sickle cell anemia -- 2/19/20

Today's selection -- from The Mosquito by Timothy Winegard. One in 12, or 4.2 million, African Americans currently possess the sickle cell trait. Sickle cell disease predominantly affects people of African descent, and is believed to have evolved as a defense against a particularly lethal form of malaria. The trade-off was a highly shortened lifespan. One carrier is the American football player Ryan Clark, who was diagnosed with the disease in 2007. Sickle cell nearly killed him and forced his retirement from football in 2014:

"The evolutionary de­sign that nearly killed Ryan Clark was initially a lifesaving human ge­netic adaptation. Clark's sickle cell, which first appeared in Africa 7,300 years ago in a female known to anthropologists as 'Sickle Cell Eve,' is the most recent and well-known genetic response to falciparum malaria.

"This first appearance of sickle cell was a direct result of expansive agriculture encroaching on formerly undisturbed mosquito habitats. Roughly 8,000 years ago, pioneering Bantu farmers began concentrated yam and plantain cultivation. This pastoral intensification in West Cen­tral Africa along the Niger River delta, slashing south to the Congo River, awoke the mosquito from her isolated slumber. The consequences could hardly have been more catastrophic: Vampiric falciparum malaria was unleashed on its new human host. Within only 700 years, our im­mediate evolutionary counteroffensive, which bewildered the parasite, was to promote a random mutation of the hemoglobin -- the cell became sickle (or crescent) shaped. Normally, healthy red blood cells are cast from a donut or oval template. The malaria parasite cannot latch on to the strange-shaped sickle cell.

Clark with the Redskins in 2014

"Children inheriting sickle cell from one parent and the normal gene from the other, known as sickle cell trait, of which Ryan Clark is a carrier, are blessed with 90% immunity from falciparum malaria. The downside (prior to modern medicine) was that the average life span of those carrying sickle cell trait was a brief twenty-three years. This would have been a great trade-off, however, in what anthropologists call our 'ancestral environment,' where life spans were relatively short -- twenty­-three years is certainly long enough to pass on the trait to 50% of any offspring. In the modern era, however, this genetic interceptor and safety against falciparum malaria turns out to be a severe health impediment for modern NFL players, or anyone else who is a carrier and would like to live to the ripe old age of, say, twenty-four. The other downside within this Punnett square hereditary matrix is that 25% of offspring would receive no sickle cell and therefore no immunity, with the other 25% receiving two sickle cell genes. Those born with sickle cell from both parents, or sickle cell disease (which killed Ryan Clark's sister-in-law two weeks after he hoisted the NFL championship Lombardi Trophy), inherit a death sentence, with the vast majority dying in infancy.

"While it now seems inconceivable, in areas of Africa that were dev­astated by unremitting falciparum malaria, the death toll from sickle cell resulted from an advantage, or alternatively was an acceptable cost, for survival, compared to what must have been apocalyptic rates of malarial mortality. Despite the influx of sickle cell, the preadult death rate prior to 1500 was upwards of 55% in sub-Saharan Africa. ...

"The genetic distribution of sickle cell shadowed the spread of hu­mans, mosquitoes, and malaria in and out of Africa. Today, there are about 50-60 million carriers of sickle cell worldwide, with 80% still liv­ing in its birthplace, sub-Saharan Africa. Regionally, there are pockets in Africa, the Middle East, and South Asia where upwards of 40% of the population harbor the sickle cell gene. The modern global diffusion of sickle cell is a hereditary reminder of our deadly and protracted war with the mosquito."


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author:

Timothy C. Winegard

title:

The Mosquito: A Human History of Our Deadliest Predator

publisher:

Dutton, an impirnt of Penguin Random House

date:

Copyright 2019 by Timothy C. Winegard

pages:

32-33
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