Aedes Rides Again: Mosquitoes and Flaviviruses in the Americas

In recent months, millions of Aedes aegypti have been at work spreading the Zika virus in South and Central America. The effects amount to an unexpected but sizeable public health challenge that potentially threatens every landscape and population where A aegypti live (and possibly Aedes albopictus as well, which may be a competent vector for Zika). This is not the first time that an imported flavivirus spread by A aegypti has broken loose in the Americas. Zika belongs to a formidable family of mosquito-borne viruses, which, over the last few centuries, has shaped the social and political history of the Americas.¹

Zika is following in distinguished footsteps. Dengue, also transmitted chiefly by A aegypti and A albopictus, long affected tropical Africa and Asia. Sometime in the last 500 years (no one can say just when) it made a seaborne crossing to the Americas. Some dengue strains arrived only recently, in the 1970s and 1980s, via air traffic. Yellow fever, for which A aegypti also serve as the primary vector, is African in origin and has rarely, if ever, been observed in Asia. It arrived in the Americas no later than 1647 and perhaps in the wake of Christopher Columbus. No one knows precisely how these viruses and their mosquito vector crossed from Africa to the Americas, but the likeliest hypothesis is that they hitched a ride on some of the many thousands of slave ships that crossed the Atlantic. Dengue and yellow fever started to shape history in the Americas at the end of the 17th century.

PREVENTING NEW COLONIES

In 1697 to 1698 the still-independent kingdom of Scotland attempted to establish a trading colony on the Caribbean shore of Panama at Darien. Caledonia, as they called their prospective colony, was intended to position Scots to take advantage of Pacific and Atlantic trade networks. Some 2500 eager volunteers, and a large share of the liquid capital of Scotland, went into the effort. Within two years, however, some 70% of the Scots were dead of “fever.” That probably means a variety of infections, likely including dengue, yellow fever, and malaria (also a newcomer to the Americas). Virtually no one from Scotland had ever encountered any of these diseases before. So their immune systems were unprepared for what they found, and they paid the price. So did Scotland, which in 1707 accepted union with England partly to pay debts incurred by the disaster at Darien.

In 1763, the French government decided to plant a sizeable colony along the banks of the Kourou River in what is now French Guiana. France had just lost Canada in war and hoped to regain its position in the Americas with a new colony. Some 11 000 hopeful souls were found for the scheme, recruited from France and elsewhere in Europe. Like the hapless Scots, their immune systems had no previous experience of yellow fever or dengue (and in most cases none of malaria either). They too sailed into excellent Aedes and Anopheles habitat. Within 18 months, 85% to 90% of them had died from disease, with yellow fever playing the largest role. Unlike Scotland, France kept a tenuous hold on the region, but never met its goal of restoring its power in the American hemisphere.

In these two examples, mosquito-borne diseases, including yellow fever and dengue, prevented the establishment of new colonies in the American tropics. In the next examples, the same infections helped to preserve colonies, specifically Spanish ones, against British attack.

PRESERVING SPANISH COLONIES

Cartagena, in Colombia, was one of the key ports in the Spanish empire in the Americas. As such, it excited the attention of military planners in London, England, in 1739 when Britain and Spain went to war. In 1741, Britain mounted the largest amphibious force in the history of warfare to that date in an assault on Cartagena, some 29 000 men in all. They came mainly from the British Isles, meaning, once again, that few if any were survivors of yellow fever or dengue, and those with malarial experience carried resistance to vivax but not falciparum. They laid siege to Cartagena in March of 1741 and in April survivors sailed away, having lost thousands of comrades to fevers. They tried their luck at another port, Santiago de Cuba, but again failed in their objective. When they called a halt to the whole expedition, 22 000 were dead, of whom 21 000 (72% of the total) died from disease. As usual, several acute infectious diseases were surely involved, but the documentary evidence makes it clear that yellow fever was most prominent among them.

British war planners did not learn their lesson. In 1762, in another war against Spain, they launched another amphibious operation, this time against Havana, Cuba, the linchpin of Spain’s defense system in the Americas. This time fortune favored the British—for a while. They conquered Havana just as fevers took hold among their soldiers and sailors. Within weeks of the capture, 10 000 of 14 000 were dead (fewer than 700 died in combat). Again, the documentation makes it clear the yellow fever led the charge against the British invasion. 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.

LIBERATING COLONIES

Yellow fever, dengue, and malaria also helped to liberate colonies in the Americas. By the end of the 18th century, populations born and raised in the Americas had childhood experience of yellow fever and malaria, and perhaps of dengue. Most adults were immune to yellow fever and resistant to malaria, and some of them were probably immune to some dengue strains. When they began to seek independence from colonial powers, they had disease on their side—and learned how to use it. Haiti provides the best example.

In the 1780s, St. Domingue (as Haiti was then called) was the most lucrative sugar colony in the world, and the most valuable of France’s colonial possessions. Half a million slaves, most of them born in Africa, powered its economy. In 1791, they took advantage of the French Revolution to mount their own insurrection. Their rising threatened the entire sugar plantation economy of the Caribbean, and so was unwelcome to Britain and Spain as well as to France. The European powers in effect took turns trying to suppress the slave insurgency, sending tens of thousands of European soldiers to St. Domingue, almost none of them immune or resistant to the viruses and plasmodia awaiting them.

The British effort cost 15 000 lives out of 24 000. The French, led by Napoleon’s brother-in-law, followed with nearly 65 000 men. Nearly 55 000 died in St. Domingue, almost all of them from fever. The defeat, at the hand of ill-equipped ex-slaves and mosquito-borne diseases, so soured Napoleon on the Americas that he sold the midsection of North America to the United States on the cheap in 1804.

In Mosquito Empires, J. R. McNeill explores the role of mosquito-borne infections such as yellow fever and malaria in shaping the history of the American hemisphere.

The commander of the uprising that created Haiti was an ex-slave named Toussaint L’Ouverture. His background included working in a Jesuit hospital. He was especially attentive to matters of health and disease. He knew that European newcomers to the Caribbean were extremely vulnerable to yellow fever whereas those born and raised there were not. He folded this awareness into his military strategy, and instructed his subordinates to wait for the rainy season to act, when (thanks to disease) their enemies would “die like flies.”

ZIKA

Zika appeared in the Americas only in 2014. A competent vector was already present, thanks to the slave trade centuries before. Zika is not lethal on the scale that yellow fever was before mosquito control and vaccines, but it is already writing its own chapter in the history of imported mosquito-borne disease in the Americas. It remains to be seen how deep its imprint will be on the hemisphere’s social and political history.