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In recent months, millions of Aedes aegypti mosquitos have been at work spreading the Zika virus in South and Central America. This summer, millions more, all capable of conveying the virus, will flit and bite throughout the southern United States. Congress just approved funding to battle its spread. This is not the first time a mosquito-borne virus has broken loose in the Americas and it will not likely be the last. Indeed, mosquitoes and viruses have shaped the history of our hemisphere in surprising ways for centuries.
Before 1492, Aedes aegypti did not live in the Americas. It came from West Africa as part of the Columbian Exchange, probably on ships of the transatlantic slave trade. The mosquito gradually colonized those parts of the Americas that suited its feeding and breeding requirements, and for centuries served as the primary carrier for yellow fever and dengue, viruses that are cousins of Zika.
Aedes aegypti is a peculiar and fussy mosquito. It has a strong preference for human blood—rare but not unique among mosquitoes—which makes it an efficient spreader of human disease. It lays its eggs in artificial water containers such as pots, cans, barrels, wells or cisterns. This preference for human activities distinguishes it from the thousands of other mosquito species. Aedes aegypti is, in effect, a domesticated animal.
Together these mosquitos and their fevers decided the fate of empires. In 1697 the kingdom of Scotland attempted to establish a trading colony on the Caribbean shore of Panama. New Caledonia was intended to position Scots to take advantage of Pacific and Atlantic trade networks. A large share of the liquid capital of Scotland and 2,500 eager volunteers went into the effort. Within two years, however, some 70 percent of the Scots were dead of “fever.” The Scots’ immune systems were unprepared for yellow fever, dengue and malaria—any or all of which might have attacked them—and they paid the price. So did Scotland, which in 1707 accepted union with England partly to pay debts incurred by the disaster.
These tiny mosquitos and their tinier viruses helped to undermine the grand plans of empires in the Americas for the next century. In 1763, France had just lost Canada in war to Britain and hoped to regain its position in the Americas with a new colony in what is now French Guiana. Some 11,000 hopeful souls were recruited from France and elsewhere in Europe. Like the hapless Scots, their immune systems had no previous experience with yellow fever or dengue (and in most cases none with malaria either). They too sailed into prime Aedes habitat. Within 18 months, 85 to 90 percent of them had died from disease, with yellow fever playing the largest role.
The British also lost thousands of troops to mosquito-borne fevers. They tried to take the Spanish strongholds of Cartegena (Colombia) and Santiago de Cuba in 1741 and ’42, but gave up after diseases killed most of their soldiers. Twenty years later, in another war, yellow fever proved a disaster when they finally took Havana. The lexicographer and man of letters Samuel Johnson wrote, “May my country never be cursed with another such conquest!” At the subsequent peace conference, Britain eagerly handed Havana back to Spain.
By the end of the 18th century, the mosquitoes were not just intervening in imperial schemes, they were helping the Americas win their liberty. Yellow fever and malaria ravaged European armies sent to prevent revolution in what is now Haiti and Venezuela, leading to the creation of independent countries.
Even the U.S. owes its independence in part to mosquitoes and malaria. In 1780, the southern colonies, a region with widespread malaria, became a decisive theater in the American Revolution. British troops had almost no experience with malaria, and thus no resistance to it. American militiamen, and much of the Continental Army, had grown up in the South and faced malaria every summer of their lives. So in the summer of 1780, the British Army hosted its own malaria epidemic, which was particularly intense in the South Carolina Lowcountry. At times, half the British Army was too sick to move. No one knew that mosquitoes carried malaria, and the British did not have the means to combat it.
In 1781, the British commander in the South, Lord Cornwallis, decided to move his army north, into the hills of Virginia, in order to avoid “the fatal sickness which so nearly ruined the army” the summer before. His superiors, however, ordered him to move to the tidewater, and so in June, Cornwallis dug in at Yorktown.
In the warm months, mosquitoes (including a malaria vector species called Anopheles quadrimaculatus) started to bite and by late summer of 1781, malaria had taken hold of his army once again. Some 51 percent of his men were too sick to stand duty, unable to conduct the counter-siege operations that Cornwallis knew were required. American and French forces penned the troops in until Cornwallis surrendered in October, which in effect decided the outcome of the American Revolution.
The Continental Army and its French allies stayed healthy until the surrender, mainly because they had only recently arrived in Virginia (from New England) and malaria had not had time to do its worst. (Many of them were also resistant from prior experience with malaria). Thus, mosquitoes and malaria helped win American independence.
Mosquitoes only lost their political importance after medical researchers realized that they were spreading the fevers. The first to publish the idea that Aedes aegypti could carry yellow fever was a Cuban doctor, Carlos Finlay. U.S. military doctors led by Walter Reed confirmed Finlay’s hypothesis. Armed with this knowledge, when the U.S. Army occupied Cuba (after 1898) and Panama (after 1903) they made life miserable for Aedes aegypti—covering up water containers and putting a drop of kerosene into those without covers. Within a couple of years, mosquito control had banished yellow fever from Cuba and Panama’s Canal Zone.
Over the next 70 years or so, mosquito control acquired ever more weapons. Insecticides, such as DDT—brought to bear in the 1940s—proved deadly to all mosquitoes (and many other creatures too). Aedes aegypti, because of its fondness for human settlements, fell victim to spraying campaigns more easily than did most other mosquitoes.
But Aedes aegypti control proved too successful for its own good. Once the mosquito populations had fallen drastically, and the risk of yellow fever and dengue diminished, the logic of paying for continued mosquito control weakened. Budgets were redirected away from mosquito control all over the Americas. On top of that, the nasty side effects of DDT and other insecticides became well known in the 1960s.
Had the Zika virus come to the Americas in the 1930s or 1950s its prospects would have been poor—Aedes aegypti was under control. But since the 1980s Aedes aegypti has made a dramatic comeback in the Americas. While the main reason is the lapse in mosquito control, another reason is the warming climate, which slowly extends the range of the mosquito. Today, Zika’s chances of spreading widely among human populations via Aedes aegypti are far greater. And it will have help from Aedes albopictus, another mosquito capable of transmitting the virus, which arrived from East Asia in the 1980s. Aedes albopictus has a wider range in the U.S. than Aedes aegypti and potentially could spread Zika to more northerly states. Fortunately it is less efficient as a disease vector.
Combatting Zika will require mosquito control, and the political difficulty that arouses shows a defiant aspect of the American character to which mosquitoes and malaria gave free rein. Malaria may have helped Americans win the revolution in 1780-81, but their descendants cherish their liberty and say, in effect, "don't tread on me" when told to cover water containers. Any attempt to spray pesticides in our democracy quickly excites opposition. Eventually, perhaps, a vaccine will sideline Zika, but until then these coming summers give the virus a chance to run amok and mosquitos to again make history.
John R McNeill is professor of history at Georgetown University. His book Mosquito Empires: Ecology and War in the Greater Caribbean 1620-1914, won the 2010 Albert J. Beveridge Award from the American Historical Association.