The Natural History of Disease
Diseases infected our ancient ancestors, followed as they evolved to become Homo sapiens, and continue to infect us today. For instance--
Tuberculosis and malaria are epidemic diseases common in developing nations and both have been epidemic here in the past. Today we no longer fear them but are we wise to dismiss them? Dr. Paul Epstein, associate director of the Harvard Medical School Center for Health and the Global Environment, warned in the August 2000 issue of Scientific American that malaria has begun to expand its range as the climate grows warmer. Currently it kills 3,000 people worldwide everyday. By the end of the century, malaria's "zone of transmission" will reach 60% of the world's population, up from the current 45%.
Malaria is transmitted from person to person by the bite of a female mosquito. In San Diego County, thousands of immigrants cross the international border every year. Many are coming from areas where malaria and other common mosquito transmitted diseases, like dengue fever and encephalitis, are epidemic. The San Diego County Department of Environmental Health is aware of the health implications. The Vector Surveillance Program monitors and manages seasonal and year round mosquito breeding areas. This includes ponds, wetlands, and backyards, places where discarded tires and containers can hold water for mosquitoes to breed in. They program tracks breeding conditions in 1200 large water sources with a sophisticated geographic information system.
On the other side of the world, the breakup of the Soviet Union, with continued political unrest and a depressed economy, has impacted health care. Once nearly eradicated tuberculosis is now back in the population. Without access to proper medicine the disease is now epidemic and frequently drug resistant. Consider also--:
Jared Diamond in the Pulitzer Prize winning Guns, Germs, and Steel: The Fates of Human Society states that "questions of the animal origins of human disease lie behind the broadest pattern of human history, and behind some of the most important issues in human health today."
To trace the animal origins of human diseases (called zoonoses) we should understand the evolutionary requirements of infectious organisms, including their host relationships, and adaptive strategies. All organisms, including bacteria, viruses, fungi, and parasitic microbes, follow the same force of natural selection to reproduce successful offspring. Populations of organisms must also adapt to changing environmental conditions. Some microbes, such as viruses, require a living host; others, such as bacteria, fungal spores, and parasites, can also survive in soil and water while waiting for the proper host. Microbes must be able to adapt quickly, reproduce rapidly (bacteria reproduce every 20 minutes), and infect broadly. With any change in the environment, whether in the host, the recipient or their ecological setting, those able to adapt most effectively survive. To be successful a microbe must--
Immediately as a microbe infects a host, the host begins to respond. Host responses include fever and antibody production by the immune system, or more long-term responses which change behavior or build genetic barriers to infection.
Malaria, another heritage disease, provides excellent examples of cultural and genetic reactions. Vietnamese hill tribes construct their homes on stilts above the flying level of malaria transmitting mosquitoes. Other cultures use natural remedies, such as eating fava beans for their antimalarial effect, or wrapping in heavy cloth at night. The people of West Africa evolved the sickle-cell trait that starves the malarial protozoon of hemoglobin but can also weaken the child who carries the trait.
Modern medicine provides a wide range of disease-fighting agents including antibiotics that are temporary and vaccinations that bring permanent immunity. Microbes respond by evolving new strains resistant to our wonder drugs. Even domestic animals are now medicated in an attempt to cure or prevent their infection, and by extension, our own. Seems like a good idea but, as the Food and Drug Administration recently announced, feeding two antibiotics to chickens and turkeys was causing the development of antibiotic resistant strains of bacteria which are showing up in humans. The agency has moved to withdraw its 1995 approval of the practice. Similar effects have been found with cattle (see E.coli above).
Closer to home, many of our local desert bighorn sheep (federally endangered) have been vaccinated against diseases brought into their range by feral cattle. These diseases are devastating to these isolated wild animals with no natural immunity for protection. Humans are not part of this desert cycle but such exchanges also existed in the past. As soon as we domesticated herding animals like goats, sheep, pigs, and cows, we entered the chain of infection.
Animal Origins of Human Diseases
Farmers and Herders
Trade Routes and Epidemics
With the availability of surplus grain, animals, and manufactured goods, worldwide trade routes developed, joining together populations over large geographical areas. These trade routes became the avenues for disease transmittal, beginning with Europe, Asia, and North Africa. Smallpox first appeared in 1600 B.C. but did not reach the Roman Empire until A.D.165. Between then and A.D.180, millions of Romans died of smallpox. Bubonic plague appeared in Europe in A.D.542 but did not decimate the population until A.D.1346 when new routes for overland trade with China provided transit for flea-infested furs from plague-ridden Central Asia. After 1492, when Columbus reached the West Indies, North, Central, and South Americans fell easy victim to Eurasian diseases with many cities and villages suffering nearly 100% mortality. Prior to Columbus there were no epidemic diseases in the New World (except possibly syphilis, although that debate continues) because there was no breeding ground for them with few domestic animals and none that tolerated crowded living conditions. Spaniards brought the ancient diseases, like smallpox and measles, with them, but these diseases had long ago severed the connection with their animal hosts to become directly transmittable from human to human.
