Science Tribune - Article - September 1997
Will microbes conquer the human race?
Daniel E. Wexler
E-mail : email@example.com.
We humans are a resilient flock. The year was 1343 and the bubonic plague was busily decimating Europe. A present day reminder of that time, an innocent children's play song (a):
Ring around the rosy
Pockets full of posies.
And we all fall down!
It is a bare shadow of the horror that a 80-90% mortality could inflict on Italian cities such as Venice and Florence. Surprisingly, the loss of up to a third of the European populace in a few short years did little to stem the tide of human growth which accelerated to fill that brief void. The plague returned many times, although with diminishing ferocity and, even today, it can be found in its natural reservoir of fleas, occasionally rearing its head against the unwary wanderer in the U.S. and elsewhere in the world.
An indelicate balance
James Lovelock and Lynn Margulis proposed in their 'Gaia Hypothesis'(1) that it is the interplay of multitudinous organisms that maintains the earth's homeostasis - climatic and otherwise - which means that living forces act with and against each other to establish a constant, or at least predictable, set of living conditions (2). The genesis of Gaia, according to Lovelock, was the slow accretion of evolutionary building blocks that added to already existing complexities. Like an interlocking puzzle, the loss of a single piece is disturbing, and the whole is no longer complete or as strong a scaffold.
The monkey wrench
Homo sapiens is the wrench thrown into the Gaian gears. Until the late Middle Ages, humans existed in small numbers punctuated by aggregations within city-states such as Athens or Rome. Later, the European and Asian populations only gradually increased but, as the 19th century approached, populations blossomed with a vigor unmatched at any time in the past (3). This surge coincided with a set of breakthroughs in knowledge and technology, especially in agriculture, which increased the amount of food available at harvest. Of profound importance beyond food production was an increase in child survival and adult longevity due to improved sanitation and waste disposal methods. These reduced the incidence of diseases such as cholera, one of many which served to check human population growth. At this juncture in the evolution of human civilization the population leapt dramatically, entering an exponential phase of Malthusian proportions which we are still furiously birthing. However, like any closed system, a ceiling will soon be reached beyond which further growth will not occur (3).
A disturbed climate
An additional factor contributing to the disastrous spread of the plague in the 14th century was a natural change in the European climate - a mini ice age. A chill that was to persist until the 18th century descended on the continent, only a few degrees, but enough to cause crop failure, widespread famine, and disease (4). Could such a change in temperature happen again? There are two alternative and not mutually exclusive phenomena that make this likely if not inevitable : nature in flux, and the effect of humans.
Nature in flux
Examination of Antarctic ice cores have led to the not unexpected finding that global temperatures have fluctuated greatly over geologic time. In fact, periods of climatic stability are the exception not the rule. Our civilization owes a profound debt to an unusually extended period of constant global temperatures which could cease at any time. A minor change in the earth's orbit could easily cause the average global temperature to drop as much as 4°C within the span of centuries, sufficient to propel us into a deep ice age. Even a much smaller change would have a dramatic effect on local temperature ranges and weather patterns. And it is clear that climate can have a dramatic effect on the incidence of some arthropod-mediated diseases (4).
The effect of humans
Astronauts aboard the space shuttle have reported, on looking down from orbit at South America, massive plumes of smoke emanating from various locations on the northern part of the continent, primarily Brazil. This is stark testimony of our ability to effect environmental change. There, farmers with government encouragement are embarked on a pattern of slash and burn agriculture despite the inability of nutrient-poor tropical soil to act as cropland for more than a few years. The consequences are devastating for local plants and wildlife, many of which are found nowhere else, not to mention migratory birds that summer in the northern hemisphere. All this sets the stage for massive species extinctions. To complicate matters, the burning of millions of acres of rainforest results in the injection of CO2 and other greenhouse gases into the atmosphere. Combined with industrial and automotive pollution, temperatures worldwide could potentially be induced to increase (5) (6). Paradoxically, this would mask for a time any natural climatic cooling that may be occuring.
