Death by Chocolate There’s something romantic about riding on a train…



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Death by Chocolate

  • There’s something romantic about riding on a train…

  • Trains can whisk us to another city in an hour or two, or across the country in a matter of days

  • But we aren’t the only ones riding the rails

  • Traveling with us are untold millions of microbes, hitching a ride on our technological coat tails

  • Microbes quickly evolve to take advantage of new environmental opportunities

  • Our modern transportation networks can deliver the next plague as efficiently as an overnight package

  • In this lecture, and the one that follows, we’ll look at some of the ways that we’ve changed the world, and how microbes have responded to those changes

  • Along the way, we’ll consider new dispersal routes, like highways, transcontinental railroads, and hypodermic needles, and new habitats, like kitty litter and air conditioners

  • We’ll look at changes caused by agriculture, warfare, and global climate change

  • And finally, we’ll examine the emergence of new diseases, like Ebola and Lassa Fever

  • All of these changes present new challenges, and new opportunities, for the microbes that surround us

  • The trade routes and roads of antiquity were interstate highways for microbes

  • We’ve already discussed several ancient plagues that arose in the Orient and Africa and were carried throughout the Mediterranean

  • The Plague of Antonine, which ran from 165 to 180 AD, for example, was carried to Rome from the orient by returning legions, killing up to 5 million

  • The Plague of Cyprian in 250 AD was also an oriental plague, reaching Europe from Egypt

  • One reason we were so vulnerable to these oriental plagues, is that the western world had been isolated from the eastern world for a very long time

  • We had never been exposed to many of these oriental microbes

  • They fell on us like a hammer blow - we had no herd immunity, we were a naïve population

  • They were “virgin soil” epidemics, comparable to the effects of the 1918 flu on naïve native populations like the Inuit

  • The casualties were so high in both of these ancient plagues that the historian William McNeill believes they were caused by two different diseases, smallpox and measles

  • And witness the plagues introduced by early Pacific explorers, like Captain Cook

  • When Captain Cook arrived in Hawaii in 1778, he carried more than trade goods

  • His cargo may have included syphilis or gonorrhea

  • He admits that his crew had an unknown venereal disease, but claims he did not spread it to the natives

  • The optimistic Cook writes:

  • “As there were some venereal complaints on board both the ships in order to prevent its being communicated to the people, I gave orders that no women on any account whatever were to be admitted on board the ships, I also forbid all manner of connection with them, and ordered that none who had the venereal upon them should go out of the ships. But whether these regulations had the desired effect or no time can only discover.”

  • And the disease broke out as soon as he left - a tribute to the ingenuity of sailors

  • Later traders and settlers brought a horrible plague to Hawaii in 1804, probably cholera

  • Hawaiians called it the “Great Sickness”, or the “squatting sickness” - half the population died

  • These and other epidemics reduced the native Hawaiian population from between 250,000 and a million in 1778, to about 84,00 by 1850, and to 37,500 by 1900

  • Spanish explorers carried smallpox to the New World, along with measles, flu, bubonic plague, diphtheria, typhus, cholera, scarlet fever, chickenpox, yellow fever and whooping cough

  • The Pre-Columbian New World native population of 18-20 million was down to about 530,000 by the late 1800’s!

  • We’ll talk more about the role of microbes in the Columbian Exchange in lecture 22, and consider why it seems to have been a one-way street

  • The transatlantic slave trade brought many Old World diseases to the New World, like yellow fever and hookworms

  • The filthy, crowded slave ships were breeding grounds for microbes

  • The slave trade spread infected slaves all along the Atlantic seaboard

  • The most infamous hitchhikers of them all were the diminutive creatures that rode to Europe in the furs and fabrics of traders following the great Silk Road to the Far East

  • I’m talking, of course, about fleas, and their even tinier riders, Yersinia pestis, the bacterium that causes bubonic plague

