Humans still seek weapons in battle against disease-infested mosquitoes

By a wide margin, the mosquito is the most deadly nonhuman animal on the planet. Malaria, largely spread by mosquitoes, infected 225 million people in 2009 and killed 781,000 of them, according to the World Health Organization. To put that into perspective, such ferocious predators as lions and crocodiles are responsible for only a few hundred deaths per year. Fortunately, malaria was largely eradicated from the United States in the 1950s. We still get malaria outbreaks in this country, but they are usually localized and come from an international traveler whose blood is spread by a mosquito or two.

That doesn't mean you shouldn't worry about mosquitoes. Malaria is just one of many diseases these insects carry, and some are a concern in the United States. There are several forms of encephalitis, such as the now-familiar West Nile virus and Eastern equine encephalitis, a rare but exceedingly deadly disease that uses mosquitoes to jump from horses to humans. Mosquitoes can also carry worm larvae that infect the human lymphatic system and cause elephantiasis, a tropical disease characterized by swelling and thickening of skin and tissue.

Before looking at mosquito control efforts, let's start with some Mosquito 101.

There are more than 3,000 species of mosquito. Around 200 of them prey on humans. Only the females bite: They need blood every time they lay eggs. (Mosquitoes get most of their energy from sugar sources such as plant nectar.) The itchiness associated with a mosquito bite is your allergic reaction to the injection of the insect's saliva, which contains an anti-clotting agent to keep your blood flowing. Certain types of mosquito are associated with particular human diseases. Anopheles, for example, carries malaria. Aedes is primarily responsible for yellow fever. Culex carries West Nile.

If you're wondering — as my wife does ruefully every summer and on every trip she takes to the tropics — why mosquitoes seem to bite some humans more than others, no one really knows the answer. The little vampires rely on the carbon dioxide in your breath to find you, and they use some chemical cues such as folic acid to decide whether they will extract your blood. Researchers speculate that certain kinds of detergents and perfumes can either attract or repel mosquitoes and that people whose skin surface is especially warm are more attractive.

Americans got serious about mosquito control around World War II. (Government efforts coincided with lifestyle changes — such as installing window screens, using air conditioners and staying inside to watch television — that helped enormously.) It's mostly done on the local level, but some states operate statewide control programs.

The key to keeping the little buggers down is to kill them during their larval stage. Government monitors pore over satellite images and observe via helicopter to see where standing water exists. Different species prefer discarded tires, stagnant ponds or something as small as a soda can. If they see an area of particular concern, teams on the ground sample the water to check for the presence of larvae.

If the larvae are caught early, control officers can stop the spread of mosquitoes with little damage to the environment, because they have very mosquito-specific tools. Draining their habitat is sometimes possible. Another option is releasing mosquitofish, which feed on mosquito larvae, into the breeding ground. There are also bacteria that can be introduced into the area to damage the larvae.

If chemicals are required, scientists have developed pesticides that target mosquito growth hormones. They stop the larvae from growing into the adult bloodsuckers we despise, with very little impact on other insects.

The system doesn't always work. In years with sustained rains and high temperatures, mosquitoes beat our early defenses.

In such years, monitors assess the severity of the problem by comparing the population in certain areas with historical data by tracking complaints from residents. Most important, they catch and kill mosquitoes, then screen them for diseases.

If things are bad enough, they send out trucks and helicopters to spray what the workers call “adulticide” — including malathion and permethrin — to distinguish its chemicals from those that kill larvae. In large enough doses they can be harmful to ducks, bees, fish and other animals. They are a last resort.

As any toxicologist knows, however, the dose makes the poison, and we've gotten pretty good at minimizing the volume of adulticide we spray into the air. These days, the drops are only 10 to 20 microns across, and the trucks spray just one ounce of adulticide over an entire acre. At that volume, the Environmental Protection Agency has found no significant evidence that the adulticide is harmful to humans, mammals or amphibians.

Of course, no one can forget the days of DDT, which most mosquito control experts thought was a gift from God, until Rachel Carson's book “Silent Spring” in 1962 showed that it was devastating generations of birds.

But every creature is a piece of the world's environmental puzzle. If scientists could find a way to wipe out all mosquitoes — recent studies have shown that it may be possible to chemically sterilize males and dupe the females into mating with them — would that be a good idea?

There's some evidence that mosquitoes are a useful food for many species of fish, insects, birds, reptiles and amphibians. They also rid waterways of decaying matter. Of course, some other animals might fill those niches in their absence.

I asked Roger Nasci, a mosquito control officer with the Centers for Disease Control and Prevention, for his opinion.

“I've made a good living off them,” he quipped, “but the disease burden of mosquitoes is truly phenomenal. From a humanitarian standpoint, I would eliminate them in a heartbeat.”