The1960s Hollywood movie 'Soylent Green' (b) described a vastly overpopulated future in which human pollution nearly obliterated oceanic life. Marine life depends on light energy captured from the sun by algae, which uses that energy to power the manufacture of sugars from CO2. This establishes algae as the main course on the menu for a host of sea creatures. In addition, the algae excrete a peculiar waste product - oxygen. The 21% oxygen content of the atmosphere is largely owed to these creatures which live precariously close to the edge of human interference. Humans are intimately adapted to this concentration of oxygen, although the capacity in most to adapt to thin atmospheres found at high altitudes suggests some plasticity. Nevertheless, a large enough depletion of atmospheric oxygen would be disastrous. In addition to the known CO2 concentration increase, we have evidence that human activity has altered levels of another atmospheric gas - methane. Interestingly, this is presumed to result in part from the flatulence of domesticated cattle which generate methane with unknowing abandon.
Spoiled nest syndrome
Industrialized societies are built upon the use of fossil fuels for both energy and the generation of ubiquitous materials such as rubber and plastics. Estimates of the world's oil reserves indicate that at current consumption levels supplies will tighten within the next decade and vanish altogether by mid-century. Certain metals are similarly important to society and nonrenewable, even scarce. One question we face : do we have the inventiveness to find cheap and plentiful substitutes for such diminishing resources? Imagine a scenario in which the international scientific enterprise fails to provide economically viable substitutes for fossil fuels, plastics, stainless steel - a future dominated by slippage towards a less technological existence. This process of deindustrialization would likely occur in such a case, given sufficient time. Along with a decrease in the availability of technology, the human population of the future would almost certainly be insupportably high relative to available resources, resulting in a general decline in living standards. This would set the stage for a renewed role of pathogenic microorganisms in shaping the tenor of human society.
We have long taken for granted the miracle of antibiotics. When penicillin use became widespread following World War II, public reliance on drugs, vaccines and other health-safety measures became a given. Common and devastating diseases such as measles, tuberculosis, dyptheria and whooping cough became rarities in industrialized societies. In 1977, smallpox was eliminated from the face of the earth. In 1945, DDT was sprayed upon vast areas of the southern U.S., successfully destroying malaria-laden mosquitoes. These early victories, however, led to complacency about the ability of disease-causing microbes to adapt to such onsloughts.
"The Antibiotic Paradox" was coined by Stuart Levy in 1993 to describe the seeming anomaly that protective measures inevitably lead to evasive action by bacterial pathogens (7). Penicillin-resistant organisms appeared almost immediately following the adoption of antibiotic therapy. It is now recognized that soil microbes serve not only as a source of antibiotics, but as a source of "contagious" antibiotic resistance genes that are easily passed to the gut bacteria of farm animals and from there, to humans (7). The misuse of antibiotics in farm industries as well as human medicine directly selects for the evolution of multidrug-resistant pathogens. Therefore, despite the advent of many natural and synthetic drugs initially effective in the onslought against disease, "superbugs" have evolved which cannot be controlled by any means currently at our disposal.
The zoo factor
The furor over "Mad Cow Disease" in Britain underscores the reality that most diseases, unlike smallpox, are not specific to humans. Rather, they inhabit a spectrum of creatures from mammals to reptiles to birds (8). Moreover, insect and arthropod vectors, never far away and promiscuous in their pursuit of blood, are effective go-betweens. Influenza is a virus with an unusual property : it mixes and matches chromosomes with others of its kind until new combinations arise that are capable of evading the combined immunities of human populations to the older strains that have already done the global circuit. Interestingly, the stewpot for this virus is not at all human. For example, chinese pig-duck farms, while an effective combination for the production of food stuffs, are also a pair known to spawn new varieties of flu virus (9). It has recently been proposed that sheep scrapies, a fatal brain disease, crosses species barriers to evolve both mad cow disease and in humans, Creutzfeldt-Jacob disease (10). The elimination of smallpox, heralded as a victory in the war on disease, may be shortlived; the genetic heritage of a similar virus in monkeys, monkeypox, is already adapting to humans and could be genetically capable of forming a version that is more highly contagious between humans (11). Although controllable by vaccine, it would be virtually impossible to eradicate since unlike smallpox it is maintained in both monkey and rodent reservoirs. One final example of the zoo factor is HIV, the causative agent of AIDS. HIV2 is a form of AIDS virus found exclusively in Africa. HIV2's DNA sequence is remarkably similar to the simian version, SIV, suggesting that HIV may have originated from monkey populations (12). The above observations point out that drug intervention and vaccination programs, while capable of reducing disease incidence, do so without disturbing the animal reservoirs from which the same or novel diseases can emanate.
