Originally published 22 July 1996
Here’s something to think about while you’re slapping mosquitoes this summer. We have more in common with these creatures than you might think.
We share an ancestor in deep time. Our kinship is revealed by molecular biologists at the level of the genes; it appears that we share some basic genetic machinery. Certainly, we are chemically enough alike so that human blood protein is useful to the mosquito.
We share the overarching purpose of all living creatures — to propagate our species.
And our lives are bound together by the malaria parasite.
In our part of the world, we have more or less forgotten about malaria. However, several hundred million people worldwide suffer from this debilitating illness. More than 1 million die each year, mostly children. All because of a microscopic creature called Plasmodium that has a remarkable lifestyle.
Follow along for a moment.
The mosquito that is pestering you has only one thing on her mind: a blood meal.
It is definitely a she. Male mosquitoes have two things on their minds: sex and nectar. And only enough nectar to have the energy for sex. The male mosquito isn’t interested in blood.
The female needs protein-rich blood to nourish her eggs. It need not be human blood, but increasingly we have made ourselves the most readily available item on her menu.
She may take more than one blood meal during her lifetime, which, as we shall see, can have unfortunate consequences for her second victim.
When a malaria-infected mosquito of the genus Anopheles bites a human, she injects a bit a saliva into the wound to keep the victim’s blood flowing freely. In the saliva are thousands of tiny threadlike creatures called sporozoites.
The sporozoites are carried by the bloodstream to the liver, where they leave the blood and penetrate a liver cell. Inside, the sporozoite transforms itself into a spore-like form, called a merozoite, and replicates itself over and over, destroying the liver cell and building a cyst that is jam-packed with merozoites.
After two weeks, the cyst bursts and spews its contents into the bloodstream. Each merozoite attaches itself to a human red blood cell and enters it. The merozoite feeds on hemoglobin, growing bigger, until it shatters into bits, each of which forms another merozoite. At last, the teeming blood cell explodes its contents back into the blood stream.
The process is repeated over and over, while the human host goes through bouts of fever. But now, an amazing thing happens. After several cycles of replication, some of the merozoites become sexual forms called gametocytes, male and female. These circulate in the bloodstream until the host is bitten by another Anopheles mosquito. The Plasmodium gametocytes are sucked up by the mosquito along with their asexual companions.
The blood-gorged Anopheles flies away from its human victim. Inside the mosquito’s stomach the asexual merozoites die. Male gametocytes turn themselves into swarms of lashing, sperm-like filaments, which penetrate the female gametocytes and fertilize them.
Each fertilized “egg” now transforms itself into a creepy-crawly thing that bores through the mosquito’s stomach wall, where it attaches itself on the outside of the stomach wall and becomes a cyst. Within the cyst, the material of the “egg” reorganizes itself into thousands of threadlike forms, the sporozoites. The cyst bursts, the sporozoites make their way to the mosquito’s salivary gland, and…
…and wait like bullets in a loaded gun for the mosquito to have her second blood meal.
A new victim, a new life cycle for Plasmodium.
This is the condensed version of the story. Left out, for example, are the tricks Plasmodium employs to outwit our immune system.
Humans are necessary to Plasmodium’s life cycle, as cafeterias for feeding and places of explosive asexual reproduction. The mosquito is necessary as a vehicle for transport and a bower for sexual expression.
It is often said that mosquitoes kill more humans than any other animal, but it is not mosquitoes that kill. It is the parasites they carry, most particularly the malaria pathogen, Plasmodium.
With our clever brains and scientific skills we are not passive victims. We marshal our resources. We discover or invent drugs to attack the pathogen or mitigate the symptoms of disease. We drain the swamps where Anopheles lays her eggs. We spray with DDT.
And for a while, these stratagems worked. Some places, the United States for example, have become malaria-free. The incidence of malaria worldwide temporarily diminished. But Anopheles and Plasmodium are not without resources of their own — most particularly their quick reproductive cycles. The spirit of Darwin brushes them with its favoring wings.
Within mere decades, Anopheles has evolved immunity against pesticides. Plasmodium has evolved resistance to drugs. After a period of decline, malaria is resurgent. Today, malaria is the leading cause of death worldwide for children under five.
All of this is something to ponder this summer as mosquitoes buzz. The evolutionary river that flows out of Eden binds us together in a common fate — human, mosquito, malaria protozoa. Each of us is doing everything we can to increase the odds for our own survival.
Slap!
