The Fever: How Malaria Has Ruled Humankind for 500,000 Years (10 page)

The bodies of black people were considered “tinctured with a shade of the pervading darkness,” as a prominent Louisiana physician wrote in 1851,
⁹¹
and blacks were on “the lowest point in the scale of human beings,” as the Alabama physician Josiah Clark Nott put it.
⁹²
Malaria was considered tolerable for blacks, and intolerable for whites. “Negroes” were “lower animals,” the American malariologist
Lewis Hackett wrote in 1937, who could withstand malaria, while “human beings of the white race” could tolerate no malaria whatsoever.
⁹³
In part, these notions stemmed from misunderstandings about Europeans’ and Africans’ different immune responses to falciparum malaria; they also rationalized the casual disregard that a racist culture propagated.

Today, attitudes of Northern superiority and white supremacy may have softened a bit, but the dense population and relative prosperity of the North compared to the South remain. Population density in some of the northeastern states today rivals that of India and Japan.
⁹⁴
We live cheek by jowl, importing nearly all our food. In Alabama, by contrast, there are but thirty souls per square kilometer, a dispersal of humans more similar to, say, Madagascar than to New Jersey. As established during malaria’s reign, today’s African American populations are larger than average, and the economy relatively impoverished.
⁹⁵

Drive through northern Alabama, as I did a while back. One passes thick forests, burbling brooks: the very picture of fertility and easy living. And yet the roads are empty. The motel I checked into was unoccupied, the restaurant vacant, the expansive Wal-Mart parking lot only lightly used. For hours, the only evidence of human habitation is a hand-lettered sign that reads like a paranoia-tinged shriek from the solitude: “Go to Church, or the Devil will get you!”

Malaria is not a disease of the environment in the way that, say, asthma is or certain kinds of cancer are. And yet its transmission depends upon an exacting set of environmental conditions. The protozoan parasite, despite all its sophisticated wiles and cunning, is more like a seed than a self-sufficient predator. Like a pip on the wind, it must alight in a fertile bed, be enveloped in the proper amount of moisture, and be bathed in the correct level of sunlight. The right mosquito must bite at the right time and with the correct frequency. If a local mosquito bites the wrong host, or if the insect’s body becomes too cool or too warm, or if it dies or fails to bite before the parasite has time to develop inside its body,
Plasmodium
, one of the world’s most deadly pathogens, might as well be an inert gas.

The circumstances that decide malaria’s fate are contingent upon other circumstances equally beyond the parasite’s control. The biting behavior of the mosquito, for example, depends partly on the species and partly on the variable availability of blood-filled hosts. Some species, such as
Anopheles gambiae
, are deeply connected to human hosts. Others are not so picky and will happily feed on the blood of cows or horses, if these happen to be available. The longevity of a
mosquito, too, depends on multiple factors, such as the mosquito’s habits and where she takes her blood meal. The female must rest soon after a feast, to excrete the excess liquid from her body. Will this siesta occur nearby, in a safe, snug place, or will it require some dodgy flight, in which she crosses paths with a swatting hand or swooping predators? What kind of weather will the blood-engorged insect encounter? Arid conditions, for example, can be deadly.

Of all the micro-geographic and climatic forces affecting malaria, the single most important factor is the species of the local population of mosquito. Of the planet’s 430 different species of
Anopheles
mosquito, some 70 species transmit malaria. Each specializes in a specific geographic and climactic zone, be it the temperate Americas or the Asian tropics. For example, you won’t often find a tropical African
Anopheles
in Northern Europe. But within each zone, you will find perhaps a handful of different
Anopheles
species, some of which are unreliable malaria carriers, while others are fabulous at it.

