Killer mosquitoes - mosquito bites can be deadly - includes article on how malaria is spread and also fighting a mighty bite - Cover Story

Science World, March 11, 1998 by Rachel Rivera

Its buzz in your ear can make your skin crawl. Its bite can make you scratch for days. And for growing millions of people around the world, the mosquito can be deadly.

Simon Osoro lives in Kisumu, a large city in western Kenya, like teens everywhere, the 16-year-old African loves to go to the movies, play hockey, spend time with friends, and read novels. But Simon had to put his life on hold last November, when he became sick with a disease that will attack a shocking 500 million people around the world this year: malaria

The disease is caused by a single-celled parasite (an organism that lives in or on other organisms), transmitted by a single bite of the mosquito, that tiny winged insect everyone loves to hate. Usually mosquito bites are a harmless nuisance. But when the insect carries parasites that cause malaria, or viruses that cause yellow fever, encephalitis, and dengue (DEN-gee) fever, one bite spells trouble. Malaria is rampant where Simon lives, and in other regions of Africa, Asia, the Caribbean, and South America.

"I suspect I got the bite in the dormitory in school," Simon told Science World.

"After the El Nino rains, the grassy area around our dormitory was full of stagnant water. Mosquitoes breed there. Other boys were also sick with malaria. I think the mosquito that bit me bit other sick boys first. Then I got sick, too."

In the 1950s and '60s, insecticide programs seemed to squelch the disease. Now malaria is back in frightening force. The number of new cases (90 percent of them in Africa) has quadrupled in the last five years. The disease kills nearly 3 million people globally every year, claims the World Health Organization.

In the U.S., about 80 cases of locally acquired malaria have been reported in the last 20 years. In these cases, mosquitoes may have first bitten someone already infected with malaria from abroad.

Scientists are desperately trying to fight back. They're developing strategies to decimate the world's 3,000 species of mosquitoes.

Why are mosquitoes--and the diseases they spread--so tough to beat?

BLOOD SPORT

To begin with, it's the female mosquito who's after blood. She needs the protein that animal and human blood provides to produce her eggs. Males feed only on plant juices. They're not equipped with the delicate hardware--piercing mouthparts--that females have evolved to do their dirty work.

To locate a host, a human to prey on, the female mosquito employs receptor cells on her antennae. She can whiff the carbon dioxide a potential blood donor exhales as far away as 60 meters (200 ft). As she flies closer, body heat, sweat, lactic acid, and about 40 other chemical compounds emitted by skin lure her to her prey.

If a female mosquito lands on your skin, she uses her proboscis (pruh-BAW-sis), a strong, needlelike sucking tube, to puncture flesh and probe for a blood vessel. That's not as easy as it sounds. Less than 5 percent of the dermis, skin's middle layer, contains blood vessels, so the mosquito prods around until she pinpoints a blood vessel to pierce.

Once she scores, she injects you with her saliva, which contains anti-coagulants, agents that stop blood from clotting and keep it flowing while the mosquito gorges herself. It's the allergic reaction to mosquito drool that results in swelling and itching. And if the mosquito is infected with a parasite such as malaria, her saliva can transmit the infection to her victim.

One blood meal supplies the nutritional boost a female mosquito needs to produce as many as 250 eggs! She lays eggs in water anywhere, from buckets to marshlands. Her eggs hatch in two days to a few months, but some species' eggs can lay dormant for years until water conditions are right for hatching.

How do mosquitoes go from water babies to flying insects? The larvae, wormlike forms that hatch from insect eggs, feed on microorganisms from water, and breathe air through a siphon that pierces the water's surface. After four to 10 days a larva curls into a pupa, a non-feeding stage, during which the mosquito metamorphoses (changes life form) into a flying adult. The pupa rises to the water surface, and the adult mosquito Pulls itself out of the pupal husk and whizzes into the air.

MALARIA WARS

Anopheles gambiae is the main mosquito species that carries malaria in Africa. How did Simon know he was infected? "First I felt unusually tired," he says. "Then I got chills, a headache, and a high temperature." Where Simon lives, those symptoms often indicate malaria.

Simon's bloodstream circulated the parasite into the liver, where the parasite invades cells and multiplies. After a week or two, parasites recirculate in the blood, penetrate red blood cells, and reproduce thousands of times. The disease leads to bouts of fever and anemia (depleted red blood cells), clogged blood vessels, and damaged vital organs, all of which can cause death if not treated quickly.

Simon's early symptoms were followed by vomiting, diarrhea, and joint pains. His skin became scaly and he lost a lot of weight. At first the medications he took didn't work; malaria parasites have become increasingly resistant to drugs. Finally, Simon was given quinine, a drug used since the 1600s to relieve severe malaria symptoms. "This made me feel better, and finally I got well," he says.