The spread of pathogens no longer depends on the speed of a caravan or long sea voyages. Transatlantic flights allow tourists, business people, political refugees, migrant workers, and soldiers to travel around the world in hours, with the potential for bringing new diseases with them. Container ships carrying goods from around the world can import everything from trucks to rats and mosquitoes. International trade in laboratory animals and pet species creates the potential for pathogens to travel as carry-on luggage. Several new cat and dog infections (feline and canine parvoviruses) have spread worldwide in a few years.
The tiger mosquito (Aedes albopictus) is suspected of entering the United States in the millions of worn tires shipped annually from Japan for retreading. Tiger mosquitoes are container-breeding specialists and tires are perfect habitat. Based on their Asian behavior, health officials' fear that as its range increases and contact with humans intensifies, the likelihood of transmission of indigenous North American viruses will increase. Mosquito expert William A. Hawley believes the likeliest candidate is the LaCrosse virus, which causes encephalitis in children and circulates between native tree-hole breeding mosquitoes and chipmunks and squirrels.
With this background, the story of the West Nile Virus is instructive. This encephalitis outbreak dramatically surfaced in New York during the summer of 1999. Not only humans were infected with the flu-like symptoms, but there were a large number of bird deaths (mostly crows) as well. Continued investigation implicates a causal chain that includes imported wild frogs for laboratory use, the tiger mosquito with a fancy for frog's blood, wild crows, and humans.
What can we do? Can we quarantine our borders? As author Arno Karlen observes, a rigorous international quarantine system seems out of the question. We have to accept becoming part of a bigger and more diverse disease pool.
Adaptive Stages of Disease
There are three adaptive stages of disease in a population, grading from the most severe to the benign: epidemic, endemic, and symbiotic. Medical science defines epidemic as an unusual or increased adverse effect on the health of a population. The word is derived from the Greek demos for "the people". Endemic means "native" to the population and epidemic means "visiting" the population.
When an epidemic disease (such as measles when it arrived in the New World) enters a new population that has few defenses, it causes acute symptoms in people of all ages. The survivors are left with improved defenses against new infection. Epidemics recur when there is a new population to infect. When an epidemic disease (again think of measles) becomes widespread in a population, its virulence decreases to be replaced by a lower-grade infection or as a childhood disease. The epidemic has become endemic, it has stopped being just a visitor. When, after long continued adaptation, the disease and host relationship develops into mutual tolerance or even benefits, it is considered symbiotic.
In 1992 a panel of distinguished researchers chosen by the National Institute of Medicine produced a landmark study concentrating on six major causes of disease emergence:
Public health professionals are concerned that events growing from any of these six causes could cause an endemic disease to reenter the epidemic category. Although we have what seem to be new diseases appearing every day, careful analysis shows that they may not be new only newly encountered (hence a new name), newly arrived, or perhaps an old acquaintance, newly resistant and eager to infect.
Zoonoses is the inclusive term for diseases that pass from animals to humans and are spread by animal bites, saliva, and scratches, or through contact with an animal's dander or waste in polluted water, or other filthy conditions. By "animals" we are referring to pets, such as dogs, cats, birds, and small reptiles, domesticated animals like cows, horses, pigs, and goats, and wild animals including rodents, canids (wild dogs), mustelids (skunks and ferrets), birds, primates, bats, mosquitoes, fleas, and ticks. It is characteristic that severe acute diseases will, over time, lessen in severity and become chronic. Syphillis is a good example. In its original form it killed people rapidly and in an ugly manner. Today, without treatment, it works its way through the body and brain over a period of years, remaining infective all the while.
For a thorough exploration of zoonotic diseases, their role in the spectrum of human disease, in medicine as biomedical models, their economic importance, and control, consult the Encyclopedia Britannica website.
Crosby, Alfred W. Jr. 1972. The Columbian Exchange: Biological and Cultural
Consequences of 1492. Greenwood Press, Westport. Connecticut.