In our last lecture, we looked at the ways that trade, travel, technology and agriculture can provide new habitats and new dispersal routes for microbes



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Bambi’s Revenge

  • In our last lecture, we looked at the ways that trade, travel, technology and agriculture can provide new habitats and new dispersal routes for microbes

  • In this lecture we’ll consider the effects of warfare on the spread of disease

  • We’ll look at several additional examples of how deforestation can cause the emergence or re-emergence of dangerous diseases

  • We’ll also see how natural changes like El Niño can spread disease

  • And finally, we’ll learn where the next great epidemic might come from

  • Few environmental changes are more dramatic than those caused by war

  • Up until World War II, microbes killed far more soldiers than ever died in battles

  • Warfare provides many opportunities for diseases to spread

  • Dengue fever, for example, was a relatively minor disease, restricted to a few isolated areas, until the second World War

  • WWII Cargo ships spread the Aedes mosquito and its tiny cargo far and wide

  • During wars, public health and sanitation grind to a halt, and malnutrition becomes widespread

  • Soldiers, prisoners, refugees are massed together under unsanitary conditions

  • And local environments suffer profound disturbance

  • Soldiers become both victims and carriers

  • Invading troops bring new diseases to infect native populations

  • Invaders take new diseases home with them after the battle

  • Consider the case of tuberculosis…

  • TB was epidemic during the 19th Century

  • Long working hours during the Industrial Revolution, coupled with poor nutrition, and the low standard of living among the working class, encouraged the spread of TB

  • But improvements in public health, nutrition, and the workplace caused TB to decline

  • TB steadily declines through the early Twentieth Century, then spikes sharply upward during World War I – Australia is the only place that escaped it, perhaps because it was dry, arid, isolated, and sparsely populated

  • As living and working conditions improved after the war, mortality from TB declined

  • Later on, in our lectures on the 1918 flu, we’ll take a closer look at the how war can spread contagious diseases

  • The Vietnam War caused bubonic plague to re-emerge in Southeast Asia

  • Ironically, the current plague pandemic (the third) was originally ignited by warfare

  • In 1894, Chinese troops were sent to quell a Muslim rebellion in a plague-infested region – that plague outbreak continues to this day

  • Although plague was endemic in Vietnam, few cases had been reported since the early 1900’s

  • Part of the plague’s resurgence can be attributed to the general decline in public health and sanitation following the departure of the French from Indochina

  • The ecosystem was also severely disturbed by over 30 million tons of defoliants like Agent Orange, which were used in a vain attempt to deny food and cover to the Viet Cong

  • This deforestation drove rodent species that were previously isolated into closer contact with one another

  • And forest rats exchanged fleas with bandicoots and other rodent species

  • There were only 8 cases of plague in 1961, but between 1965 and 1969, at the height of the Vietnam War, there were 4,500 cases

  • By 1970, plague was widespread in South Vietnam, although reports of plague were censored by the military until 1972

  • Mortality was 60-90% among those left untreated

  • Our troops were routinely vaccinated for plague, and only 8 of those vaccinated acquired it

  • In much the same fashion, King Henry III brought the plague to Sherwood Forest, although it wasn’t war but greed that helped the disease to spread

  • Although Sherwood Forest today is reduced to a mere 1.6 square miles, in Henry’s day it spread over several counties

  • In 1216, King Henry III was forced by greedy and powerful lords to allow extensive clear cutting of the English forest

  • This widespread clearing of land, and the resulting expansion of agriculture, brought peasants all across England into closer contact with rodents, and allowed forest rodents to exchange fleas with rural and urban rodents

  • Deforestation caused a similar problem right in our own back yard, in 1975…

  • Children in Old Lyme Connecticut began reporting strange symptoms

  • They had aches and pains in their bones and joints, mild fevers, a general malaise – they couldn’t concentrate, and they had memory problems

  • The symptoms were so general, it took doctors a long time to recognize that they were seeing something different – so early victims paid a big price!