It would be nice if the variety of local species of
Anopheles
depended on some unchanging factor in the landscape, so we could simply avoid those places where the worst mosquitoes lived, just as we avoid living around, say, alligators or grizzly bears. Unfortunately, the peculiar mix of species in a given locale depends mostly upon a rather more mutable part of the landscape. The impregnated female mosquito must lay her eggs in bodies of water where they will hatch and feed on whatever debris floats by. The larvae’s survival depends on being deposited in an amenable place to which its kind has been specifically adapted. Some thrive in salty water; others must have fresh. Some require shade; others, sun. Some demand flowing waters; others, stagnant.

The trouble is that the hydrology of puddles, streams, and ponds is one of the more mercurial aspects of the environment, vulnerable to any number of disruptive influences. We remake mosquitoes’ microhabitats ourselves, mindlessly and routinely, by felling a few trees or digging a few holes. In so doing, we alter the temperature, rate of flow, and chemical composition of puddles, streams, and pond
edges. To us these small alterations seem like nothing. But for the mosquitoes, they are the difference between life and death.

When the landscape is static, there’s only so much of each kind of larval habitat available, and the mix of local
Anopheles
species can thus remain relatively stable. All things being the same, the larvae of the dominant species will fight off any intruders, and its populace is likely to become increasingly adapted to its specific niche. Once malaria transmission is established, the local people will, too, in time grow accustomed to the parasite, acquiring a patina of partial immunity. A relatively stable malarial ecology is established. Mortality declines.

But when the local malaria ecology twists and turns, new opportunities arise for the malaria parasite. Perhaps the local vector’s habitat is extended, allowing the parasite to reach into new human populations. Or maybe the local vector is crowded out and a new, more efficient
Anopheles
population takes root, allowing the parasite to penetrate the local humans in more robust ways. Then the parasite’s gains are the local humans’ loss, for it can adapt much more quickly to the changed conditions than can the humans upon which it preys. In the lag between exposure to the new pattern of malaria transmission and the acquisition of immunity, the death toll rises.

Take the Roman Empire. The founding of the ancient city of Rome upon the banks of the Tiber in 753
BC
created a prime habitat for the European malaria vector,
Anopheles atroparvus
. The Tiber, an actively migrating stream back then, regularly flooded its banks, leaving behind scores of puddles and pools in which
A. atroparvus
’s young thrived.
¹
The Roman penchant for vegetable gardens, fountains, and impluvia provided even more mosquito nurseries. With an abundance of available larval sites and plenty of Romans to feast on,
A. atroparvus
abounded in Rome.
²
By 200
BC
, a stable malarial ecology had been established, with
A. atroparvus
regularly passing on
P. vivax
parasites to the locals.
³

Luckily for Rome, while
A. atroparvus
ably carried
P. vivax
parasites, that mosquito wasn’t a reliable carrier of
P. falciparum
. Unlike
P. vivax
,
P. falciparum
’s survival depends on continuous transmission, without which it dies out, trapped inside its host. A stream of
P. falciparum
parasites regularly trickled into the Italian peninsula, in the bodies of traders and slaves from Africa. But
A. atroparvus
successfully foiled it before it took root. For one thing, the
A. atroparvus
mosquito is as attracted to animals for its blood meal as it is to humans, so its carriage of malaria to the correct host is not the most reliable. Every now and again, a falciparum-infected
A. atroparvus
mosquito would deposit
P. falciparum
parasites inside a cow or horse, which meant certain death for the parasite. Worse,
A. atroparvus
hibernates all winter. Once the cool weather arrives, it stops biting and repairs to some dark, warm corner for weeks at a time. For
P. vivax
parasites, which can go dormant, this wasn’t a problem. But pauses are a deal-breaker for
P. falciparum.
After a few weeks inside the body of a human or an insect without access to new blood,
P. falciparum
parasites disintegrate.
⁴

More reliable, nonhibernating, human-loving biters such as
Anopheles labranchiae
flit in North Africa, on the other side of the Mediterranean. Ancient Rome increasingly relied on imported grain from the second century onward. Stowaway
A. labranchiae
would have regularly arrived in Rome on the grain ships from North Africa, and were able travelers. While traders loaded the ships, rain showers might fill some broken clay jars with water, into which a passing female
A. labranchiae
might lay her eggs. The tiny ornamented pods, coated in a velvety pile, would balance upon the water’s surface with two air-filled floats shaped like fans on either side.
⁵

