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Medical Ecology

By JoDean Nicolette, MD

Medical ecology is an interdisciplinary field that focuses on how human-driven environmental change affects human well-being. A central theme is the reconciliation between human health and the health of the ecosystem.

Many principles underlie the concept of medical ecology, including cooperation, sustainability, diversity, nonmaleficence and resilience. Perhaps most important is the principle of interdependence. Physicians are familiar with interdependence in the context of physiology and the intricate interactions of many complex systems in the human body. In fact, interdependence extends to all living and nonliving components of the planet, from food webs (no such linear things as “chains”) to larger biological neighborhoods.

Medical ecologists believe that we all share an interconnectedness that stems from our common evolutionary heritage. Altering one part of the ecosystem necessarily impacts many other parts of the system. Human choices ripple through connected systems in the environment and ultimately impact human health.

The relationship between human and ecological health can be clearly seen in four main areas: emerging infectious diseases, pesticides and toxins, large-scale environmental change, and biophilia.

Emerging Infectious Diseases
HIV, SARS, Ebola virus, West Nile virus and Lyme disease are classified as emerging infectious diseases. Such diseases display an increase in incidence, an expanded geographic area or host range, a new pathological effect, or a newly evolved pathogen. One of these emerging infections, Lyme disease, is likely the result of human-induced ecologic change.[1]

According to the CDC, Lyme disease is the most common tick-borne illness in the United States, with an incidence of about 40,000 cases per year.[2] The disease, first characterized in 1975, is caused by a bacterium that lives in a steady state in the environment. The hosts are a diverse group of wildlife, including white-footed mice, blue-bellied lizards and white-tailed deer. The bacteria are spread from host to host by ticks, which are known as “vectors.” The relationship between host and vector is no doubt more complex than the simple term “parasitic,” but we do know that neither the ticks nor the hosts become ill from colonization. Scientists only became aware of the bacterium’s effects when humans were infected and became ill.

What happened to disrupt the steady state in nature and cause Lyme disease to infect humans? In the view of medical ecologists, the answer lies in human activity. When people began spreading out in the Northeastern United States, they infringed on the natural habitat of many tick hosts, decreasing not only the number, but also the types of animals (decreased biodiversity). Mice, for example, have a higher survival rate in suburbia, farmland and industrial areas than lizards or deer. In addition, white-footed mice are more likely than other hosts to infect ticks with Lyme bacteria.

The mouse population in the Northeast declined when widespread use of pesticides reduced their food supply. This decline left fewer mice but more ticks. As humans spread into the habitat, people were more likely to get bitten and infected by ticks. Eventually Lyme disease emerged.
An interesting testimony to interdependence is notable here. One of the hosts whose population declined with the spread of humans, the blue-bellied lizard, harbors a component in its blood that cleanses ticks of Lyme bacteria. Ticks that bite blue-bellied lizards never harbor Lyme bacteria.[3] As biodiversity in the Northeast decreased, blue-bellied lizards were reduced or eliminated, removing a natural check against Lyme disease in a very complex and interdependent natural environment. The net result of habitat destruction, pesticide use and decreased biodiversity was that Lyme disease spread to humans.

Pesticides and Toxins
Two billion pounds of herbicides and pesticides are produced in the United States each year. Most of these have never gone through rigorous scientific testing for safety and long-term effects, often because they received expedited approval during wartime.

Two characteristics of pesticides are of particular interest to medical ecologists. First, pesticides and other organochlorines degrade slowly, with half-lives that are often decades or longer. Second, pesticides are lipophilic, meaning they can be stored in fatty tissue of the human body. An average white male in the United States is estimated to have 177 identifiable organochlorines in his body.[4] These compounds are but one small part of the total chemical burden of humans, the “body burden.”

While many synthetic chemicals have been linked to cancer, scientists have recently recognized a new concern: Many of the synthetic chemicals in widespread use display hormonal activity. These chemicals, called “endocrine disruptors,” are implicated not only in the instance of cancer, but also in thyroid disease, fertility, and fetal and child development. In fact, many experts believe that the most important exposures to these chemicals occur in utero. Pesticides have been consistently detected in amniotic fluid and breast milk.[5,6] Some chemicals—such as DDT, DDE and dioxin—have been implicated in increased rates of hypospadias and other conditions.[7,8] Unfortunately, our knowledge of the long-term effects of many chemicals is limited, but new data emerge each day.

