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Science + Tech

How Mosquitos Shaped Humanity

It’s our apex predator, and its relentless slaughter continues, as traced in this new book.

By Crawford Kilian 29 Aug 2019 | TheTyee.ca

Crawford Kilian is a contributing editor of The Tyee.

The mosquito, Timothy Winegard argues, has killed half of all humans who ever lived — 52 billion of us.

Add all the other mammals and birds it preys upon, and it is clearly the planet’s apex predator. After millions of years in that role, however, the mosquito could be overthrown by its human victims’ ingenuity.

Winegard’s research makes this an important book for anyone concerned with mosquito-borne diseases. As his subtitle suggests, he focuses on how mosquitos have shaped human history. And they have even shaped us genetically. Humans evolved in an Africa where mosquitos had prospered for millions of years. They preyed on our ancestral species, and by the time modern humans emerged we had even evolved sickle-cell anemia as a defensive measure against malaria.

Modern humans who left Africa took malaria and other mosquito-borne diseases with them, infecting the mosquito populations they encountered. Migration and settlement must have come at a terrible price in deaths and sickness. The survivors, after repeated infections, might become “seasoned” — able to recover from malaria relatively easily. But if they moved into a new region, an unfamiliar strain of malaria might still finish them off.

Winegard’s focus is on the mosquito’s role in human warfare and conquest. It could play both offence and defence. Ancient Rome, for example, was surrounded by the Pontine Marshes, so malarious that only a few sickly inhabitants scratched out a living there. Invaders who besieged the city soon fell sick and died. Even those who sacked Rome did so in hit-and-run operations rather than lose their armies to disease.

Not until Mussolini drained the Pontine Marshes in the 1930s did malaria numbers fall in the region. But when the Allies approached Rome in 1943, the Nazis filled the marshes with both water and malarious mosquitos to slow down the advance.

Winegard argues that mosquitos also influenced the Mongols’ decision not to occupy Europe, though it’s also arguable that Europe couldn’t provide enough grassland for an empire on horseback.

Less debatable was the mosquito’s role in the “Columbian exchange” after 1492, as Europe, Asia and Africa swapped plants, animals and insects. The original peoples who had settled the Americas hadn’t carried malaria with them, but the Spanish and Portuguese certainly did. Along with other European diseases, malaria swept through the Americas.

With no exposure to such diseases in perhaps 20,000 years, the millions living in the Americas were reduced by as much as 90 per cent in a century. Most of them died without ever seeing a European, killed by malaria and other diseases moving swiftly along ancient trade and travel routes.

This put the European conquest at risk: How were they to get rich out of the New World if they had to do the farming and mining themselves? The whole point was to get someone else to do the work.

Enslaved Africans were the answer, and Montezuma’s real revenge. Relatively immune to mosquito-borne diseases, they replaced the lost Indigenous peoples as a labour force while also importing malaria strains their European masters couldn’t cope with. Despite the death toll on both sides, enough Africans survived to make the European survivors (or their heirs) very rich.

In the process, however, Europeans died by the thousands of malaria and yellow fever (also known as “the black vomit”). Yellow fever forced the new American republic to abandon Philadelphia as the national capital and build a new capital (with slave labour) on the swampy banks of the Potomac River.

Winegard argues that malaria-seasoned colonials had an advantage over their imperial rulers when revolution swept the hemisphere. Fresh British troops died by the thousands, helping American colonials gain their independence. The same thing happened to Spanish soldiers sent to suppress uprisings from Mexico to Argentina.

Through all these military and public health disasters, the mosquito was seen as no more than an associated nuisance. Malaria was literally “bad air,” a miasma rising from stagnant waters like other diseases. After enduring mosquito attacks for 200,000 years or more, we were surprisingly slow to recognize the real threat they posed.

When we did, it was not to improve the health of the public, but to preserve that of soldiers. In the Civil War, the U.S. and the Confederacy had lost more men to the mosquito than to one another. The South in particular had relied on its local strains of malaria to weaken invading Union forces, but even local strains took their toll on Southern troops.

When the U.S. picked a fight with Spain in 1898, Pasteur’s germ theory of disease was revolutionizing medicine. Cuba and the Philippines were mosquito-rich, and U.S. army doctors soon confirmed mosquitos are the vectors of malaria and yellow fever.

That led to rapid reductions in the incidence of those diseases (a vaccine for yellow fever was soon developed), but mosquitos still discouraged the U.S. from annexing Cuba outright. Still, like the Philippines, it remained a client of the U.S. for half a century.

The Second World War brought renewed interest in fighting malaria, whether in North Africa, southern Europe or the Pacific — and in using the mosquito as a weapon of biological warfare. New drugs were only a partial answer: U.S. troops hated atabrine, chloroquine has nasty side effects, and mefloquine has been associated with brain damage mimicking PTSD in Canadian and other soldiers.

Worse yet, mosquitos and their malaria-causing parasites have rapidly developed immunity to such drugs, and even to DDT, which seemed for a while to have beaten them. A Chinese-developed anti-malarial, artemisinin, saw brilliant success but has also triggered resistance. As a result, more than 600,000 people have died of malaria so far this year.

Meanwhile, mosquitos have acquired other nasty diseases, like dengue, which so far this year has infected at least two million people in Latin America and the Caribbean and killed over 700. Zika virus, which emerged from an African forest only about 60 years ago, has travelled across South Asia and the Pacific into the Americas; in pregnant women it inflicts microcephaly and other deformities on their babies. (Brazil recently coped with Zika and yellow fever almost simultaneously.)

With climate change, such diseases will move back into the U.S. and even into Canada, imported first by travellers and then spreading through local mosquito populations. Mosquitos have so far been able to develop resistance to everything we throw at them, so new drugs may soon become useless.

The temptation now, Winegard tells us, is to tinker with mosquito genetics, making the insects unable to transfer malaria and other diseases to humans. Or failing that, to sterilize them and effectively wipe them off the planet.

That too could backfire: only a handful of mosquito species out of thousands are a threat, and eradicating even those might have unintended environmental consequences. Still, the opportunity to end the mosquito’s predation on humans after 200,000 years will look like one we must seize.  [Tyee]

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