The bite that binds

The bite that binds

Anopheles funestus • Photo by James Gathany, Dr. Frank Collins, University of Notre Dame (Public Domain)

Originally published 21 January 2007

Humans and mos­qui­tos share an ances­tor in deep time. Our kin­ship is revealed at the lev­el of the genes. Cer­tain­ly, we are enough alike chem­i­cal­ly so that human blood pro­tein is use­ful to the mosquito.

And, of course, we share the over­ar­ch­ing pur­pose of all liv­ing crea­tures — to prop­a­gate our species.

If that were not enough, our lives are bound togeth­er by the malar­ia par­a­site, a nasty lit­tle crea­ture named Plas­mod­i­um fal­ci­parum.

Plas­mod­i­um needs us. It needs an Anophe­les mosquito.

There was a bit of a malar­ia scare on the island this past sum­mer. Some­one showed up here with the par­a­site in his blood. He was bit­ten. The infect­ed mos­qui­to bit anoth­er. More mos­qui­tos entered the pic­ture as Plas­mod­i­um was passed around. Before it was over, a dozen peo­ple were infected.

It is only the female mos­qui­to we need wor­ry about, and only mos­qui­tos of the genus Anophe­les. Male mos­qui­tos have just 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. So you can for­get the male mos­qui­to; he isn’t part of the story.

And what a story!

The female mos­qui­to 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. Espe­cial­ly on this island where here are so few oth­er mam­mals to share the bites. She may take more than one blood meal dur­ing her life­time, which can have unfor­tu­nate con­se­quences for her sec­ond victim.

When a malar­ia-infect­ed mos­qui­to 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 spore­like form, called a mero­zoite, and repli­cates 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 mos­qui­to. The Plas­mod­i­um game­to­cytes are sucked up 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, spermlike 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.

This is the con­densed ver­sion of the sto­ry. I left out, for exam­ple, the tricks Plas­mod­i­um plays to out­wit our immune system.

Humans are nec­es­sary to Plas­mod­i­um’s life cycle as sources of food and places for explo­sive asex­u­al repro­duc­tion. The mos­qui­to is nec­es­sary as a vehi­cle for trans­port and as 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 dis­cov­er or invent drugs to attack the pathogen or mit­i­gate the symp­toms of dis­ease. We drain swamps where Anophe­les lays her eggs. We spray with DDT. We dis­trib­ute insec­ti­cide-impre­gant­ed bed nets in places where the dis­ease is endemic.

And for a while, these strat­a­gems work. Some places, the Unit­ed States for exam­ple, are malar­ia-free. Except for a brief peri­od this past sum­mer, the island of Exu­ma was malar­ia free. Jef­fery Sachs recent­ly esti­mat­ed that with a cou­ple of bil­lion dol­lars and polit­i­cal will on the part of rich nations malar­ia could be made history.

But Anophe­les and Plas­mod­i­um are not with­out resources of their own — most par­tic­u­lar­ly their quick repro­duc­tive cycles. With­in mere decades, Anophe­les has evolved immu­ni­ty against pes­ti­cides. Plas­mod­i­um has evolved resis­tance to drugs. After a peri­od of decline world­wide, malar­ia is resur­gent. Today, malar­ia is the lead­ing cause of death for chil­dren under five.

And so, the evo­lu­tion­ary riv­er that flows out of Eden binds us togeth­er in a com­mon fate — human, mos­qui­to, malar­ia pro­to­zoa. Con­scious­ly or uncon­scious­ly, each of us is doing every­thing we can to increase the odds for our own survival.

Slap!

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