  • Bubonic plague struck the Mediterranean world in the plague of Justinian, around 541 AD, although the second major wave didn’t hit until much later, about 1346 AD

  • The new overland trade routes to China probably carried flea-infested furs into Europe from somewhere in Central Asia, perhaps the Gobi Desert

  • But we have done our ancestors one better…

  • Our modern network of international trade and travel unites the entire planet into one vast microbial breeding farm, and distribution network

  • The building of the great Canadian-Pacific railroad was a triumph of engineering

  • But it brought more than trainloads of settlers – it also carried tuberculosis to Saskatchewan in the 1880’s

  • At the peak of the epidemic, the Qu’Appelle Indians were losing 9,000 per 100,000 of their tribe per year

  • Fort San, near Fort Qu’Appelle, was built in the early 1900’s as a TB sanitarium

  • Now slated for the wrecking ball, it treated thousands of patients

  • It acquired a reputation for being haunted by the victims of the “white plague” who had died within its walls

  • Among other ghostly apparitions was said to be the sound of wheelchairs rolling up and down the halls…

  • Modern airplanes can deliver microbes much faster and more efficiently than steam locomotives…

  • Cholera nearly escaped into the US in 1992, when infected shrimp were served to a planeload of Peruvians, aboard Aerolineas Argentina Flight 386

  • The shrimp were tainted from a cholera epidemic that was then killing thousands of people in Peru

  • 75 passengers were infected, and one of them died

  • The disease nearly escaped into Los Angeles - once the first cases were reported, health officials had to spend several weeks tracking down and testing all the passengers

  • But fortunately, given the current odds of being served shrimp on an airplane, this type of accident is pretty unlikely to happen again…

  • Transportation isn’t the only area where new technology has created new ways for microbes to disperse

  • Hypodermic needles, first used in 1844, have provided new evolutionary pathways for diseases to spread

  • Their use in blood transfusions, allergy injections, and illegal narcotics, have given microbes a new express route into our bloodstream

  • Hepatitis B and Hepatitis C are often spread by sharing needles among drug addicts

  • Hepatitis affects millions of Americans, and can cause severe liver damage

  • AIDS is also spread by sharing hypodermic needles, as well as by sexual contact

  • And, ironically, injections of antibiotics can also put us at risk of infection!



  • This is a really bad problem in Less Developed Countries, where needles are relatively rare and expensive, and are often reused by doctors and nurses

  • New technology has not only created new microbial highways, it has also created new microbial habitats

  • Kitty litter, for example, has created a new way for Toxoplasmosis to spread from cats to humans

  • Toxoplasma gondii occurs in most warm-blooded animals

  • As many as one-third of us carry it in our bodies, and it’s most dangerous to those with weakened immune systems, like pregnant women

  • You usually catch it from handling or eating undercooked meat that contains the cysts

  • Or, because it lasts for up to a year outside its host, it can be picked up from gardening, or any other contact with soil contaminated with feces, like the dirt in a child’s sandbox, or your cat’s litter box

  • So don’t blame it all on poor Fluffy - but because it can be transmitted to the fetus through the mother’s bloodstream, pregnant women are well advised to avoid cleaning the litter box

  • My wife informs me that this is one of the few advantages of being pregnant -other than the end result, of course…

  • New technology can be a blessing or a curse – it all depends on your point of view

  • Members of the American Legion discovered this the hard way…

  • In July of 1976, members of the American Legion were holding their annual convention in Philadelphia

  • This being the American Bicentennial, it seemed like they were in the right place at the right time

  • The conventioneers stayed at the Bellevue-Stratford Hotel

  • Upon returning home, many of them fell ill, with a disease similar to pneumonia, with fever, coughing, and in some cases, respiratory failure

  • Of the 221 victims, 34 of them died

  • Enter the CDC…

  • The Centers for Disease Control (CDC) in Atlanta is our first and strongest line of national defense against epidemic diseases

