inside homes that left residue on the walls that kills resting mosquitoes

Better drugs and better drug-treatment plans

Treatment of pregnant women to protect their young from infection

Basic but beautiful: bed nets rule!

Bed nets and residual spraying work because once engorged with blood, a mosquito needs to rest while digesting its nutritious repast. If that resting place happens to be a net or wall treated with insecticide, it becomes the bloodsucker’s final resting place. That breaks the chain of infection, which can only occur if a mosquito feeds on an infected person, and then a healthy one.

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Graph shows usage of nets increasing after 2000, and leveling off in 2011.

Nets treated with insecticide are a last-ditch defense against malaria, yet only about half of households in sub-Saharan Africa have them.

Both bed nets and the plant-derived pyrethroid insecticide are cheap; though the insecticide must be reapplied every six months or so.

These basic techniques proved far more effective than high-tech tactics that have been explored over the past couple of decades, such as releasing mosquito-killing bacteria or genetically altering mosquitoes to make them immune to malaria.

We asked mosquito expert Barnard to explain why simple is beating sophisticated. He responded in practical terms. “You need something that works, and [bed nets and spraying insecticide on home interiors] basically are tried and true.”

Although some people object to insecticide on public health or environmental grounds, “they work,” he says. “You don’t have time to wait for development of a genetic technology that may be years away.”

The pitfall of genetically engineered mosquitoes, Barnard continues, is survival and reproduction. “Success or failure depends on how competitive are the mosquitoes introduced in the natural population. Generally speaking in the real world, their competitiveness is relatively low. These ideas are always worth exploring, but we don’t understand the population dynamics of mosquitoes well enough to insert an artificial organism into the system and figure out how make it work. The real acid test is nature.”

Global average temperature of land and ocean, compared to the average for 1951-1980

Graph shows a fairly steady temperature rise since a low point in 1910.

Global warming is changing the location of mosquitoes and the diseases they carry. With the exception of 1998, the 10 warmest years in the 134-year record all followed 2000. 2010 and 2005 were the warmest years on record. The dotted black line is the annual mean and the solid red line is the five-year mean. Green bars show uncertainty estimates.
Graph: NASA-GISS

Success forces adaptation

Through its reliance on relentless, informed use of basic anti-malaria techniques, the international anti-malaria campaign has been “one of the biggest success stories in global health in the last decade,” according to S. Patrick Kachur of the U.S. Centers for Disease Control and Prevention. “The number of people who become sick or die from malaria has decreased by 25 percent globally and by 33 percent in the hardest-hit region, sub-Saharan Africa.”

The gains depend on continued infusions of money from rich people and countries. And, as Kachur wrote with colleague Laurence Slutsker1, the anti-malaria campaigns must change to reflect changes in both parasites and their vectors.

Some countries, Kachur wrote, could be moving toward a malaria-free future, while others that were uniformly dangerous now have safer zones mixed with “hot spots.”

Zambia, for example, used the four tactics to significantly reduce infections, and as the disease wanes, has started intensive surveillance in areas with low infection rates. Public health managers get reports of malaria cases via mobile phone, enabling them to distribute bed nets and drugs as needed.

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Woman holds glass jar over 1 netted bucket-shaped receptacle.

Technician Hyon Chu So removes a trap used to track mosquito populations at Osan Air Base, Republic of Korea. The mosquitoes are sorted and sent to an entomologist for disease analysis.
July 12, 2012, U.S. Air Force photo, Senior Airman Michael Battles

Patients are tested to detect outbreaks at an early, manageable stage. If patients were infected locally, health workers may visit to find other cases for treatment. “This intervention, called ‘reactive case detection,’ is one way to target efforts to reach areas and individuals most in need, be more efficient, and make progress toward decreasing transmission, and thus, the risk of malaria infection,” Kachur and Slutsker wrote.

In malaria, remember, infected people form the reservoir of parasites. All other things being equal, a falling rate of disease leads to a falling rate of new infections.

Protect yourself from flying tigers

By now, let’s assume you don’t want to a mosquito bite to make you sick; you don’t want to donate blood to the phylum arthropoda, and you don’t want to hear their signature whining in your ear. What to do?

All in the timing, Part I: Many mosquito species prefer to be out at dawn and dusk. Their host animals tend to be active at dawn and dusk, and many of the little lovelies are harmed by low mid-day humidity. You can avoid bites by staying inside when they’re outside.

All in the timing, Part II: Avoid being outside after intense mosquito hatches.

Shoot the breeze! There’s nothing like a wind to keep mosquitoes down. Moving to a windy bluff can restore sanity on a mosquito-mangled camping trip.

DEET is the standard repellent, but don’t swim in it. “We generally recommend 30 percent DEET or less,” says Barnard, “applied according to label directions. Don’t drink DEET, don’t slather on more than you need, and be especially careful with young children.” Repellants containing picaridin and paramenthane diol, or PMD, extracted from oil of lemon eucalyptus, are also proven, Barnard adds. (The Environmental Working Group recommended repellents here.)

Soak clothing in permethrin, an insecticide that will last through several washings.

Set up a house for bats or purple martins. These winged wonders gobble hundreds of mosquitoes each hour, and they’re not only non-toxic, they’re fun to watch.

– David J. Tenenbaum

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Kevin Barrett, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David J. Tenenbaum, feature writer

Bibliography

  1. It is time to rethink tactics in the fight against malaria? Laurence Slutsker and S Patrick Kachur, Malar J. 2013; 12: 140, Published online Apr 24, 2013.
  2. Anti-mosquito advice for travelers, and everybody else.
  3. Mosquito counting with Winnipeg’s interactive mosquito map.
  4. Human cases of West Nile Virus in U.S.: interactive map.