If conditions were amenable, by the time the wormlike larvae, with their giant eyes and spiky whiskers, hatched, they would have been in Rome.
⁶
Nobody would have noticed the skittish, ascetic little pupae they became. They don’t eat anything; they have no mouths. If even so much as a shadow passes over them, they flip their tails and duck out of sight, rising to the surface again only to breathe. When
the adult
A. labranchiae
emerges from the pupae, she is soft and wobbly. But after half an hour, her cuticle stiffens and she flies off to some still, dark corner. She can fly as far as eight miles in search of a meal. With the help of a strong wind, she could end up hundreds of miles away.
⁷

But Rome’s
A. atroparvus
effectively repelled such interlopers, and had already claimed all the best mosquito nurseries, those watery areas free of predatory fish. If any
A. labranchiae
mosquitoes successfully deposited a few eggs somewhere, they’d be fish food soon enough. If they dared lay claim to
A. atroparvus
turf, there’d be dire consequences.
Anopheles
actively guard their territory from encroachment by rival species, their larvae secreting deadly chemicals to kill off any newcomers that they don’t devour.
⁸

The ecology of malaria in early Rome was thus both stable and resilient. This helped strengthen the empire, for it meant that the Romans had ample opportunity to adapt to life with the parasite, and exercise an immunological advantage over foreign intruders who did not. Most powerfully, people across the Italian peninsula and around the Mediterranean developed genetic defenses against the worst ravages of the parasite. Genes that disrupted an enzyme called G6PD, required for normal functioning of red blood cells, emerged and spread. The defect impaired human bodies’ ability to repair oxygen damage, so that malaria-infected cells essentially poisoned themselves. (The main drawback: the peninsula’s famous fava beans could send G6PD-deficient Italians into a spiral of hemolytic anemia, a condition known as favism, after the bean.)
⁹

The Romans adapted culturally, too. They understood enough about the epidemiology of their malaria to minimize exposure to it. Ancient Roman scholars such as first-century
BC
writer Marcus Terentius Varro warned that animals too small to be seen (he called them
bestiolae
) entered the mouth and nostrils and caused horrible diseases.
¹⁰
He recommended that Roman houses be built on high land, where the wind would blow the beasties away. Roman elites thus built their sumptuous villas in the mosquito-free hills. Even the
peasants who worked the infested lands below the villas knew to avoid the sickly winds, building their houses with windows facing in, toward a central courtyard, rather than out into the breeze.
¹¹
The very worst mosquito-ridden regions, such as the rich wetlands of the Pontine marshes and the Roman Campagna, which ringed the metropolis, were abandoned to brigands, highwaymen, and the odd pallid peasant. Although these were the nearest and best agricultural lands, to avoid their malarious mosquitoes, the Romans (after a period of development) left them sparsely settled.
¹²

For the inevitable infections they suffered, the Romans devised a varied and fanciful welter of antimalarial therapies. The malarious might try some honeysuckle dissolved in wine to relieve their swollen spleens, or perhaps consume the liver of a seven-year-old mouse.
¹³
They might, as the emperor Caracalla’s physician, Serenus Sammonicus, recommended, wear a piece of papyrus inscribed with a powerful incantation—“abracadabra”—around their necks as an amulet. Bolder souls might try Sammonicus’s other malaria cure: bedbugs eaten with eggs and wine.
¹⁴
They might try waking at dawn three mornings in a row, facing a window, and shutting it suddenly while reciting a prayer, or, for a male sufferer, having intercourse with a woman just starting to menstruate.
¹⁵
Prominent Roman physician Galen and medical scholar Celsus advocated energetic bloodletting. Finally, when all else failed, the Romans prayed for relief to the demon goddess of malaria, Febris, in three dedicated temples around the city.
¹⁶