The internationalization of travel lends to the rapid dissemination of many types of living organisms to foreign lands (13). Carving a niche in their new location depends on a variety of environmental factors such as temperature, moisture, food supply, and predators. Microbes are exquisitely easy to seed abroad. The recent reintroduction of cholera to South America appears to have originated from contaminated bilgewater pumped out of merchant ships from Asia (14). The international used-tire trade has led to the import of an especially unwelcome customer, the Asian Tiger Mosquito (15); an aggressive biter, it is capable of hosting a variety of diseases previously unknown to the U.S., such as Dengue hemorrhagic fever, and already has been implicated in the spread of eastern equine encephalomyelitis (17).
When opportunity knocks
Microbes are notoriously opportunistic. They are known to fly through the smallest of ecological windows. Their ability to outfit themselves for survival in new environments similarly gives them an advantage in their battle with the human race. Both microbes and their brainy multicellular nemesis feint, strike, withdraw, and regroup in evolutionary patterns we are now only beginning to understand. The strongest health measures of all are not biotechnological, but good sanitation, healthy living conditions, and educated personal behavior. Unfortunately, poor economies sacrifice on both fronts of medicine and hygiene. Russia, once a strong provider of health services to its people, is in the midst of poverty and socioeconomic chaos. The lack of rubles for vaccines has allowed a resurgence of nasty diseases such as diphtheria which were previously thought conquered (17). New at-risk behaviors as well as rural flight to already congested cities provide a fertile breeding ground for human epidemics. In the short-run, the future will be calamitous if technology fails to find magic bullets against diseases, strategies which the pathogens cannot adapt to. To look further, we need to look backwards into history. For thousands of years, the multitudes of tribal societies struck a balance with their surroundings, acknowledging that illness is a part of that balance, inescapable and natural. As societies evolved and learned to interact with each other through trade and travel, new opportunities arose for pathogens to extend their territory and adopt new armaments (18). Antibiotics and other drugs dealt minor setbacks, but these are being defused as weapons. Vaccine programs are effective for only a limited set of diseases at present, and they easily fall victim to the common social afflictions of poverty and civil unrest. However, the old microbial disease status quo will not just be regained, but surpassed in terms of sheer numbers of victims unless international travel and trade are sharply curtailed - an unlikely prospect. Instead of localized pockets of disease, the causative agents are taking hold in new, supportive environments scattered across the world landscape.
It seems inevitable that microbes will win back their own territory and more. Human efforts to subvert this process will only delay it. The upside of this equation is that mankind will no doubt survive, although in a form profoundly shaped by chronic microbial onslaughts. At the heart of the problem of disease continues to be overpopulation, crowding and poverty, the exhaustion of natural resources, and climatic change. Rather than relying on temporary or economically fragile medical countermeasures for our health and well-being, it is our ability and willingness to respond to the devils that govern our susceptibility to disease that will have the greatest impact on our future (19).
(a)"Ring around the rosie" describes a major bubonic plague symptom consisting of grossly enlarged and inflamed lymph nodes (buboes) which may burst before the victim dies. "Pockets full of posies" refers to a European death ritual of the Middle Ages. "Ashes" are the remains of cremated disease victims.
(b) The film 'Soylent Green' is based on the story, 'Make Room! Make Room!', by Harry Harrison.
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