  • The syndrome became known as Lyme disease

  • It’s caused by a bacterium, Borrelia burgdorferi

  • There are 19 species of Borrelia, all of them carried by ticks


  • It causes a characteristic bulls-eye rash, with mild flu-like symptoms in its early stages, and it’s easy to treat if you diagnose it in time

  • But if left untreated, it soon fades away, only to return later on with a vengeance, causing arthritis-like symptoms and permanent crippling

  • The disease spread rapidly through New England, New York, into the Midwest and west coast, and into the deep south

  • Borrelia probably first evolved to live in forest ticks, its primary host

  • The disease later spread to birds, mammals, and humans

  • But like other vector-borne diseases, it only relies on its primary host for its continued success - it doesn’t really need us, it just uses us if we’re handy

  • Humans are useful hosts, but are not essential to bacterial survival, so the disease can be relatively virulent

  • Nor do the bacteria depend on the deer for their survival

  • Lyme disease is carried by deer ticks of the genus Ixodes

  • Borrelia is passed directly from one tick generation to the next in the tick’s eggs

  • The deer ticks spread the bacteria in their saliva when they bite


  • There is no current vaccine, though antibiotics are effective if the disease is caught in time…

  • It takes about 48 hours of feeding to transmit the disease, plenty of time to find and remove the ticks


  • The juvenile ticks use deer mice as hosts, and the adult ticks use deer as hosts

  • So if you’re out and about in an area known to have deer, it’s a very good idea to check yourself carefully for any hitch hiking ticks

  • The best prevention is to keep your legs and feet covered, and spray well with bug and tick spray before entering fields and forests

  • Why the sudden mysterious appearance and rapid spread of Lyme disease?

  • Once again, the root cause is deforestation

  • Most of the colonial American forest has been cleared for agriculture

  • White-tailed deer have been quick to take advantage of these more open habitats

  • Deer populations are exploding across North America - and once the deer are infected, they will leave a trail of infected ticks wherever they go, waiting in ambush for us to walk by (the ticks, not the deer…)


  • The clearing of forest land for agriculture caused rapid growth of tick, rodent, and deer populations

  • The expansion of suburbs into cleared farmland brought large numbers of people into regular contact with deer habitat, where ticks might be waiting in the grass

  • As all these examples remind us, deforestation completely disrupts an ecosystem, and can create new opportunities for a wide variety of microbes and vectors

  • The forest may regenerate, through the natural process of succession, but the new forest will never be quite the same…

  • It often lacks many of the checks and balances that used to keep it healthy

  • Most of the big predators are gone

  • Populations of rodents and other species that like disturbed areas - clearings, weedy or grassy spots - increase very rapidly, especially r-selected species

  • We’d better hope that the bacterium that causes Lyme disease doesn’t decide it likes us better than its current primary host, the deer tick

  • African relapsing fever has already taken that fateful step…

  • African relapsing fever is carried by Borrelia duttoni, a close relative of the species that causes Lyme Disease

  • African ticks rely on a variety of rodent vectors - wood mice, deer mice, and the white-footed mouse

  • Relapsing fever earns its name, because it keeps coming back for more

  • Symptoms include fever, headaches, muscle and joint aches, and rash, all of which can last for a few days, up to a week

  • The patient recovers, and then gets sick again - the cycle can repeat for several weeks

  • Borrelia duttoni now occurs only in ticks and humans, bypassing all the other intermediate hosts that it used to prefer

  • And Lyme disease has become so widespread, there is a very good chance it might do the same!

  • In addition to deforestation, agriculture has altered habitats by flooding them for irrigation

  • Flukes like Clonorchis, and Schistosoma have taken advantage of our growing reliance on irrigated agriculture

  • Irrigated acreage has tripled since 1950

  • There are over 260 million acres of irrigated cropland world wide

  • Flukes are highly modified parasitic flatworms, with suckers to attach to their hosts

  • They range in size from microscopic species that barely reach a millimeter, to monsters over 20 feet long!

  • They can be spread by the use of human waste as fertilizer in irrigated fields

  • Fertilizer is very rare and costly in most of the world – so you use what you have on hand

  • Clonorchis sinensis, the Chinese liver fluke, infects 20 million East Asians

  • This tiny little creature, about 10 to 25 millimeters long, can cause severe jaundice, and liver cancer

  • Its intricate life cycles includes multiple hosts - a fish, a snail, and a human

  • Schistosoma is the fluke that causes Schistosomiasis


  • The symptoms are many and varied, and include anemia, diarrhea, and brain damage

  • The fluke damages the bladder, kidney, spleen, intestines and other organs

  • At least seven of the 21 species of Schistosoma are known to infect humans, and most of them use a snail as an intermediate host

  • The eggs are passed through human urine or feces

  • It infects over 200 million people in the tropics, and over 20 million of these victims become seriously ill

  • The acute form of this disease is often fatal, with a 25% mortality rate!