  • The CDC sent a team of 23 agents to Philadelphia to track down the killer

  • It helps a disease to have a memorable name, and the press obliged by dubbing this one “Legionnaire’s Disease”

  • But after six months of thorough investigation, it was still a mystery

  • All of the victims had stayed in the hotel

  • But research at the site had ruled out contaminated food, water, rodents or other vectors

  • That left only the air itself…

  • CDC agent Joseph McDade was part of the small army marshaled to attack Legionnaire’s Disease

  • McDade later became Deputy Director of the CDC’s National Center for Infectious Diseases

  • McDade had been investigating Q fever rickettsiae, a form of bacterial pneumonia which is carried by cattle, and sometimes spread to farmers

  • But this seemed like a dead end, because the records were very clear - no farm animals had been registered at the hotel…

  • He discovered a mysterious bacterium in samples from dead victims

  • But it was a stubborn strain, and he couldn’t culture it at first

  • He almost gave up, but he kept coming back to it, and he finally got it to grow on a culture made from hen’s eggs

  • The survivors’ blood serum was tested against the bacterium

  • Over 90% of the patients had antibodies to this mystery virus

  • Named Legionella pneumophila, it was tracked to the hotel’s air-conditioning system

  • Legionella could survive for a year or more in fresh water

  • It had found a new ecological habitat in the air conditioning system’s coolant water, and the fine aerosol drops of moisture in the cool air spread the disease all over the hotel

  • Many outbreaks of Legionnaire’s disease have occurred since then

  • Between 8,000 and 18,000 people are treated for it every year in the U.S alone

  • It’s turned up in showers, fountains, hot-water towers, whirlpool baths, hospital respiratory therapy equipment, even in those artificial rain-storm misters in up-scale supermarket aisles!

  • So where did it come from?

  • Turns out, the disease had been there all along…

  • It’s fairly common in fresh water habitats, and not all of the many different species are harmful

  • This one only required a new aerosol route to reach a new host

  • New diseases often emerge in response to changes in the environment

  • Microbes quickly evolve to take advantage of both man-made changes and natural changes

  • Take global climate change for example

  • One of the most significant ways in which we’re changing our global environment is by slowly raising the Earth’s average temperature

  • Global climate change is mostly due to the release of millions of tons of CO2 from the combustion of fossil fuels

  • It ‘s a complicated business, and we could easily spend several lectures discussing how it works

  • But beneath the complexities of climate change, lies one simple fact…

  • Atmospheric gases absorb infrared radiation, heat energy, trapping it in the atmosphere

  • This is analogous to the way a greenhouse warms up…

  • The glass keeps some of the incoming infrared radiation from escaping – hence the “greenhouse effect”

  • Any and all atmospheric gasses have this property

  • Even Oxygen and water vapor can act as greenhouse gases – most of the natural greenhouse effect, about 35-70% of it, is actually due to water vapor

  • We focus on carbon dioxide and methane when we talk about global climate change because these gasses, methane in particular, retain the most heat

  • There is a very strong correlation between atmospheric levels of carbon dioxide and global temperature, with good data going back 160,000 years

  • We can be very grateful for the greenhouse effect – without it, we would be shivering under our blankets in the summertime

  • Without the greenhouse effect, surface temperatures would drop about 70 degrees

  • The real problem is too much of a good thing…

  • Carbon dioxide is actually a trace gas, making up only about 0.03% of our atmosphere

  • But we are adding billions of tons of carbon dioxide every year, which means more infrared gets trapped in the atmosphere, and the planet gets gradually warmer

  • How hot will it get?

