The Malaria Story

It begins with a bite, a painless bite. The mosquito comes in the night, alights on an exposed patch of flesh, and assumes the hunched, head-lowered posture of a sprinter in the starting blocks. Then she plunges her stiletto mouthparts into the skin.

The mosquito has long, filament-thin legs and dappled wings; she's of the genus Anopheles, the only insect capable of harboring the human malaria parasite. And she's definitely a she: Male mosquitoes have no interest in blood, while females depend on protein-rich hemoglobin to nourish their eggs. A mosquito's proboscis appears spike-solid, but it's actually a sheath of separate toolscutting blades and a feeding tube powered by two tiny pumps. She drills through the epidermis, then through a thin layer of fat, then into the network of blood-filled micro-capillaries. She starts to drink.

To inhibit the blood from coagulating, the mosquito oils the bite area with a spray of saliva. This is when it happens. Carried in the mosquito's salivary glands -- and entering the body with the lubricating squirt -- are minute, wormlike creatures. These are the one-celled malaria parasites, known as plasmodia. Fifty thousand of them could swim in a pool the size of the period at the end of this sentence. Typically, a couple of dozen slip into the bloodstream. But it takes just one. A single plasmodium is enough to kill a person.

The parasites remain in the bloodstream for only a few minutes. They ride the flume of the circulatory system to the liver. There they stop. Each plasmodium burrows into a different liver cell. Almost certainly, the person who has been bitten hardly stirs from sleep. And for the next week or two, there's no overt sign that something in the body has just gone horribly wrong.

...

From the mosquito's salivary glands to the host's liver cell: a quiet trip. Everything seems fine. Even the liver itself, that reddish sack of blood-filtering cells, shows no sign of trouble. It's only in those few rooms whose locks have been picked by falciparum where all is pandemonium. Inside these cells, the malaria parasites eat and multiply. They do this nonstop for about a week, until the cell's original contents have been entirely digested and it is bulging with parasites like a soup can gone bad. Each falciparum that entered the body has now replicated itself 40,000 times.

The cells explode. A riot of parasites is set loose in the bloodstream. Within 30 seconds, though, the parasites have again entered the safe houses of cells -- this time, each has drilled into a red blood cell, flowing through the circulatory system. Over the next two days, the parasites continue to devour and proliferate stealthily. After they have consumed the invaded cells, they burst out again, and once more there is bedlam in the blood.

For the first time, the body realizes it has been ambushed. Headache and muscle pains are a sign that the immune system has been triggered. But if this is the victim's first bout of malaria, the immune response is mostly ineffective. The alarm has sounded, but the thieves are already under the bed: The parasites swiftly invade a new set of blood cells, and the sequence of reproduction and release continues.

Now the internal temperature begins to rise as the body attempts to cook away the invaders. Shivering sets in -- muscle vibrations generate warmth. This is followed by severe fever, then drenching sweat. Cold, hot, wet; the symptoms are a hallmark of the disease. But the parasites' exponential growth continues, and after a few more cycles there are billions of them tumbling about the blood.

By this point, the fever has reached maximum intensity. The body is practically boiling itself to death -- anything to halt the attack -- but to no avail. The parasites can even commandeer blood cells to help aid their survival. In some cases of falciparum, infected cells sprout Velcro-like knobs on their surfaces, and as these cells pass through the capillaries of the brain, they latch to the sides. The adhesion keeps them from washing into the spleen, which cleans the blood by shredding damaged cells. Somehow -- no one is quite sure how -- the adhesion also causes the brain to swell. The infection has turned into cerebral malaria, the most feared manifestation of the disease.

This is when the body starts to break down. The parasites have destroyed so many oxygen-carrying red cells that too few are left to sustain vital functions. The lungs fight for breath, and the heart struggles to pump. The blood acidifies. Brain cells die. The child struggles and convulses and finally falls into a coma.

Extracted from Bedlam in the Blood: Malaria, National Geographic, July 2007. (local copy)

For you mathematicians out there, note the use of the phrase "exponential growth". I've often noted the use of this phrase, and sometimes it's appropriate; sometimes it's not. One interesting question, then, is the extent to which it's appropriate in this context?


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