  • Irrigated fields provide a new habitat, where parasites, their intermediate hosts, and humans are brought into close proximity and regular contact

  • The use of human wastes as fertilizer can complete the parasite’s life cycle

  • But humans are not the only source of environmental change…

  • Natural changes in the environment can also create new opportunities for microbes

  • Consider the attack of Hantavirus on the Navajo nation…

  • The Four Corners area, where plague is also endemic, seems to be a surprisingly hazardous place to live…

  • The Navajo believe that disease occurs when the balance of the universe is disturbed

  • The Hopi word Koyaanisqatsi means “life out of balance”

  • That balance was upset in May 1993, in the Navajo nation’s Four Corners reservation, when a young man died on his way to his fiancé's funeral

  • An autopsy showed that both he and his fiancé had died from the same mysterious disease

  • Everyone’s first thought was bubonic plague, which is endemic in the American southwest

  • But medical detectives soon pinned it to a hantavirus

  • This type of virus is named after the Hantan River in Korea, where it is endemic

  • It’s carried in Korea by striped field mice, and it’s one of many viruses that cause hemorrhagic fever (fever with internal and external bleeding)

  • The Navajo type of hantavirus, however, was unknown

  • It was appropriately called the “Sin Nombre” virus - sin nombre means no name!

  • Of the 22 types of hantavirus found around the world, 18 of them are found in the New World, and all of them are associated with rodent vectors

  • In Louisiana, hantavirus is carried by the rice rat

  • Fortunately, Bayou hantavirus is pretty rare

  • The Navajo strain of hantavirus, however, was especially deadly, with a 67% mortality rate (now down to about 35%)

  • Like many new-world strains, it causes HPS, hantavirus pulmonary syndrome

  • Hantavirus starts with a raging fever, muscle aches, headaches, and cough - like a really bad flu

  • Some strains can cause severe kidney damage – a condition known as HFRS – hemorrhagic fever with renal syndrome

  • Other strains, like the one that afflicted the Navajo, cause severe lung and respiratory system damage, a condition called HPS – hantavirus pulmonary syndrome

  • HPS strains of hantavirus are very virulent, and in about half of the victims, HPS causes respiratory failure and death

  • Since 1993, about 20 to 40 cases of HPS are reported in the US every year

  • The disease spreads in the urine, droppings, and saliva of rodents – yuck!

  • So if you live in a nice clean house like me, you might think you’re off the hook for this one…

  • But mouse urine turns out to be harder to avoid than you might think!

  • When it dries, particles of mouse urine are picked up on bits of dust, and carried through the air

  • A small army of researchers combed the Four Corners area for rodents

  • They caught and analyzed over 1,700 rodents

  • The primary vector for the Sin Nombre virus turned out to be the deer mouse Peromyscus

  • About 30% of the trapped Peromyscus were infected

  • But where did all the deer mice and the virus come from?

  • Both vector and microbe had been there all along - antibodies have been found in cases dating as far back as 1959

  • El Niño conditions caused unusually high snow fall in 1992 and 1993, after several years of drought

  • The melting snow fueled rapid growth of vegetation, including evergreen Piñon trees

  • Piñon trees produce Piñon nuts, which are eaten by deer mice

  • Exploding populations of deer mice, fueled by the bounty of Piñon nuts, came into closer contact with the scattered rural population in Four Corners

  • Plague in South Vietnam is also affected by similar natural cycles

  • Rodent populations and flea populations in Vietnam are inversely correlated with rainfall

  • Plague breaks out in Vietnam’s dry season as rodent populations expand, and declines in the wet season when rodent populations contract

  • Microbes respond quickly to any kind of environmental change

  • And we’ve been most obliging in providing it, with our fast-paced technological progress and our meteoric population growth…

  • Given the rapid expansion of human civilization into our few remaining wild areas, what does the future hold for us?

