Yellow Fever and Globalization

Back in the 17th century, were pandemics also transmitted by birds?

September 1, 2005

Back in the 17th century, were pandemics also transmitted by birds?

For comparison, malaria kills 2 million to 3 million people per year today. But before 20th century medical interventions that led to mosquito control and vaccination, yellow fever was a fearsome scourge in the tropical Atlantic world.

It was above all else the ecological and social revolutions of the sugar plantation that opened yellow fever’s reign of terror in the Americas.

With decisive help from Eurasian diseases, Spain acquired a loose — but lucrative — empire in the Americas after 1492.

By 1600, the lowland tropical segments of that empire were depopulated backwaters, but great riches flowed from silver mined in the Andes and in the highlands of Mexico.

To get silver to Spain, it had to pass through choke points in the tropical lowlands and the Caribbean Sea, such as the isthmus of Panama or the port of Veracruz, and always the port of Havana.

That fact, and the hope that great wealth might lie elsewhere in the American tropics, inspired England, France and the Netherlands to contest these Spanish dominions. They acquired several Caribbean islands and a few stretches of coastline by 1655 — usually via conquest and settlement involving, initially, only a few hundred people per adventure.

This was the age of buccaneers, when even modest efforts with minimal support from European states could change the political map of the Caribbean. That age ended when three things came to the Atlantic American tropics: sugar, slaves and sieges.

Sugar made its first major impact in the Americas in Northeastern Brazil. When the Portuguese expelled the Dutch (who controlled part of Brazil between 1630 and 1654), the Dutch (and Luso-Brazilian Sephardic Jews) brought sugar and the latest in sugar-refining technology to the Caribbean, beginning in Barbados in the 1640s.

A social revolution followed as plantations spread throughout suitable lowland regions. Eventually, after an experiment with indentured labor from Europe failed, planters turned to mass importation of slaves from West Africa. This ensured politically unreliable majorities on many islands and coastlands, changing the nature of war and politics.

The comparative scarcity of whites and their understandable fear of arming blacks led to a pattern of warfare by European expeditionary force. To protect their colonies, all European empires upgraded their fortifications.

Spanish silver and everyone’s sugar made it possible to afford such investments in the 17th century, and it made many colonies and ports too valuable not to fortify.

Spain in particular relied on masonry and local militias, more than on naval power, for imperial defense. Thus, the Vauban revolution in fortification — named for the French military engineer Sebastien le Prestre de Vauban — came to the Americas, and with it, the pattern of prolonged siege warfare.

Siege warfare in the Atlantic American tropics proceeded under conditions very different from those prevailing within Europe along its Ottoman frontier or at the scattered European outposts elsewhere around the world.

A Vauban fortress in Europe was intended to be able to hold out for six weeks, by which time — the theory went — relief columns might march to the rescue.

However, in the far-flung Portuguese, Dutch and British strongholds in the Indian Ocean, relief could never arrive in time, and so besiegers often succeeded. But in the tropical Atlantic, siege warfare after 1655 favored the defenders.

In 1655, the English took Spanish Jamaica, part of Oliver Cromwell’s “Western Design,” intended to weaken Spain. This conquest involved a force of some 7,000 men, far more than had taken part in any previous campaign in the Caribbean.

The days of buccaneers were passing, although another 70 years would elapse before they were finally extinguished.

The era of expeditionary forces, of systematic and large-scale warfare around the Atlantic, was beginning. It took Cromwell’s legions a day to take the main Spanish settlement on Jamaica, and a week to control the entire island — although guerilla resistance flickered on.

But after this easy conquest, very few successful invasions took place in tropical America, despite repeated war and upwards of 50 attempts. The main reason for this lies in another unsuspected consequence of the arrival of sugar: yellow fever.

Native to tropical West Africa, yellow fever is a viral infection, of the genus Flavivirus that also includes dengue fever, West Nile virus and Japanese encephalitis, among others. It, like all others in its genus, is an arbovirus, meaning it is communicated by mosquitoes or ticks.

Its symptoms can be mild or serious. In fortunate cases, they consist of high fever, muscular pains and headache that last for three or four days, but then disappear. In serious cases, these symptoms abate, then recur — joined by jaundice and internal hemorrhage.

In the latter stages of lethal cases, the victim vomits up partially coagulated blood, roughly the color and consistency of coffee grounds. This symptom gave the disease one of its several nicknames — the “black vomit.” When this happens, death is near. Yellow fever kills people through organ failure, normally the liver, and through circulatory collapse.

Typically, the immune system forms antibodies within a week, but that does not always help. In vulnerable human populations in times past, case mortality may have been as high as 85%, although today the range seems to be much lower, never more than 30% to 50%.

Perhaps in the past, reporting was so poor that many who had the disease, but recovered, went unnoticed. Perhaps nowadays the virus no longer gets the chance to run amok among highly susceptible populations.

In addition, it may be that the virus has evolved so as to be less virulent in recent centuries, although the evidence from analysis of the virus’ genome implies it has been genetically very stable.

The American yellow fever virus is extremely close to the West African one, and the symptoms of disease are identical everywhere.

