The Disease Environment

In addition, Africans had their own tropical illnesses: malaria, blackwater fever, yellow fever, trypanosomiasis (sleeping sickness), filariases, leishmaniasis (kala-azar), schistosomiasis (bilharziasis), onchocerciasis (river blindness), leprosy, yaws, guinea worm, helminthiasis, relapsing fever (louse- and tickborne), and tropical ulcer. A number of these are caused by parasites that are transmitted to humans by insects. Because both the parasites and the insects require very specific conditions of temperature and moisture and because many of the other diseases are water-borne, the emphasis on "tropical" is understandable. Also understandably, diseases that had such complex and fascinating etiologies and that required the combined expertise of zoologists, protozoologists, epidemiologists, chemists, and biologists overwhelmingly attracted the attention of early investigators in Africa. Unfortunately, other important causes of morbidity and mortality, such as respiratory diseases, childhood diseases, and malnutrition, were not given research priority. "Tropical" diseases were mistakenly conceptualized as distinct from other communicable diseases. In this way, tropical medicine evolved as a specialty outside of mainstream medicine.

By the mid-nineteenth century, tropical Africans were afflicted by most of the diseases of the temperate Old World, such as brucellosis (Malta fever), undulant fever, cerebrospinal meningitis, cholera, diphtheria, influenza, measles, plague, pneumonia, poliomyelitis, smallpox, tuberculosis, typhoid fever (enteric fever), typhus and other rickettsial infections, venereal diseases, and whooping cough. The causes of 50 to 90 percent of illness and death among the poor in the underdeveloped world, including Africa, fall into two main groups: nutritional deficiencies and communicable diseases (Sanders 1985). The major causes of adult mortality are infectious and parasitic diseases, acute respiratory infections, cardiovascular disease, and accidents and violence. Most childhood mortality is caused by a limited number of infectious diseases: diarrheal diseases, acute respiratory infections, malaria, measles, and neonatal tetanus (I. Timaeus, personal communication). Moreover, there is evidence that in recent years the incidence of noninfectious and degenerative diseases is increasing.

What constitutes a disease environment and what causes it to change? Every human community coexists with specific microorganisms that contribute to the disease environment of the group. Many diseases represent the failure of one organism, in this case human beings, to cope successfully with competing organisms, in this case microorganisms, in the ceaseless struggle for survival. A disease environment contains many elements: humans, animals, various other forms of flora and fauna, microorganisms, climate, and geographic features. Many disease environments are often located close to one another, and these undergo frequent and sometimes rapid change. For instance, after a period of interaction with a stable human population, disease-causing pathogens can adapt and produce new strains in response to the immune defenses produced by that population. In recent decades this has been demonstrated in parts of the world by the appearance of new strains of chloroquinine-resistant Plasmodium falciparum, the cause of the most deadly of the malarial diseases.

Conversely, human host communities develop immunity to endemic diseases. In other words, humans who survive the diseases of their environment often acquire some resistance to further infection. Epidemics of disease occasionally flare up when nonimmune members of the community are exposed to pathogens or when newcomers in large enough numbers reach a disease environment containing pathogens against which they have no immune protection. Poor hygiene resulting from ignorance of disease causation coupled with the widespread lack of safe water supplies and sewage disposal also profoundly affect a society's disease environment.

As stated previously, disease environments are affected by both natural and sociocultural factors. A natural factor of much importance in Africa is the proliferation of insects, many of which serve as vectors of disease-causing pathogens. Another factor is climate, which can be a major determinant of those diseases that have specific requirements of temperature, humidity, or aridity. An equally important factor encompasses the sociocultural practices of communities. For instance, ritual practices may bring humans into regular and intimate contact with certain features of the physical environment that results in contact with disease-causing pathogens. Shrines hidden deep in forested regions may bring humans into contact with pathogen-bearing insects such as the tsetse fly and anopheles mosquito. In addition, agricultural practices, such as the cultivation of certain crops close to homesteads, often bring humans into contact with pathogens. And changes in crops or techniques often provide new ecological niches in which disease-bearing pathogens flourish. An example of this is the introduction by a British colonial officer of a new variety of hedge, the lantana, from India into Uganda in the 1940s. The lantana flourished in its new environment, gradually spreading northward toward the southern Sudan. Unfortunately, it provided an excellent habitat for a species of tsetse fly, Glossina pallidipes, which in turn introduced new strains of trypanosomes, the cause of human sleeping sickness.

Perhaps the most important factors in disease environments are water supply and human excreta. Wa-terborne diseases include typhoid fever, poliomyelitis, infectious hepatitis, cholera, schistosomiasis, dracunculiasis, and leishmaniasis. Water supplies and sewage disposal are complex subjects of much concern to health planners in the late twentieth century. But during much of the nineteenth century these two vitally important areas of public health were given very little serious attention and even less investment. Indeed, despite John Snow's brilliant 1849 analysis of the epidemiology of cholera in London when he revealed that a water pump was responsible for the spread of the disease, the supply of safe water and the disposal of sewage remained for more than a century conceptually separate from the concerns of medicine with its focus on cure.

Rather, clean water supplies and sanitary sewage disposal, as means of disease prevention, were, until very recently, solely the concern of the engineer. Thus, a major source of much human disease was neglected by medical people. The colonial legacy included very little in the way of safe/water supplies and effective sewage disposal systems, except in a few of the urban areas where the Europeans themselves resided. And Africans are suffering the effects to the present day.

Last but hardly least among the African "pathogenic factors" is protein-energy malnutrition, for the excessive mortality of African children under 5 years of age is due largely to the synergism between protein-energy malnutrition and infectious and parasitic diseases. Until fairly recently, nutrition has been largely ignored in relation to morbidity, yet it is a vitally important key to understanding the widespread perception of Africa as a "disease-ridden" continent.

Earlier theories of epidemiology held that exposure to new pathogens was sufficient cause for epidemic outbreaks, and much emphasis was laid on the increased interactions of African populations, witnessed by Europeans from the mid-nineteenth century on, as an explanation for apparently increased rates of morbidity and mortality. More recently, however, it has been argued that more than mere exposure is needed to explain why humans fall ill and why epidemics flare. Thomas McKeown (1979) argued convincingly that, for almost the whole of human existence, disease and early death have resulted from basic deficiencies or from hazards related to them and that the role of nutrition is of paramount importance.

Thus, medical services, specific remedies, and the increasingly complex technology of biomedicine may not provide the best solutions to African health problems; rather, the total ecology of health and disease may be a more fruitful area of study for future health planners.

Maryinez Lyons

The author thanks Ian Timaeus for his help.

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