Parasites: Tales of Humanity's Most Unwelcome Guestsby Rosemary Drisdelle
Hidden away within living tissues, parasites are all around usand inside us. Yet, despite their unsavory characteristics, as we find in this compulsively readable book, parasites have played an enormous role in civilizations through time and around the globe. Parasites: Tales of Humanity’s Most Unwelcome Guests puts amoebae, roundworms,/i>
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Hidden away within living tissues, parasites are all around usand inside us. Yet, despite their unsavory characteristics, as we find in this compulsively readable book, parasites have played an enormous role in civilizations through time and around the globe. Parasites: Tales of Humanity’s Most Unwelcome Guests puts amoebae, roundworms, tapeworms, mites, and others at the center of the action as human cultures have evolved and declined. It shows their role in exploration, war, and even terrorist plots, often through an unpredictable ripple effect. It reveals them as invisible threats in our food, water, and luggage; as invaders that have shaped behaviors and taboos; and as unexpected partners in such venues as crime scene investigations. Parasites also describes their evolution and life histories and considers their significant benefits. Deftly blending the sociological with the scientific, this natural and social history of parasites looks closely at a fascinating, often disgusting group of organisms and discovers that they are in fact an integral thread in the web of life.
“Drisdelle has written one of those rare books that is fun to read but does not skimp on scholarly rigor.”
- University of California Press
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Tales of Humanity's Most Unwelcome Guests
By Rosemary Drisdelle
UNIVERSITY OF CALIFORNIA PRESSCopyright © 2010 Rosemary Drisdelle
All rights reserved.
If I cause noisome beasts to pass through the land, and they spoil it ..., that no man may pass through because of the beasts ... the land shall be desolate.
EZEKIEL 14:15—16, Polyglott
THE DEGREE TO WHICH VARIOUS parasites have interfered in human affairs cannot be overstated. Parasitic diseases such as schistosomiasis, sleeping sickness, and malaria affect millions and muddle plans in everything from exploration to foreign aid. Others cast a more subtle shadow but still wield enormous power. They wipe out populations; they topple fragile ecosystems. The monsters hiding under our beds when we were children grew up to be parasites. For those of us who grew up in the cloistered "first world," they fill us with a distant dread, and rightly so; for everyone else, the monsters have always had one cold and slippery foot under the blanket.
Where did these monsters come from, and how did they gain such power? Millennia ago, when humans were nomadic hunter-gatherers, continually moving from place to place, the only really successful human parasites were those that spent their entire lives on human hosts, moving from one body to the next as opportunity arose. Lice, fleas, skin mites, and pinworms are their contemporary equivalents. The rest, if they infected humans at all, were probably either rare or responsible for relatively light infections.
People who move about a lot tend to leave their parasitic troubles behind them: by the time the malaria-carrying mosquito is ready to bite again, or the worm egg has matured in warm moist soil, the potential hosts have moved on. True, some worm eggs survive for years in the soil under the right conditions, and malaria can remain silent in the liver for years, but in prehistoric times, by the time a band of nomads returned to a favorite campsite, weather and animals would have dispersed eggs in feces, and blood levels of malaria parasites would likely have been minimal. Levels of infection remained low.
The big break for parasites came when humans began to stay in one place. With the appearance of permanent settlements, mosquitoes had a food supply to which they could return again and again—and the malaria parasites they carried could complete their life cycles in continuous rhythm, at least where the climate was warm enough throughout the year. Meanwhile, trade between cultures helped parasites spread from one population to another.
Organic waste was an issue too. At best, sewage treatment amounted to spreading human and animal waste on cropland as fertilizer. The level of contamination of settled land and nearby water rose steadily. As food and drinking water became progressively more contaminated with feces, intestinal parasites boomed. Soon people domesticated animals and moved them into human spaces: the rate at which humans and animals passed pathogens back and forth accelerated. The stage was set for some real monsters to emerge as threats to human and animal health.
Thousands of years later, we have yet to figure out how to return our parasites to the margins. Like a creeping mist, they spread through communities, invading one body after another. Some parasitic diseases are quick and lethal; others bring slow decay, chipping away at daily life—but only the effects are visible. Most of our parasites are well hidden: we can't see them, we don't understand them, and we are afraid that they, like the monsters under the bed, will "get us."
