What is tropical medicine?
Humankind evolved in tropical Africa, and it is presumed that many of the diseases and parasites characteristic of the tropical environment were inherited from nonhuman primate ancestors. Over the centuries, tropical diseases have challenged the limited resources of tribal medical practitioners and the more sophisticated medical learning of Chinese and Arab physicians. Western involvement in tropical medicine developed as a consequence of colonial expansion. The great number and diversity of tropical diseases may be grouped according to the nature of the causative agent, the symptoms involved, their mode of transmission, or their geographical distribution. The causative agent is the focus of this entry.
Viral diseases have become the most important infectious diseases in the temperate zone since the introduction of antibiotics. In the tropics, they are important but not preeminent. The principal tropical viral diseases are yellow fever and dengue fever, which are transmitted by arthropods . The transmitted viruses are called arboviruses. Yellow fever and dengue fever are acute illnesses with mortality (death) rates that exceed 50 percent if left untreated. The diseases occur sporadically when mosquitoes transmit the virus from a primate reservoir. There is potential for devastating epidemics to occur if a breakdown in health care delivery prevents prompt immunization of the population in affected areas. Arboviruses are distributed throughout tropical areas of both the New and Old Worlds. Until a massive eradication campaign orchestrated by the World Health Organization (WHO) eliminated smallpox, they exceeded other viral diseases in mortality and morbidity (illness).
Other viral diseases affect tropical regions. Human immunodeficiency virus (HIV), the agent that causes acquired immunodeficiency syndrome (AIDS), is prevalent and spreading in East and Central Africa, as well as in Haiti and Brazil. Influenza and measles, although not predominantly tropical, have high mortality rates among poor tropical populations. Hepatitis is endemic throughout the developing world. Until recently, poliomyelitis was virtually universal in the tropics, although paralytic cases were infrequent. Rabies claims a small number of victims. Finally, scientists are constantly encountering new viral diseases. Some (such as filoviruses) are exceedingly virulent. Ebola, Marbourg, and Bolivian hemorrhagic fevers are examples. Their distribution is confined to relatively small areas, probably because of the rapidly fatal nature of these diseases.
Tropical diseases caused by bacteria include some of the diseases most feared by humanity: cholera, bacillary dysentery, typhoid fever, tuberculosis, leprosy, and bubonic plague. As a result of the lack of safe sources of drinking water, there has been a resurgence of epidemic cholera in urban slums in the developing world. Fortunately, the discovery of inexpensive methods of oral rehydration therapy has reduced mortality from cholera and dysentery. Leprosy is surprisingly common and, because of its low infectivity and slow onset of debilitating symptoms, is not always perceived as a major menace. Bubonic plague occurs in isolated outbreaks within and outside the tropics where rodent reservoirs exist. In the southwest United States, approximately one hundred cases of plague are reported each year. Trachoma, an inflammation of the eyelids, affects large numbers of people—as many as a million in Brazil alone—and is a leading cause of blindness. Vaccines and antibiotics exist for the most prevalent bacterial diseases.
Spirochetes of the genus Treponema cause syphilis and yaws, which are chronic, endemic, and degenerative illnesses characterized by skin ulcers and neurological involvement. Yaws was the most common major tropical disease reported by WHO in the 1950s, affecting twelve million people in Southeast Asia alone. The disease is rarely fatal. Since then, aggressive wholesale treatment campaigns employing penicillin have reduced the incidence of yaws considerably in Asia and the Americas. Typhus and relapsing fever, arthropod-transmitted rickettsial diseases, can occur in epidemics.
Parasitic diseases caused by protozoa and helminths constitute the classic tropical diseases. These groups of organisms typically have complex life cycles that involve invertebrate vectors (flies, worms, and amoebas). Since many organisms are intolerant of freezing, human parasites are much more common in tropical areas.
A mosquito-transmitted protozoan causes malaria. Schistosomiasis is caused by a flatworm whose alternate hosts are aquatic snails that inhabit rice paddies and irrigation canals. These two diseases are arguably the greatest threats to human health anywhere in the world today. They affect enormous numbers of people throughout the tropics and warm temperature regions. Both cause chronic infections that may persist for decades, undermining the health and vigor of the host. In poorer tropical nations, malaria, schistosomiasis, and ancylostomiasis (hookworm) affect much of the adult population. These victims are chronically malnourished and may harbor other parasites. Trypanosomiasis, African sleeping sickness and its South American counterpart, Chagas disease, are caused by insect-transmitted protozoa. Both diseases declined in frequency and geographical distribution following aggressive attempts to eliminate vectors.
