The chief transmission route is the familiar trail through human feces, via either contaminated food or soil. When an infected person defecates in the open, the soil becomes a breeding ground. The contaminated soil is then carried into homes on the soles of the feet. Not surprisingly, worm infections are on the rise in urban slums and shanty towns of the developing world, according to WHO (108).
Rarely fatal, worm infections nonetheless exact a tremendous and diverse toll. Infection takes place repeatedly, and the disease becomes more serious as the worm burden in the body builds up. The most vulnerable group is school-aged children, who may harbor multiple infections simultaneously. The effects on children are especially pernicious because chronic infections impair both physical and intellectual growth and development.
The worm infection that is most deadly is schistosomiasis; it kills some 20,000 people a year and causes chronic ill health among hundreds of millions more. Safe and effective drugs cost 30 cents to treat one patient, but this cost is prohibitive in many countries (109). The disease is caused by parasitic flatworms, called flukes, of the genus Schistosoma, whose intermediate hosts are snails common throughout Asia, Africa, and other tropical regions. These snails flourish in relatively calm supplies of fresh water. WHO estimates that about 200 million people in tropical countries have acquired the disease from bathing or wading in infested rivers, lakes, and irrigation systems, and as many as 500 million to 600 million more are at risk (110). Schistosomiasis is on the upswing in developing countries, in part because it spreads to previously unaffected areas through water development projects, such as dams or irrigation.(See Chaging Environments, Changing Health.)
As with diarrhea, providing for the sanitary disposal of feces would do much to prevent the transmission of schistosomiasis and other worm infections. The gains from improved sanitation may take years to materialize, however; in the near term, experts recommend a combined strategy that includes snail control, sanitary improvements, education, and repeated treatment with medicines to keep the worm burden low (111).
Malaria is the most deadly of the insect-borne diseases, claiming the lives of some 1 million to 3 million people each year – 90 percent of them in Africa, and most of them children (112). (Because of different calculation methods, the Global Burden of Disease study estimates malaria mortality at roughly 800,000 per year (113).) WHO estimates that some 300 million to 500 million cases of malaria occur each year – and the number is on the rise – and that 40 percent of the world’s population is at risk for malaria (114). Malaria has been called the “laziness” disease because it is so debilitating. The most prevalent disease in poor rural regions, malaria produces recurrent infections with attacks of fever in warm and rainy seasons, just when workers are needed to collect crops (115).
Caused by four different species of protozoal parasites, malaria is transmitted through the bite of the Anopheles mosquito, which harbors the parasites. Thus, the geographic range of the disease is determined by the environmental conditions that the mosquito in question requires – a particular combination of altitude, rainfall, heat, and humidity, and available surface water. Because ideal conditions vary for the 60 or so species of Anopheles that transmit the disease, strategies designed to control one vector won’t necessarily work for another.
In the tropics, where malaria is most entrenched and deadly, spread of the disease is abetted by conditions of heat, humidity, poverty, and inadequate waste disposal. (See Figure 1.13.) Malaria is a particular problem in newly cultivated or frontier areas such as Amazonia- (See Malaria in the Brazilian Amazon.) and areas of recent agricultural expansion. Once targeted for global eradication, malaria has turned out to be more than a worthy adversary. A more realistic goal is simply to control the disease(116).
WHO has declared the mosquito “Public Enemy Number One” because it is involved in so many deadly or debilitating diseases – not just malaria but also dengue fever, yellow fever, filariasis (elephantiasis), and Japanese encephalitis (117). These diseases, too, are exquisitely attuned to changing environmental conditions. The Aedes aegypti mosquitoes that carry dengue and yellow fever, for instance, thrive in urban settings, where they lay their eggs in water storage containers or in discarded plastic bottles or tires. (See The Spread of Dengue Hemorrhagic Fever.)
References and notes
108. World Health Organization (WHO), The World Health Report 1996: Fighting Disease, Fostering Development (WHO, Geneva, 1996), p. 43.
109. World Health Organization (WHO), The World Health Report 1995: Bridging the Gaps (WHO, Geneva, 1995), p. 28.
110. World Health Organization (WHO), The World Health Report 1996: Fighting Disease, Fostering Development (WHO, Geneva, 1996), p. 39.
111. Kenneth S. Warren, et al., “Helminth Infection” in Disease Control Priorities in Developing Countries, Dean T. Jamison, et al., eds. (Oxford University Press, Oxford, U.K., 1993), pp. 141, 156.
112. World Health Organization (WHO), The World Health Report 1997: Conquering Suffering, Enriching Humanity (WHO, Geneva, 1997), p. 15.
113. Christopher J. L. Murray and Alan D. Lopez, eds., The Global Burden of Disease: Volume 1 (World Health Organization, Harvard School of Public Health, and The World Bank, Geneva, 1996), p. 465.
114. World Health Organization (WHO), The World Health Report 1996: Fighting Disease, Fostering Development (WHO, Geneva, 1996), p. 47.
115. Jose A. Najera, Bernhard H. Liese, and Jeffrey Hammer, “Malaria,” in Disease Control Priorities in Developing Countries, Dean T. Jamison et al., eds. (Oxford University Press, Oxford, U.K., 1993), p. 281.
116. World Health Organization (WHO), The World Health Report 1997: Conquering Suffering, Enriching Humanity (WHO, Geneva, 1997), p. 122.
117. World Health Organization (WHO), The World Health Report 1996: Fighting Disease, Fostering Development (WHO, Geneva, 1996), p. 46.