  • Where will the great epidemics of tomorrow come from?

  • They may already be lurking in some forgotten corner of the Earth, waiting for an unwary victim to carry them far and wide

  • We have already had several close calls…

  • Jan. 12, 1969, in the sleepy village of Lassa, Nigeria, something came out of the jungle

  • Laura Wine began complaining of a backache

  • Within two weeks she was seriously ill, and began showing unusual symptoms

  • Her skin began to swell and hemorrhage, and she was unable to speak

  • A day later she died…

  • Three days later, one of the nurses who had treated her fell ill, with the same mystery disease

  • Nurse Charlotte Shaw had scratched herself on the thorn of a rose, the day before treating Laura

  • That scratch spelled her doom…

  • An autopsy showed that Shaw’s body had been thoroughly ravaged by the disease

  • Her lungs filled with fluid

  • Her arteries were clogged with platelets

  • Her lymph system was drained completely dry

  • And she had suffered massive internal damage

  • Lily Pinneo, one of the nurses completing the autopsy, soon lay dying herself…

  • Day after day she drifted away, drained by her constant fever

  • She lost control of her muscles, her eyes twitching uncontrollably

  • Her temperature reached 107 degrees!

  • But her high fever probably saved her life, by burning out the infection

  • After several weeks, she began a slow and miraculous recovery

  • Samples of the virus, now known as the cause of Lassa fever, were kept for awhile in a lab at Yale

  • In retrospect, not the most secure location…

  • Because the virus nearly escaped into the New England countryside!

  • Dr. Jordi Casals-Ariet was searching for the virus in lab samples, and somehow contracted it

  • We still don’t know how it could have happened

  • His family came home, and found him ill

  • He was taken to an isolation chamber at Columbia University’s Presbyterian Hospital, where he was injected with serum from the recovered nurse Pinneo

  • Casals-Ariet quickly recovered…

  • Yale officials later pressured him to get the stuff off campus (no kidding! – imagine all the frantic parents of those college students)

  • He finally sent the samples to the CDC

  • But it was too late to save Juan Roman, one of his lab technicians

  • Roman fell ill while going home to York, Pennsylvania for a family Thanksgiving

  • He was admitted to a local hospital - without any quarantine, because no one knew what he had – and he quickly died

  • How he got it remains a total mystery – he never worked first-hand with the virus

  • Sporadic outbreaks of Lassa fever have occurred across Africa ever since - in Gabon, Nigeria, Guinea, Liberia…

  • There are an estimated 300,000-500,000 infections per year in West Africa, with about 5,000 deaths each year

  • The disease is carried by Mastomys rodents, which are very common

  • Because there is no realistic way to control the vector, public health campaigns in Africa stress better control of food preparation and food storage to keep rodents away, and discourage the use of rodents as food

  • Lassa Fever has never succeeded in invading the US, but it continues to be a threat

  • Just one of the many mysterious diseases that await us in the forest…

  • The virus turned out to be another member of the diverse group that causes hemorrhagic fever

  • Hantavirus, and the virus that causes yellow fever are members of this same group, which spans four different Families of viruses

  • One of the latest members of this group to emerge is Ebola hemorrhagic fever, named after the Ebola River in the Congo, where it was first described in 1976

  • Like many of the diseases discussed in this lecture, it is zoonotic – it’s carried by animals, although in the case of Ebola, we still haven’t discovered the vector

  • The Ebola virus is an example of a zoonosis (pl. zoonoses) – a microbe that spend part of its life cycle on us, and part on a different species of vertebrate

  • The symptoms include high fever, severe headaches, muscle aches, and diarrhea

  • It can rapidly progress to severe internal and external bleeding, and death

  • Ebola is frightening in its rapid onslaught and its utter devastation of the human body

  • It dissolves internal tissues, and its victims ooze blood from every orifice

  • Victims often become delirious and hard to control, tearing their clothes off, running about, talking to invisible relatives…

  • Ebola is a savage killer, with a fatality rate of 50-90%!!