Trying to determine how many people parasites "got" in prehistoric, and even historic, times is a bit like looking for a needle in a blazing forge: almost no evidence remains. We find Egyptian mummies condemned to spend eternity with an intestinal tapeworm, fossilized human feces containing various worm eggs (if you don't want your intestinal contents analyzed thousands of years after your death, do not defecate in caves), and Ötzi, the enigmatic iceman who emerged from an Italian glacier after more than five thousand years with whipworm eggs still in his gut.
But how much trouble were these parasites to their collective hosts? The first tale for which we have credible evidence of a serious parasite menace took place more than three millennia ago in the Jordan Valley, where a young woman named Rahab catapulted herself to biblical infamy—and a parasitic worm may have made her do it.
* * *
Rahab lived on the site of perhaps the oldest permanent settlement on earth, home to humans for more than seven thousand years. Once, her village had been a much larger place, a walled town of several thousand people and a bustling center of agriculture and trade. Visitors brought trade goods and innovations from Egypt in the southwest and from the Mesopotamian cities in Syria and the Fertile Crescent in the northeast.
In Rahab's time, the village stood on a mound of old sun-dried brick, the remains of earlier buildings and protective walls now broken into rubble. Since the earliest settlers, the mound had grown, layer upon layer of dust and debris. New houses were built on top of the mound with the same adobe brick, made with mud and water from the nearby pond. This was Late Bronze Age Jericho, City of Palms.
Jericho was a lush haven, a green paradise in a harsh, arid land and an oasis of date palms, figs, and wheat, with a perpetual spring-fed pool. Where the green of the oasis faded, the deeply eroded limestone of the Judean hills climbed thousands of feet to the western horizon. Away to the south, a distant blue haze marked the Dead Sea, where the Jordan River ended. No river ran out to sea: under a baking sun, the water in the Dead Sea simply evaporated, leaving salts and minerals behind. The water was so salty that it could not be used.
Nobody lived by the Dead Sea, and without the oasis, no City of Palms could have taken root. The water in the oasis was clear and sweet, and it never ran out, but vital as it was to the Canaanites of Jericho, it may also have been the source of their ruin: unknown to them, scientists think, the precious springwater hid a deadly, invisible menace, one that had been imported from far away.
By Rahab's time, travelers and traders had been stopping at Jericho for thousands of years. At some point in that long past, and possibly on many occasions, visitors had arrived bearing more than items for trade. Some researchers believe they had arrived with a parasitic infection, a debilitating worm living in the blood. The theory is that when these visitors contaminated the oasis waters with their urine, Schistosoma hematobium came to Jericho, and it had been flourishing there ever since. Generation after generation, it made the people of Jericho sick.
The village, in fact, was known for its dwindling population. Women had many miscarriages and few births. Too many of the children died in their teens, and even the adults were afflicted with a mysterious malady that drained them. Listless and weak, many lacked the energy to do a day's work. Some of the men had a grossly enlarged penis and were impotent. Rahab, a prostitute, probably knew this better than most. She likely knew, too, how much it hurt to urinate and saw that her own urine was cloudy and red with blood.
On the day her story begins, we can imagine that Rahab made her daily trip to the pool for water. Perhaps she paused to rest on the way back, feeling impossibly weary. The air was humid and still, a stillness made more noticeable by the lack of activity in the village. A small group of pallid, thin children played in the dust; others moved listlessly about in the street. A few adults were at work in the fields and orchards. Setting her vessel down and shading her eyes with one hand, Rahab gazed out past the village walls.
Rahab was not the only one watching the plain; others glanced nervously over their shoulders or simply stood and looked out. Despite the pall of lassitude, tension was growing in the village. To the east, the dry and dusty Jordan Valley stretched to the river, about five miles away, then across the Plains of Moab on the other side. An army had recently marched across those plains and forded the Jordan. Now Joshua and the Hebrews were camped on the inhospitable plain, preparing to attack. The villagers were far too few and far too tired. The fight wouldn't last long.
