Mosquitoes helped Americans defeat British in Revolutionary War

Major combat operations in the American Revolution ended 229 years ago this week, at Yorktown, Va. For that we can thank the fortitude of American forces under George Washington, the siegecraft of French troops of Gen. Jean-Baptiste Donatien de Vimeur, the count of Rochambeau — and the relentless bloodthirstiness of female Anopheles quadrimaculatus mosquitoes.

Those tiny amazons conducted covert biological warfare against the British army. Female mosquitoes seek mammalian blood to provide the proteins they need to make eggs. No blood meal, no reproduction. It makes them bold and determined to bite.

Some anopheles mosquitoes carry the malaria parasite, which they can inject into human bloodstreams when taking their meals. In eastern North America, A. quadrimaculatus was the sole important malaria vector. It carried malaria from person to person, and susceptible humans carried it from mosquito to mosquito. In the 18th century, no one suspected that mosquitoes carried diseases.

Malaria, still one of the most deadly infectious diseases in the world, was a widespread scourge in North America until little more than a century ago. The only people resistant to it were either those of African descent — many of whom had inherited genetic traits that blocked malaria from doing its worst — or folks who had been infected many times, acquiring resistance the hard way. In general, the more bouts you survive, the more resistant you are.

Malaria was all over the American South but especially prevalent in the warm, humid coastlands from Georgia to Maryland, where the climate suited mosquitoes and there were plenty of people (and other mammals) to bite.

In 1779, the British chose a "southern strategy" in their war against rebellious Americans. They had fought inconclusively since 1775, with the British controlling the main ports but unable to hold the countryside.

To break the deadlock, they sent an army of 9,000 men, British and German (known as Hessians to the Americans), to besiege Charleston, S.C. A few victories in the South, they hoped, would inflame Southerners loyal to King George III, causing them to rise up and allow London to "Americanize" the war.

But in the South, A. quadrimaculatus were more numerous and more determined than Loyalists. South Carolina's irrigated rice-plantation economy had made good mosquito country better by creating excellent breeding habitat. Every summer, hungry mosquitoes injected malaria parasites many times into almost everyone in the Lowcountry.

As one German visitor put it, "Carolina in the spring is a paradise, in the summer a hell and in the autumn a hospital." Death rates, especially among small children, spiked every year from August to October as a result of malaria. Those who survived to adulthood were highly resistant.

The British army, commanded by Gen. Charles Cornwallis, consisted of lads from Britain and Germany. Few had grown up with malaria. Most were highly susceptible. Cornwallis' army, although a superior fighting force, suffered from a malaria-resistance gap.

To treat malaria, military physicians normally recommended venesection — draining 20 ounces of blood, about 10 percent of an adult's supply — sometimes supplementing that with doses of mercury or opium, and in one case applying freshly killed pigeons to the soles of patients' feet.

Fortunately, doctors were almost as scarce as hen's teeth. Soldiers could try English folk remedies for ague (as malaria was known) such as eating cobwebs and spiders, drinking one's urine or tying one's hair to a tree trunk and yanking one's head so violently as to leave their hair — and illness — with the tree. These measures did no good but surely did less harm than venesection or a swig of mercury.

Only one thing 18th-century doctors prescribed against malaria did any good: bark. Powdered bark from the cinchona tree, found only on the eastern slopes of the Peruvian Andes, contained alkaloids that checked malaria. But the bark was expensive, and by 1779 increasingly hard for the British to get: Spain controlled the supply and had entered the war against Britain.

Cornwallis' army won most of its battles but suffered heavily from malaria in the summer and fall of 1780. After recovering their health in the winter, the British fled the Carolinas in April 1781 for Virginia, a move that Cornwallis believed might allow him to "preserve the troops from the fatal sickness, which so nearly ruined the Army last autumn."

He headed for healthier upland regions, but his commander in New York ordered him to the Tidewater — malaria country. Cornwallis objected, wondering about the logic of occupying a "sickly defensive post in this Bay." But orders were orders, so Cornwallis started to dig in around Yorktown in midsummer.

By late September, he was besieged by a Franco-American army, recently arrived from New York and New England. After 21 days, Cornwallis surrendered one-quarter of the British forces in North America, quashing British hopes in the war. A British fleet arrived five days later — too late. Cornwallis explained to his superiors that with his "force daily diminished by sickness," he could not resist the siege. He claimed that half his men were too sick to stand duty.

Why didn't the French and Americans fall ill, too? Some did, but far fewer and too late to matter. With malaria, it takes about a month between infectious bite and the onset of symptoms. The British had been absorbing the parasite since June, but the Franco-Americans arrived in the Tidewater in September. So malaria had two extra months to work its mischief in British ranks.

Moreover, most of the Americans had grown up with malaria. Those who had not suffered heavily in the week before the surrender. Once committed to Yorktown, Cornwallis faced a biological-warfare campaign he could not counter. Mosquitoes helped the Americans snatch victory from the jaws of stalemate and win the Revolutionary War, without which there would be no United States of America.

McNeill is a Georgetown University professor of environmental history and author of many books, including "Mosquito Empires: Ecology and War in the Greater Caribbean, 1620-1914."