Forgotten People, Forgotten Diseases. Peter J. Hotez
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world leaders and global health advocates are only now waking up to. Beginning in 2005, an original core group of the 13 major so-called neglected tropical diseases, or NTDs, was proposed,3 which has since been expanded by the WHO to a list of 17 diseases (Table 1.2). They include the major parasitic worm infections of humans, such as ascariasis (roundworm infection), hookworm infection, trichuriasis (whipworm infection), lymphatic filariasis (LF or elephantiasis), schistosomiasis (snail fever), onchocerciasis (river blindness), food-borne trematode infections (liver fluke, lung fluke, and intestinal fluke), cysticercosis, echinococcosis, and dracunculiasis (guinea worm infection); an important group of infections caused by single-celled protozoan parasites, such as Chagas disease, leishmaniasis, and human African trypanosomiasis (sleeping sickness); some atypical bacterial infections, such as trachoma, yaws and endemic treponematoses, and the mycobacterial infections Buruli ulcer and leprosy; and selected viral infections, such as dengue and rabies. Additional tropical infections can also be considered NTDs, and there is an expanded list of these conditions included in the appendix.
Table 1.2 The NTDs (core group of 17)a
aCompiled from Molyneux et al., 2005; Hotez et al., 2006a; Hotez et al., 2007; and World Health Organization, 2010.
While many educated people have by now learned something about HIV/AIDS and malaria and their impact in Africa and elsewhere in the developing world, far fewer have heard about this core group of NTDs. Therefore, it may come as a surprise to learn that the NTDs represent some of the most common infections of the world’s poorest people. Today, of the 7 billion people living on our planet, an estimated 1.3 billion people (20%) live on less than US$1.25 per day, which is considered the World Bank poverty threshold. Paul Collier, the Oxford University economist, helped to popularize the term “the bottom billion” to describe this group of people living in extreme poverty. As shown in Table 1.3, most of the bottom billion suffer from ascariasis, trichuriasis, or hookworm infection, parasitic worm infections that are transmitted through the contaminated warm and moist soil of tropical developing countries (and are known as the soil-transmitted helminth infections), while roughly one-third of the world’s poorest people suffer from schistosomiasis and 1 in 10 from LF.3,4 Essentially all of the bottom billion are affected by one or more of the eight most common NTDs—ascariasis, trichuriasis, hookworm infection, schistosomiasis, LF, food-borne trematode (fluke) infections, trachoma, and onchocerciasis. While dengue disproportionately affects large numbers of people living in poverty, this viral infection can also affect people living in wealthy countries.
Table 1.3 The 17 NTDs ranked by prevalencea
aCompiled from Hotez et al., 2007; Hotez, 2012; Bethony et al., 2006; Furst et al., 2012; Nash and Garcia, 2011; Rajshekhar et al., 2003; Budke et al., 2006; and www.who.int/blindness/causes/priority/en/index2.html
Figure 1.1 Burden of NTDs (blinding trachoma, river blindness, Chagas disease, soil-transmitted helminth infections, guinea worm infection, schistosomiasis, sleeping sickness, visceral leishmaniasis, and lymphatic filariasis). This map displays countries where one or more of these diseases are endemic, based on 2009–2010 data and international borders. (Interactive version available at www.unitingtocombatntds.org/ntd-burden-map-interactive [© Global Health Strategies/Neglected Tropical Diseases, WHO].)
Shown in Fig. 1.1 are the countries in which the NTDs occur.3 The extensive geographic overlap of these conditions means that many of the NTDs are coendemic and that it is common for poor people to be simultaneously infected with multiple NTDs. Of the 56 nations with five or more coendemic NTDs, 40 are found in Africa, 9 in Asia, 5 in the Americas, and 2 in the Middle East. Today, Africa accounts for 100% of all of the world’s few remaining cases of dracunculiasis, 99% of the cases of onchocerciasis, more than 90% of the world’s cases of schistosomiasis, approximately 40% of the cases of LF and trachoma, and one-third of the world’s hookworm infections.5 The impoverished areas of Asia, especially Southeast Asia and the Indian subcontinent, account for more than one-half of the world’s cases of hookworm, ascariasis, and LF. Hookworm, schistosomiasis, LF, and onchocerciasis also remain highly endemic in focal regions of American tropics and subtropics, especially in Central America and Brazil, where it has been suggested that these NTDs represent a living legacy of the transatlantic slave trade.5 Today, these NTDs still primarily afflict the poor and marginalized people living in the region.5
In addition to their geographic overlap and coendemicity, the major NTDs exhibit a remarkable set of common features, all of which adversely affect the health and socioeconomic status of the world’s poorest people (Table 1.4).6
To summarize these common features:
1 The NTDs have high prevalence. As discussed above, today the NTDs are among the most common infections of the poorest people in developing countries.3
2 The NTDs are linked to rural poverty. The high prevalence of the NTDs is frequently not widely appreciated by policymakers or sometimes even by many government officials from the countries where NTDs are endemic. An important reason for the lack of awareness about these conditions is that the NTDs are seldom found in capital cities, where the government officials work and live. Instead, the NTDs are primarily found in poor rural and agricultural areas, particularly in regions where subsistence farming is practiced.6 Therefore, unlike HIV/AIDS or other better-known infections, the NTDs are frequently both out of sight and out of mind. They truly are forgotten diseases afflicting forgotten people. There are exceptions, such as dengue fever and leptospirosis, which are also found in urban slums. These conditions will be addressed separately (in chapter 8), but for the most part the NTDs occur in the setting of rural poverty.
3 The NTDs are ancient conditions. Another interesting feature of the NTDs is their nonemerging character. By this phrase, I mean the NTDs are just the opposite of better-known emerging infections, such as avian influenza, SARS, Ebola, Lyme disease, and HIV/AIDS, which have either newly appeared in the population or have rapidly increased in incidence or geographic range. Instead, the NTDs have been around