The microscopic eggs of the blood fluke S. hematobium are found in urine. When passed into freshwater, the larvae (miracidia) inside the eggs sense the change in environment and begin to move, flipping over and over until they burst out and swim freely. Like tiny revolving torpedoes, they spin briskly, searching for a welcoming species of snail. Quiet ponds like the one in Jericho, with stable water levels, relatively still water, and plant life around the shallow edge, are a perfect habitat for snails.
If a suitable snail is not available, the miracidia spin on like windup toys until they run out of energy and die, but if the right snail turns up, the larvae plunge in and begin a period of asexual multiplication. After about six weeks, the parasites (now called cercariae) emerge from the snails and hang about like jellyfish, slowly rising to the surface and then slowly sinking down again, waiting for humans to come to the water.
In Jericho, the humans came every day. Unless the people walked across the parched valley to the banks of the Jordan, the oasis was virtually the only local source of water, and the villagers came in contact with it repeatedly. The infant Rahab, like all the babies of Jericho, was no doubt bathed in the springwater. From childhood on, she drank the water, cooked with it, and washed with it. Every time human skin came in contact with the water, cercariae that had recently emerged from snails had a chance to latch on. Each little parasite crept about briefly and then plunged its head through the skin, shedding its wriggling forked tail and disappearing in less than half a minute.
Cercariae that got through Rahab's skin would have set themselves adrift in her veins and traveled to her heart. From there they followed the bloodstream through lungs and intestines, eventually arriving in her liver, where they spent about three weeks maturing. Each adult male worm wrapped itself around a female like a thick, knobby blanket—two tiny worms that could easily curl up on the tip of Rahab's little finger. Theirs was a perpetual embrace that would last thirty years, if Rahab lived that long. Their final move took the worms, now young adults, to the blood vessels around Rahab's bladder. The female worm soon began producing eggs.
Up to this point, baby Rahab probably felt fine, but as egg production began, she developed a fever and headache. She was tired and suffered chills, aching muscles, and pain in her abdomen, where the eggs, deposited in tiny vessels, were gradually moving through her bladder wall. Breaking through, the eggs mixed with blood and urine in the bladder, and Rahab began passing bloody urine.
Most of these symptoms passed, but the bleeding grew worse, and Rahab felt a burning sensation when she urinated. In time, bladder ulcers developed, and she suffered from painful bladder infections. Some eggs never reached the bladder and were swept away in the blood to lodge elsewhere, particularly in her liver and lungs. Rahab was probably always tired, and she was thin.
The story was likely the same for all the people of Jericho. Obliged to use water from a single source, they were continually exposed to the parasite, and they continually contaminated the oasis. Every one of the people of Jericho may have had the worms and suffered from various degrees of emaciation and fatigue. Some slowly wasted away and died from bladder cancer or heart disease. Others slipped helplessly into a state of chronic illness. The children suffered most.
Now Rahab looked up and down the almost deserted street in despair. The Hebrews were threatening, prepared for battle, and the worm was on their side. Rahab knew nothing of the parasite, but she knew of the sickness and understood what the outcome would be if the village were attacked. As she stooped to retrieve her water, she noticed two strangers in the street. As the book of Joshua reports, she guessed that they were Hebrew spies, come to scout the village and report back.
Rahab took a chance and approached the two men, quickly leading them away to her home at the edge of the village. Then, buying her life and the lives of her family, she told them about the poor state of the villagers. When searchers came knocking, she bought time, claiming the men had already left; then she helped the spies slip away, telling them where to hide to avoid detection.
The Hebrew scouts escaped the search parties, returning safely to Joshua in the plain. The biblical account relates that, after the ensuing battle, not a soul was left alive in Jericho except Rahab and her family, and that Joshua cursed the site: "Cursed be the man before the Lord, that riseth up and buildeth this city of Jericho" (Joshua 6:26). Thereafter, the dusty streets of Jericho were silent, and though the spring still flowed and filled the pool, only the breeze ruffled the waters where S. hematobium waited in vain.
