stunting!
Moreover, the unholy trinity also adversely affects the neuropsychiatric activities of children, in turn damaging school performance and reducing school attendance.10,11 The mechanisms by which school performance is impaired are not well established, but a number of clinical studies have shown that STHs can adversely affect cognition and memory and in some cases possibly lower intelligence. 11 Therefore, chronic infections with STHs destroy the lives of children not by shortening their lives but instead by impairing their physical growth, mental development, and ability to learn in school. Each of the NTDs not only occurs in the setting of poverty but also promotes poverty. In the case of STH infections, roundworms, whipworms, and hookworms promote poverty primarily through their impact on overall child development. Presumably, these processes account for the observation that chronic infection with hookworm during childhood is associated with a 43% reduction in future wage-earning capacity (similar studies for ascariasis and trichuriasis are not yet available).12 Therefore, STH infections have a huge impact not only on health but also on education, and like other NTDs they are economic threats.
As suggested by the opening quotation from the late Norman Stoll, hookworm is probably the most significant STH. New Global Burden of Disease 2010 information (published at the end of 2012) confirms this observation, with preliminary indications that hookworm is responsible for almost two-thirds of the disability-adjusted life years (DALYs) lost from all of the STH infections.13 Hookworms are 1-cm-long parasites that live in the small intestine, where they suck blood from the small blood vessels lining the gut mucosa and submucosa. Approximately 700 million people, about one-half of the world’s poorest people, are infected with hookworm. The greatest concentration of cases occurs in rural areas of sub-Saharan Africa, East Asia and the Pacific region, the Indian subcontinent, and tropical regions of the Americas, especially Brazil and Central America (Fig. 2.5).2,14 Infection rates are often particularly high in coastal areas, an observation that most likely reflects the unique requirements of the soil-dwelling environmental stages of these parasites.
Figure 2.5 Global distribution of human hookworm infection. (From Hotez et al., 2005.)
Nearly as striking as the high prevalence of hookworm in developing countries is the almost complete absence of hookworm in highly developed countries, including the United States. However, up until the 1930s, hookworm infection (as well as many other NTDs, such as malaria and typhoid fever) was endemic in the southern United States.15 Shown in Fig. 2.6 is a map of the distribution of hookworm in the American South during the first decades of the 20th century, when high rates of hookworm infection occurred along the Gulf Coast and the Atlantic seaboard (the basis for the high rates of hookworm in coastal areas will become clearer when we discuss the hookworm life cycle). In some regions where more than 50% of the children were infected, it was shown that hookworm was a major reason why children were malnourished, why their growth was stunted, and why they did poorly in school and were prevented from reaching their full economic potential.12,15
After Charles Wardell Stiles and Bailey K. Ashford identified Necator americanus as the predominant hookworm in the United States, it became known as the “germ of laziness” or the “vampire of the South.”1,15 It is believed that hookworm was introduced into the United States when N. americanus was imported by infected slaves from sub-Saharan Africa during the 17th, 18th, and 19th centuries.1,15 Up until the 1950s, hookworm was also common in Japan and South Korea. In each of these now developed countries, reductions in the prevalence of tropical infections occurred primarily because of overall reductions in poverty and a shift to a more urbanized economy. In her book Malaria, Poverty, Race, and Public Health in the United States, the medical historian Margaret Humphreys argues that the Agricultural Adjustment Act and other New Deal legislation, which Congress passed in 1933, promoted rural depopulation by providing investment capital for the purchase of machinery, which took agricultural workers out of cotton and tobacco production.16 Such legislation caused landlords to tear down rural shacks and forced former dwellers to move either north or into southern cities.16 There is a common misconception that during the first 2 decades of the 20th century, the Rockefeller Foundation and its forerunner, the Rockefeller Sanitary Commission, eradicated hookworm in the American South and later in parts of Asia and South America through a combination of aggressive sanitation and the widespread distribution of shoes. For reasons that we will see below, it turns out that shoes are not an effective hookworm prevention measure, while sanitation in the absence of parallel economic development frequently has little impact on the transmission of STH infections.17 Instead, rural depopulation, urbanization, and economic development in the United States during the 1930s and in Japan and Korea in the years following World War II were probably the major elements leading to control of STH infections. In Asia, control was further hastened through widespread deworming by using first-generation anthelmintic drugs. Similar changes in human ecology probably account for the reductions observed in eastern China over the last 2 decades. Therefore, urbanization and economic development represent two of the most powerful forces responsible for the control of hookworm infection and other NTDs. Far more than the Sanitary Commission, the major health legacy of John Rockefeller was his foresight in establishing The Rockefeller University as a biomedical research powerhouse and in endowing the first generation of public health schools in the United States, beginning with the flagship school at Johns Hopkins University.
Figure 2.6 Distribution of human hookworm infection in the American South at the turn of the 20th century. The map displays the rates of hookworm infection among children by county groups. Red areas indicate the highest infection rates, followed by orange, yellow, and green. (Data from Bleakley, 2006.)
Humans become infected with hookworm through contact with infective larvae that live in the soil.18 The major cause of human hookworm infection is the nematode parasite N. americanus, although a second but less common species, Ancylostoma duodenale, also causes hookworm infection. The life cycle of N. americanus is shown in Fig. 2.7. Soil-dwelling infective hookworm larvae exhibit the ability to directly penetrate human skin. The larvae are less than 1 mm long (Fig. 2.8) and are therefore largely invisible to people working in the fields or children playing on the ground. Larvae enter through any exposed skin, including the hands, the arms, the buttocks, the legs, and yes, sometimes even the feet. The ability of N. americanus larvae to penetrate all aspects of the skin explains why shoes have minimal if any impact on reducing the hookworm prevalence in affected communities. The higher rates of hookworm infection in coastal areas reflect the sandy soils present in these regions. Hookworm larvae can migrate through sandy soils better than through soils with a high clay content.19