  • The answer varies from model to model…

  • By the end of the 21st Century, predictions range from 2.5o F - 10.4o F degrees

  • More than hot enough to trigger many environmental changes that will benefit harmful microbes

  • In fact, those changes are already underway…

  • Global climate change will create some new health problems, and worsen some old ones, like allergies

  • It’s already changing the natural range or distribution of a wide variety of species, including many microbes and their vectors

  • Mosquitoes, ticks, and other vectors that need tropical or subtropical temperatures to survive, are being found farther north, at higher altitudes, and in greater numbers

  • The Asian tiger mosquito, for example, Aedes albopictus, has taken advantage of the warmer temperatures, and is already firmly established in the southern US, reaching as far north as Maine

  • What makes this aggressive biter especially dangerous is that it feeds in the daytime, not just at dawn and dusk, which greatly increases the chances of finding a victim

  • Tropical vector-borne diseases like West Nile virus are becoming more and more common in the United States

  • West Nile virus is carried by mosquitoes, and is generally benign

  • Birds are its intermediate hosts, and viral populations build up in their bodies until the virus reaches infective levels

  • Mosquitoes then carry it from infected birds to humans

  • Most people who are infected are asymptomatic (about 90% of them)

  • But about one in ten develop West Nile fever, suffering from flu-like symptoms, with drowsiness and heavy sweating

  • One in every 150 victims, however, has a severe and potentially fatal reaction, either:

  • West Nile encephalitis – (encephalitis is an inflammation of the brain)

  • or West Nile meningitis – (meningitis is an infection of the membranes that protect the spinal cord and brain)

  • Because most people don’t even know they’ve been infected, we can only judge the extent of the disease by the more serious infections that require medical intervention

  • The CDC reports that there were 663 cases reported in 2009, and 335 of them were serious, with 30 deaths

  • More frequent bites from uninfected mosquitoes, incidentally, actually seem to increase the chances of acquiring the virus

  • Some components of mosquito saliva seem to compromise the immune system in a way that makes us more vulnerable to viral infections from other mosquitoes!

  • Warmer temperatures allow mosquitoes to build up larger seasonal populations

  • Higher temperatures also allow the young insects to reach biting age more rapidly

  • All of which results in more effective transmission of the microbe

  • Malarial and dengue mosquitoes will also benefit from global climate change

  • Dengue fever, and the more severe form called dengue hemorrhagic fever, or DHF, evolved as a tropical forest species in Asia, spread by mosquitoes to monkeys

  • It relies for dispersal on the mosquito Aedes egypti

  • As the mosquito’s range increases, so does the range of dengue fever

  • Also known as breakbone fever, dengue is nasty stuff!

  • The viral disease starts with a sudden fever, chills, and headaches, severe pain behind the eyes, and agonizing joint pains

  • A characteristic rash spreads over the body, and in extreme cases internal bleeding can lead to shock and death

  • There is no cure, nor any vaccine

  • WHO estimates that over 2.5 billion people in over 100 countries are at risk for dengue

  • 50 to 100 million people get dengue fever every year, and 22,000 of them will die from it

  • It’s a rare disease in the United States, but with the mosquito expanding its range in response to global climate change, it may soon become a major problem in Southern states

  • We are rapidly changing the global environment

  • And many of these environmental changes will provide new opportunities for the evolution or dispersal of pathogens…

  • We’ve already talked about some of the effects of agriculture on our relationship to microbes

  • One of the most profound effects of agriculture is how it changes habitats, primarily through deforestation, clearing land for pastures and crops

  • Deforestation often brings us into contact with new species of pathogens

  • Over time, ecosystems evolve a fragile balance between species

  • And disrupting that balance can give some species the opportunity for rapid population growth

  • Which brings us to the subject of …

  • Sadly, money doesn’t grow on trees

  • But fortunately for us, chocolate does!

  • And like citrus, it has to be harvested by hand

  • Chocolate is a dry fruit, whose flowers are pollinated by tiny midges

  • The fruit has a fleshy pulp, the part that’s designed to bribe animals to disperse it

  • When we pick it, however, we throw the pulp away, and only keep the seeds

  • We leave the seeds to ferment and dry in the sun, and then we roast them

  • Now it helps to appreciate that seeds in general are often bitter or even mildly toxic, which is a neat adaptive mechanism to keep animals like us from feeding on them (apple seeds contain cyanide!)