Kala-azar is a lethal, disseminated form of the disease leishmaniasis. It is comparatively rare; some experts predict a resurgence because of increases in vector populations and drug resistance. Cutaneous leishmaniasis is widespread throughout the tropics. The organism responsible for amebic dysentery is universal in contaminated water in the tropics; infection is extremely common. Carriers are often asymptomatic; unsuspecting tourists and natives with compromised immune systems contract the most serious forms of the disease. Filariasis, called elephantiasis in its extreme form, is caused by a mosquito-transmitted nematode. It is common in Africa and the Indian subcontinent. In 1998, the pharmaceutical company SmithKline Beecham announced that it would donate its antiparasitic drug albendazole for use by the one billion people at risk for contracting filiarisis until the disease is eliminated completely. Onchocerciasis, also called river blindness, is a parasitic disease found primarily in Africa. It is one of the leading causes of blindness in the world. Approximately one million people become blind each year as a result of this disease. Economic limitations and politics have hampered campaigns to treat onchocerciasis. There are many other parasitic diseases, including guinea worm, that occur locally in the tropics.
Nutritional deficiencies primarily result from extreme poverty and parasitic infection but also from ignorance and poor dietary practices. Characteristic of tropical regions is kwashiorkor, a protein deficiency disorder affecting primarily very young children and increasing in regions where modernization encourages early weaning. Children with kwashiorkor are fed carbohydrates but inadequate amounts of protein. Marasmus (wasting) is usually caused by a combination of insufficient total calorie intake and very inadequate intake of protein. It is made worse by dysentery or other parasitic infections and the B-complex vitamin deficiencies beriberi and pellagra.
Diseases caused by other factors are not as prevalent in tropical countries. While severe systemic fungal diseases are predominantly tropical, the total number of cases is not high. Sickle cell disease, an inherited disorder, is frequent in central Africa because the mild, heterozygous form confers immunity to malaria. Little is known, however, about other genetic disorders in the tropics as a whole. In South Africa, genetic diseases are more common in white than in black populations. Typical so-called diseases of affluence may also pose a particular threat to people who make an abrupt transition from tribal to urban life in developing countries. The urban black population of South Africa is experiencing high rates of obesity and adult-onset diabetes.
Tropical medicine is a vast field in which progress is slow and sporadic, with gains in one area often offset by losses in another. Tropical medicine, especially in developed countries that are located in temperate climates, is hampered at every stage, from research to clinical practice, by the low proportion of resources devoted to basic research in tropical disease etiology and ecology. Pharmaceutical companies often assign low priorities to the development of medicines and therapeutic agents for tropical diseases because of the potentially low returns on their investments. In affected countries, governmental policies are often established that ignore human health consequences. Extreme poverty among affected populations slows efforts to improve sanitation and general health. Explosive population growth and social and political instability slow efforts to improve infrastructures that, in turn, would improve human health. Persistent customs and attitudes in many tropical regions, which may once have been adaptive, are often harmful in modern settings. Finally, health care facilities and professionals are in short supply and unevenly distributed throughout the world and within countries affected by tropical diseases.
All these factors are important. No tropical country enjoys an average life span as long or an infant mortality rate as low as Western Europe, North America, or Japan. Nevertheless, the overwhelming influence of poverty and lack of health care access is well illustrated by contrasting the health status of the populations of Puerto Rico, Okinawa, or Taiwan with the conditions in Central Africa and the Indian subcontinent. In relatively prosperous, politically stable countries, diseases that can be prevented by immunization (such as polio) or easily cured by chemotherapy (such as yaws) are unimportant. Education, better sanitation, and environmental management have dramatically reduced the incidence of the parasites that cause malaria, schistosomiasis, and hookworm. In developed countries, severe malnutrition is rare.
Existing medical knowledge is constantly being refined, and an increased commitment to tropical medical research is needed. It is worth noting, however, that many typical tropical diseases were prevalent in the southeastern United States before World War I. Furthermore, the medical knowledge and governmental agencies available in the 1920s proved effective in combating malaria, yellow fever, hookworm, and pellagra in the United States.
The development of drugs to combat disease is largely the business of a pharmaceutical industry based in developed countries. These companies have been accused of neglecting tropical diseases because of a low potential for profit. WHO has provided incentives in some cases but has also opposed exclusive licensing of drugs developed under its aegis, a potential disincentive. Less than 5 percent of health research worldwide is devoted to the health problems of developing nations.
The pharmaceutical industry has also been implicated in marketing drugs to developing nations that have been banned as ineffective or dangerous in the United States. This practice underscores a wider and growing health threat: multinational corporations exporting pesticides, industrial chemicals, and manufacturing processes that undermine human health to the developing world. Any process resulting in wholesale environmental disruption produces an increase in disease in ways that are unpredictable.
Drug-resistant strains of pathogens are most likely to evolve when large populations are treated with a single therapeutic agent and when treatment is insufficient to cure patients completely. The risk of this pattern occurring is high in tropical countries. Drug-resistant strains of malaria are already increasing and complicate the task of combating this dangerous disease.
Poverty among individuals leads to malnutrition, overcrowding, poor sanitation, lowered resistance to infection, and an inability to avoid exposure to vectors and contaminated water. In many tropical areas, access to medical care ranges from limited to nonexistent. On a regional and national level, poverty leads to political and social instability and an inability to implement public health programs. Political upheaval facilitates the interregional spread of pathogens.