Environmental Change
Human health has been affected by several examples of large-scale environmental change, including regional ecosystem change, atmospheric pollution, ozone depletion, and global warming. Many impacts on human health seem relatively obvious. Ozone depletion, which allows more intense UV light to reach the earth’s surface, has likely resulted in increased incidence of dermatologic neoplasms and cataracts. Global warming can cause heat extremes, and temperature increases of even a few degrees increase mortality rates, especially in the elderly and in people with cardiopulmonary disease.[9]

Other examples are more subtle. When large areas of rainforest in Trinidad were cleared for cacao plantations, a deadly malaria epidemic broke out. Eventually, local health officials discovered that erythrina trees, introduced to shade the cacao trees, harbored cup-like bromeliad plants on their trunks. The bromeliads offered a breeding ground for a previously uncommon type of mosquito, which began to spread malaria. Harvesting the bromeliads halted the epidemic. [10]

Biophilia
The term biophilia, first introduced by zoologist E.O. Wilson, describes the natural affinity human beings feel toward other living organisms. Wilson contends that “those deprived of nature and natural things suffer psychologically and manifest a measurable decline in well-being.”[11] Human relationships with animals are one aspect of biophilia, and the impacts of these relationships on health and healing are well documented.

Many studies, for example, document lower blood pressures, longer lives, and better postoperative survival rates in individuals with pets.[12] People with developmental and physical disabilities make otherwise unseen strides in emotional, psychological and physical arenas when using animal-based therapy, such as therapeutic riding (hippotherapy).[13]

A recent article in the Santa Rosa Press Democrat described a 40-year-old developmentally delayed man who had never spoken a word in his life, yet said “horse” after participating in the Giant Steps hippotherapy program in Petaluma.[14] The profound effect that daily contact with animals has on the well-being of humans suggests an interdependence resulting from biological evolution.

Next Steps
Human-driven ecologic change impacts our health and well-being and requires a new approach to our interaction with the environment. Like any intellectual and behavioral change, awareness comes first. As physicians, we can educate ourselves about how human health is affected by our ecological choices and policies. We can then introduce this concept of interdependence to our patients.

Your patients should understand that many choices regarding health have implications larger than personal well-being. When discussing organic produce, for example, you can raise the issue of reducing pesticide use rather than just the individual benefits of eating organic food. Walking to work becomes a means of decreasing the use of fossil fuels, not just a tool for preventing coronary artery disease. No change is too small.

The effects of human-driven ecological change have not only diminished our planet’s environmental capacity, but have also profoundly influenced our health and well-being. The primary tenet of the Hippocratic oath—“First, do no harm”—can be interpreted to mean that no improvements to the human condition should be sought if they cause harm to the greater biological environment. If we exist in a state of interdependence, we cannot harm our biologic neighborhood without harming ourselves. If physicians cannot speak out for our planet, who will?

References

1. Ostfeld RS, et al, “Biodiversity and disease risk,” Conserv Biol, 14:722-728 (2000).
   
2. Centers for Disease Control and Prevention, “Surveillance for Lyme Disease,” MMWR, 49:1-11 (2000).
   
3. Apperson CS et al, “Relative utilization of reptiles and rodents as hosts by immature Ixodes Scapularis,” Systematic & Applied Acarology, 17:719-731 (1993).
   
4. Steingraber S, Living Downstream, Random House (1998).
   
5. Bradman A, et al, “Measurement of pesticides and other toxicants in amniotic fluid,” Environ Health Persp, 111:1779-82 (2003).
   
6. Damgaard IN, et al, “Persistent pesticides in human breast milk,” Environ Health Persp, 114:1133-38 (2006).
   
7. Coburn TS, et al, “Developmental effects of endocrine-disrupting chemicals in wildlife and humans,” Environ Health Perspect, 101:378-384 (1993).
   
8. Paulozzi LD, et al, “Hypospadius trends in two US surveillance systems,” Pediatrics, 100:831-834 (1997).
   
9. Huynen MTE, et al, “Impact of heat waves and cold spells on mortality in the Dutch population,” Environ Health Perspect, 109:463-470 (2001).
   
10. Downs WG, et al, “Bromeliad malaria in Trinidad, British West Indies,” Am J Trop Med Hyg, 26:47-66 (1946).
    
11. Wilson EO, et al, The Biophilia Hypothesis, Island Press (1993).
    
12. McNicholas J, et al, “Pet ownership and human health,” BMJ, 331:1254-55 (2005).
    
13. Sterba JA, et al, “Horseback riding in children with cerebral palsy,” Dev Med Child Neurol, 44:301-308 (2002).
    
14. Wolfe R, “Giant steps for all,” Santa Rosa Press Democrat (Aug. 5, 2007).