  • But where chocolate is concerned, common sense goes out the window

  • We carefully extract the bitter alkaloid-laden seeds and throw away the tasty fleshy pulp

  • A nice example of evolutionary irony!

  • Chocolate comes from Theobroma cacao, and was first used by the Mayans and Aztecs

  • Theobroma is an apt scientific name for chocolate, it means “food of the gods”

  • A recent discovery dates the first use of chocolate to around 1100 BC

  • People then used the fermented pulp, rather than the seeds, to make chocolate beer!

  • We later discovered that the fermented and roasted seeds made a much tastier drink

  • The Aztecs invented hot chocolate!

  • Montezuma was said to drink 50 cups a day (from golden goblets, of course)

  • It was given to Cortez and his men, in return for smallpox and syphilis (gee, thanks…)

  • The bitter and spicy Aztec version, however, wasn’t popular in Europe, but the sweetened version was an instant hit!

  • Milk chocolate was invented by Daniel Peter in 1867, using a process that his neighbor, Henri Nestlé, had developed to make baby food

  • Chocolate, incidentally, qualifies as a recreational drug, a stimulant

  • The chemical formula of theobromine is nearly identical to that of caffeine

  • Chocolate has been cultivated for over 3,000 years, and it is still one of the planet’s most valuable cash crops

  • Although most chocolate comes from West Africa, a large portion comes from the Caribbean and from South America…

  • In Belém, Brazil, in the early 1960’s, over 11,000 residents came down with an unknown flu-like illness

  • The disease turned out to be Oropouche Fever, a nasty but rarely fatal disease

  • Okay, I admit it…I pulled you in with a lurid lecture title

  • The only way you can really die from chocolate is by eating way too much of it

  • Which, for a few of us, unfortunately, may be a distinct possibility...

  • While trying to determine the correct pronunciation of Oropouche Fever, I ran across a letter in a health forum that said “If you can’t pronounce it, and it ends in fever, WORRY!”

  • And Oropouche Fever can be serious stuff…

  • The symptoms are similar to dengue fever, with severe bone and joint pains, chills and headaches – a few victims develop meningitis

  • This viral disease is named after the Oropouche River in Trinidad and Tobago, where it was first described, and is carried by midges or mosquitoes from sloths to humans

  • But why the sudden outbreak at Belém?

  • It took 19 years to solve the mystery

  • The Belém-Brasilia road, one of the first major state highways into the rain forest, was constructed in the 1960’s

  • Settlers followed the highway, clearing large areas for chocolate plantations – and in the process, disturbing the habitat of a forest midge

  • The midge, Culicoides paraensis, turned out to be the vector for Oropouche Fever

  • It had found a new habitat in the discarded fruit shells, building up huge populations near farms and villages

  • The cultivation of chocolate reminds us that ecosystem disruption often represents an evolutionary opportunity for many species

  • It’s what ecologists call a disturbance

  • A disturbance is any force or factor that disrupts an ecosystem

  • Remember that microbes are r-selected organisms, poised to take quick advantage of an environmental disturbance

  • Disturbances can be natural – like fires and floods - or man-made, like land conversion and deforestation

  • The historic pattern in diseases like Oropouche Fever is:

  • The ecosystem is disturbed

  • The disturbance leads to first contact between humans and formerly separated species of microbes, or microbe vectors

  • Then, finally, the microbe enters and adapts to its new human host


  • Man-made changes in global trade and travel have opened up new pathways and habitats for microbes

  • And changes in technology, like hypodermic needles and air conditioning, have also provided new routes of dispersal, and new microbial habitats


  • We are rapidly changing the global environment

  • And many of these environmental changes will provide new opportunities for the evolution or dispersal of pathogens

  • In our next lecture we’ll look at further examples of how ecological disturbances, both natural and man-made, can benefit harmful microbes



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