In much of the tropics, efforts to improve infant health and agricultural productivity backfire because social and economic customs favor large families. The resulting population increase exacerbates poverty-dependent variables responsible for disease. Training skilled medical personnel and maintaining clinics are costly. In the developing world, however, this cost is less because labor costs are low.
If market forces alone dictate the allocation of medical resources, health care providers gravitate toward the most prosperous areas. Medically trained developing world nationals frequently emigrate to Europe or the United States. Paramedic personnel, such as the so-called barefoot doctors of China, are effective only to the extent that their training, supervision, and support services are adequate. The number of doctors relative to the population in many tropical countries is too low to provide adequate medical care even without the additional factors of uneven distribution and higher incidence of serious disease.
The Old World tropics, especially tropical Africa, were dubbed the “white man’s grave” because European military personnel and colonists lacked both inherited resistance and customary methods of avoidance to protect themselves from the ravages of malaria, yellow fever, Asiatic cholera, and scores of other life-threatening diseases. Occasionally, disease resulted in the failure of colonial enterprise. In Hispaniola (Haiti), black slaves were successful in their bid for independence after yellow fever killed all but six thousand of the thirty thousand troops sent by Napoleon to quell the rebellion. In Africa, large areas in which sleeping sickness was endemic were inaccessible to European colonization.
The period of maximum European colonial expansion (from 1830 to 1914) coincided with great strides in the understanding of disease causation and prevention, although effective cures lagged until the beginning of World War II. In 1900, Walter Reed, in the course of investigating a devastating epidemic of yellow fever among workers and soldiers digging the Panama Canal, discovered that the disease was mosquito-borne. In 1898, the Italian researchers Amico Bignami, Giovanni Grassi, and Giuseppe Bastianelli, who were also working with malaria, made a similar discovery. These observations paved the way for effective control through exclusion and eradication of the vectors.
European activities in the tropics have often exacerbated tropical disease problems. The African slave trade introduced a number of tropical diseases into the southern United States, including schistosomiasis, one of the most debilitating and intractable of tropical parasitic infections. Bengalis in India dubbed kala-azar “the British government disease,” since road-building and improved communication introduced a previously localized pathogen into large areas, with disastrous results. Dam-building and artificial irrigation dramatically increase the incidence of schistosomiasis in tropical areas. Paradoxically, medical intervention reducing infant mortality and allowing rapid population growth tends to undermine health and increases the incidence of nutritional disorders and parasitic infections. Inadequate sanitation in rural clinics, especially the use of poorly sterilized hypodermic needles, contributes to the spread of disease and has been implicated in the spread of AIDS in Africa.
The examples of Brazil and China, however, illustrate how nationally coordinated efforts within a tropical region of low per capita income can be effective. In China’s regimented society, a blanket application of environmental control measures with mobilization of a large rural workforce, mass screening and treatment, education at all levels, and coordinated targeted research have been shown to be feasible. Between 1956 and 1987, the areas in which schistosomiasis occurred and the number of people at risk of exposure were halved. At the same time, the number of infected individuals declined by a factor of ten. In Brazil, the Superintendency for Public Health Campaigns (SUCAM) works in frontier areas where social control is slower and channels for education and communication do not function well. SUCAM relies on the Guarda, a field staff composed of a large number of paraprofessionals trained to recognize symptoms and risk factors in a particular disease and to implement control measures. SUCAM has had notable success against Chagas’ disease, which can be controlled by eliminating the insect vector from houses through the use of insecticides and renovation. Egypt, Zimbabwe, and the Philippines have also mounted successful campaigns that have reduced the incidence of parasitic diseases.
Education of women is an important factor in the health of poor populations. Women play a crucial role in maintaining the health of infants and children. They are also more likely to be vulnerable to neglect and ill health than are adult males as a result of cultural attitudes. In the Indian state of Kerala, longevity and infant mortality statistics are now comparable to those of African Americans because the government has devoted a large proportion of resources to universal education and public health. Nearby Bangladesh, which has a comparable climate and per capita income but lacks the commitment to education and health access, has some of the worst statistics on survival and longevity in the developing world.
The net result of improving public health in the developing world has been an increase in average life expectancy of approximately ten years, from fifty to sixty, between 1970 and 1990. Increased longevity results in an increase in diseases of old age and their demands on the health care system. This is most true in China, where life expectancy approaches seventy years and aggressive measures have reduced the birth rate dramatically.
In an age of international travel, virtually any communicable disease has the potential to spread rapidly. The worldwide epidemic of AIDS clearly illustrates this phenomenon. Some medical ecologists view the large AIDS-infected population in Africa as a medical time bomb in which a novel, virulent pathogen, such as the Ebola virus, could gain a foothold. Military incursions and tourism in the tropics expose people from the temperate zone to tropical ailments. These factors should provide an additional impetus for research into tropical diseases and general awareness of them by physicians.
Tropical diseases extract an enormous toll in human productivity that, by perpetuating poverty and hindering all forms of development, contributes to global political instability. The interaction of tropical disease processes and European colonialism was destructive for both Europeans and colonial subjects. Medical science and social policy are a long way from solving the medical problems of the tropics.
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