Worldwide, 210 million people are infected // Half a million children are killed each year // But the question remains: How do you wipe out a deadly disease?
Smoking Out Malaria
Living in the United States, it’s easy not to think about malaria, which ceased being a real problem here more than half a century ago. Yet this ancient scourge, which is thought to have killed more people than any other single cause, still ravages parts of Africa, Asia and Latin America. Half of the world’s population lives in malarial zones, and some 210 million people harbor malaria every year. Malaria kills more than half a million children under the age of five each year, mostly in sub-Saharan Africa. Yet many experts now think that as complex and dangerous a disease as malaria is, the time may be at hand to put an end to it—even though failure could be disastrous.
A previous global campaign to eradicate malaria began with enthusiasm, hubris and elation in the 1950s. It ended a decade later in defeat and despair. So it was a shock when the philanthropists Melinda and Bill Gates announced at their charitable foundation’s Malaria Forum in October 2007 a brazen goal to dig out malaria’s roots everywhere on earth, even in the farthest reaches of sub-Saharan Africa (where the 1950s campaign did not aspire to go). Others at the forum were even more surprised when Margaret Chan, director general of the World Health Organization, pledged her support and dared everyone to join. “The research community was galvanized,” says Ed Ryan, director of tropical medicine at Massachusetts General Hospital. “How do you wipe a disease off the face of the earth?”
For smallpox, polio and most other infectious diseases, that question had a relatively simple answer. Because they’re caused by viruses and are passed from person to person, everyone within an infected person’s circle of interactions can be vaccinated or treated. When no more circles appear, the disease is gone forever. But malaria is different. It is a mosquito-borne illness caused by a parasite that cycles relentlessly between humans and the mosquitoes that frequent rainy seasons and tropical regions. You can’t draw circles around swarming mosquitoes, and just one parasite from one infected mosquito can generate billions of offspring within weeks—and if just one infected person transmits a parasite to one mosquito, that mosquito can infect many more people. What’s more, that’s only the beginning of the challenges of eradicating malaria.
When a mosquito transmits Plasmodium parasites to a human, other mosquitoes pick up the parasites when they bite that same victim. Infected mosquitoes carry tiny parasitic forms called sporozoites in their salivary glands, and when they bite, 100 or so sporozoites penetrate the skin and invade blood vessels, swimming to the liver. That initial transmission causes infection, but not yet disease, and this “pre-blood” stage is a bottleneck in the malarial life cycle when it might be possible for a drug or vaccine to kill the limited number of parasites that have been transmitted.