That suggests the virus has remained genetically stable since its transmission from Africa to the Americas. It also confirms the West African origin of the virus, which had formerly been a controversial question.

The yellow fever virus has long been endemic in tropical West African forests and is now endemic in tropical American ones as well, circulating among monkeys and species of mosquito that are not much attracted to human blood.

From the virus’ point of view, despite the warmth and rainfall, conditions in the Atlantic American tropics before 1640 left a lot to be desired.

There was not enough water vessels for breeding sites, not enough — if any — A. aegypti (the mosquito vector of yellow fever), and not enough human bloodstreams. Among those bloodstreams, not enough people spent their childhoods in places where cold temperatures precluded exposure and, therefore, immunity to the virus.

But after 1640, sugar and geopolitics set the table very nicely for the yellow fever virus. Sugar wrought an ecological revolution upon dozens of islands and numerous patches of adjacent continental lowlands. Soon, armies of slaves hacked down and burned off millions of hectares of forest in order to plant cane.

Their efforts led to multiple ecological changes. Soil erosion accelerated and wildlife vanished. More important from the human point of view, as plantations replaced forest, conditions came to favor the transmission of yellow fever. Falling trees brought canopy-dwelling mosquitoes down to ground level, where their chances of biting a person improved.

Deforestation meant fewer birds, and fewer birds meant fewer predators for all mosquitoes. Plantations made excellent incubators. Meanwhile, sugar meant slaves and population growth.

The Caribbean population had crashed after 1492, and by 1640 was perhaps 200,000. By 1800, it had surpassed two million, improving mosquito nutrition handsomely. For that matter, there were more and more slave ships arriving from West Africa, bringing as stowaways more mosquitoes.

Conditions for the yellow fever virus improved too, with one catch that geopolitics soon addressed. More mosquitoes, more human bloodstreams and more ships from Africa favored the establishment of the yellow fever virus in the neotropics.

Indeed, the first clear epidemic of yellow fever in the Americas came in 1647, striking first in Barbados — then the main sugar island — and over the ensuing months and years, Guadeloupe, St. Kitts, Cuba, the Yucatan and the east coasts of Central America generally. It killed perhaps 20% to 30% of the local populations.

But after this outbreak, yellow fever disappeared for almost 40 years. The virus’ problem was compounded by the resistance of West Africans.

Yellow fever confers immunity upon all survivors. Almost all slaves arriving in the Caribbean from Africa had grown up in endemic yellow fever zones, and hence were immunes and virus stoppers.

Beyond that, West Africans and people of West African descent may carry an inherited partial immunity to yellow fever whether or not they carry conferred immunity.

So while the population growth of the sugar zones helped the mosquitoes find food, it did not provoke many epidemics because so many of the people bitten by mosquitoes were West Africans (or, possibly, of West African descent).

Raging epidemics required an influx of inexperienced immune systems. This is what expeditionary warfare provided. Participants and observers in the inter-imperial wars of the 17th and 18th centuries normally regarded yellow fever epidemics as acts of God.

Modern military historians tend to see them as random events. But differential immunity made yellow fever decidedly and systematically partisan.

Yellow fever went easy on entire populations that included numerous individuals with either conferred or (if it existed) inherited immunity.

In this way, a large contingent of Africans or — perhaps somewhat less effectively, of Caribbean-born whites — could serve as a shield for individuals highly vulnerable themselves to yellow fever by interrupting the transmission cycle (this is known as “herd immunity” to epidemiologists).

Yellow fever strongly favored local populations over invaders and immigrants, strongly favored populations with West Africans as opposed to those without them and even favored populations with children as opposed to those made up exclusively of adults.

Yellow fever was most dangerous to unadulterated populations of young adult Europeans — precisely the composition of expeditionary forces.

After their one-week, Oliver Cromwell-inspired conquest of Jamaica in May 1655, the English troops fell victim to disease. By November, 47% were dead and half the remainder were ill.

Henceforth, British garrisons in Jamaica died off at a rate of about 20% annually in peacetime, almost entirely from diseases (malaria and others, as well as yellow fever). This was about seven-times the peacetime death rate of British garrisons in Canada.

Beginning in the 1680s, in the context of the struggles between England and Louis XIV’s France, expeditions to the West Indies became more frequent — and so did yellow fever outbreaks.

Before 1713, Spain often fought on the same side as Britain, but after the accession of a Bourbon king in Madrid, Spain normally allied with France against Britain.

Most West Indies expeditions were British, but some were French, especially before Louis XIV scaled back his navy in the 1690s. Almost all were failures.

After the successes, victors usually evacuated quickly, suffering from epidemics, and at the next peace treaty, conquered ports or islands were restored to their previous masters.

It is flattering to our species to suppose that our wiles and valor shape our geopolitical history. But in some cases, mosquitoes and viruses brought to new places via trade and forced migration affected the contests among states every bit as much as men did — even if no one recognized it at the time.

In the 17th and 18th centuries, accelerating trade and migration associated with the installation of a new economy in the Atlantic world based on sugar created new ecological conditions that, in turn, conditioned war. It turns out that war was the sport of mosquitoes, as well as kings.