  • Transmission is through infected blood, semen, and other bodily fluids

  • Without knowing the “natural reservoir”, however, we don’t know how the initial victims get the disease

  • Since it was first discovered, 1,850 cases of Ebola have been reported, with over 1,200 deaths

  • There is no cure, although a promising new vaccine is currently being tested

  • From June to November 1976, Ebola infected 284 people in Sudan, causing 151 deaths

  • An outbreak in the Congo, in 1995, infected 318 people, with 280 fatalities

  • The last major outbreaks were in Gabon in 1994 and in 1996 (116 cases, 75 deaths), and in Gabon and the Congo between 2001 and 2003 (302 cases, 254 deaths)

  • Ebola affects monkeys, chimps and gorillas as well, and is suspected of causing massive mortality among wild primate populations

  • Up to 88% of the Central African chimpanzee population may have died of Ebola since 1993

  • We may finally be on the right track for finding the vector

  • Following up on outbreaks in 2001 and 2003, a research team in 2006 caught over 1,030 small animals for testing

  • Most turned out to be negative, except for several species of fruit bats who were asymptomatic for Ebola – they showed no symptoms, but were infected with the virus

  • Fruit bats are known to drop partially eaten fruit

  • Maybe foraging primates find the fruit, contaminated with bat saliva, and eat it

  • Humans might get the disease by handling the bodies of primates

  • Gorillas and chimps suffer from it just like we do, and we consider them victims, not vectors – hey, they’re one of us – or more accurately, biologicall speaking, we’re one of them…

  • Most natural reservoirs of pathogens are asymptomatic – they carry the disease, but don’t suffer from it

  • Bats might also be capable of passing the disease directly to humans

  • One of the first outbreaks in Sudan was in a cotton factory that was infested with bats

  • Fruit bats might also turn out to be vectors for Marburg virus and SARS virus

  • But why the sudden emergence of Ebola?

  • We may be witnessing another example of agriculture moving into new areas, bringing people into more frequent contact with bats

  • One of the five known types of Ebola, called Ebola-Reston, almost escaped into the US in October, 1989

  • Hazleton Laboratories, in Reston, Virginia, was studying Simian hemorrhagic fever (SHFV)

  • They found Ebola virus in three of their monkeys, part of a large shipment of Crab-eating Macaques that had been sent from the Philippines by Ferlite Farms to New York City, Mexico City, and Alice, Texas

  • As the disease continued to spread, one-third of the Reston monkeys died of Ebola (none of the other labs reported a problem)

  • Six lab workers tested positive for Ebola antibodies, but they displayed no symptoms

  • This particular strain, as it turned out, seems to be fatal to monkeys, but benign in humans

  • Whew…

  • Several monkeys from Ferlite Farms also died at Hazleton’s Texas Primate Center in Alice, Texas, and at the Reston site, in January of 1990

  • The Alice, Texas lab had another outbreak in 1996, once again from Ferlite Farms – you’d think at this point, folks would be shopping at Monkeys-R-Us instead…

  • This one was a little scarier, because one of the two monkeys that died was located at the far end of the cage block from the other one

  • The virus might have gone airborne, traveling through the ventilation system

  • Dustin Hoffman’s 1995 thriller Outbreak was inspired in part by the first Reston incident

  • The film turned out to have anticipated this deadly evolutionary twist, with its story of an Ebola-like virus that spreads from a smuggled monkey, and launches an airborne epidemic

  • We must always remember that we are not alone on this planet

  • When we make radical changes in our environment, we change the rules for all the species we share it with, including the microscopic ones

  • Our farms, our factories, and our battlefields often alter the environment in favor of dangerous microbes, like those that cause TB, bubonic plague, and Lyme disease

  • Natural changes, like El Niño, can also change local environments in ways that favor microbes or their vectors


  • And so we’ve learned, in our last five lectures, how humans and microorganisms have coevolved with one another, a complex and intricate dance of strategy and counter-strategy, of change and adaptation

  • We’ve changed the world that microbes live in, and they in turn, have changed our world in many ways


  • We’ll never know how many Mozarts or Newtons were nipped in their prime by a microbial killer, how many Rembrandts or Shakespeares were killed in their cradles by a creature too small to see

  • It’s a delicate balance - but there are times in human history when microbial strategies prevail, and microbes carry the day



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