Is any of this true? It may be true, for the remains of Bulinus truncatus, an intermediate snail host of S. hematobium, have been recovered from the mud brick used for construction in Bronze Age Jericho. Other archaeological finds, including seashells and pottery, prove that the community traded with people from Egypt and Mesopotamia, areas where S. hematobium was well established. Biblical texts also suggest that Jericho was known for its low birthrate and that people suspected the water was tainted.
Archaeological evidence indicates that, despite the city's attractiveness, no one lived in Jericho for about four hundred years after the Hebrews passed through in their quest for the Promised Land. In that time, with no infected humans to keep the cycle going, S. hematobium would have died out in the water. Droughts may have eradicated the snail as well. Today, Jericho is once again inhabited, and the parasite is absent.
Ironically, Joshua's act—destroying the village and leaving it deserted for generations—was the only thing that, at that time, could rid the lovely oasis of a lurking menace, making it once again a desirable place to live. Perhaps Joshua deserves some credit for being the first public health official, albeit inadvertently.
Because we now understand the life cycle of S. hematobium, we can indulge in "what if" considerations of this scenario of Jericho. What if the people had had more than one water source? What if they had troubled to keep the oasis waters free of urine? Levels of infection would have been much lower, if the worms thrived at all. An Old Testament Bible story might have turned out rather differently. In fact, the whole history of the place would probably have changed dramatically.
Humans do unwittingly clear the way for parasites, and in the case of schistosomes, we keep making the same mistake and getting the same results. In the past century, however, we're even better at making the mistake: we don't just contaminate the water supply; we also create the ideal habitat for the snail's intermediate hosts—a process innocently known as "water resources development." Because of this "development," the people who live on the shores of Lake Volta in Ghana have much in common with the Canaanites of Late Bronze Age Jericho.
* * *
The Akosombo Dam, on the Volta River in Ghana, was completed in 1964. When the river was interrupted, an enormous fern-leaf–shaped lake formed, stretching more than three hundred miles into the heart of the country. The flooding created the largest artificial lake in the world and set off a chain of unintended consequences that, in hindsight, were completely predictable.
The relatively still, nutrient-rich water in the reservoir provided a perfect environment for aquatic plants, which soon flourished on and under the surface and at the edges of the lake. Snail species that live on aquatic plants increased along with their food supply. On the bottom, meanwhile, decaying organic matter fueled an increase in plankton, which in turn fueled an increase in fish.
Many of the people displaced by the rising water were unable to farm as they had in the past and turned to fishing instead. Others came from more distant regions, drawn to the lake by rumors of plentiful fish. Communities grew on the lake's shores. People bathed in the lake, used it for drinking water, and cooked with the water. The lake was the center of their world. They fished in it, swam in it, and contaminated it with their urine and feces.
In such a scenario, children and fishermen are at particularly serious risk of schistosomiasis because of their intense water contact; however, all of the people living by Lake Volta had frequent contact with the contaminated reservoir. Before the lake was created, between 5 and 10 percent of children in the region had adult worms of S. hematobium living in the blood vessels around their bladders. By 1969, 90 percent of children living in lakeside communities had them.
Excerpted from Parasites by Rosemary Drisdelle. Copyright © 2010 Rosemary Drisdelle. Excerpted by permission of UNIVERSITY OF CALIFORNIA PRESS.
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What People are Saying About This
"I highly recommend taking a deeper look into Drisdelle's new book . . . You'll be surprised by how much you learn and even more by how much you enjoy the read!"Bedbug.com
"Drisdelle describes biological processes lovingly and beautifully . . . read Drisdelle for an education."Times Literary Supplement (Tls)
"Hookworm, roundworm, bed bugs, lice, trichinosis, sleeping sickness, scabies: these are some of the parasites and diseases that Drisdelle ably describes with mirth, occasional poetry, and an infectious scientific fascination, where the human story is an essential element of the natural history."Scitech Book News
"An interesting guide to what's eating you, literally! Not for the squeamish!"The Guardian (Uk) / the Birdbooker Report
Meet the Author
Rosemary Drisdelle is a writer and a clinical parasitologist living in Nova Scotia.
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