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INTRODUCTION

Hookworms are soil-transmitted helminths (STHs) that infect humans in association with poor sanitation in tropical and subtropical climates.1 An estimated 451 million people are infected with hookworms,2 with the highest prevalence in sub-Saharan Africa, South East Asia, and the Pacific.3 The prevalence and intensity of hookworm infection in these populations steadily rises with age and plateaus in adults, although moderate and heavy pediatric hookworm infections are also common.4,5 In areas where overcrowding, poverty, and unsanitary living conditions combined with inadequate healthcare and education prevail, moderate and heavy hookworm infections leading to disease results in enormous human misery and suffering, as well as economic loss. It is estimated that up to 4 million disability adjusted life-years (DALYs) and U.S. $139 billion of productivity are lost annually as a direct result of hookworm disease.6 With this in mind, the World Health Organization (WHO) has established a global target of elimination of morbidity due to STHs (including hookworm), in children in endemic areas by 2020.7

Three species of hookworms commonly infect the human gut, these being Necator americanus, Ancylostoma duodenale, and A. ceylanicum. Most human cases are caused by N. americanus, with A. duodenale infections also scattered throughout the world.4 Significant numbers of human A. ceylanicum infections have been reported in South Asia, South East Asia,8 and Melanesia.9,10 N. americanus is the dominant species in southern China, southern India, South East Asia, sub-Saharan Africa11 and it is considered to be almost the exclusive species of human hookworm found in West Africa.12 N. americanus is also the dominant species in the Americas and, historically, this was the only human hookworm species found in the United States.11 Some investigators postulate that N. americanus was introduced into the Americas during the Middle Passage of the Atlantic slave trade.13

GEOGRAPHIC DISTRIBUTION

A. duodenale coexists with N. americanus in many regions of the world. In areas where the soil conditions and temperatures are favorable to hookworm larval development, N. americanus is typically the predominant hookworm species. However, due to the unique ability of A. duodenale larval stages to undergo hypobiosis (arrested development) during times of environmental stress from low temperatures and other factors, A. duodenale has the ability to survive in some areas that are unfavorable to N. americanus larvae in soil.11 A. duodenale is historically more prevalent and widely distributed than N. americanus in Europe, parts of India, North Africa, the Middle East, some areas of China (north of the Yangtze River), Peru, Chile, northern Australia and northern Japan.14–16 Early researchers suggested that A. duodenale was native to the northern hemisphere, being introduced to more southern regions via the historical migrations of the European, Arab, Indian, and Chinese peoples.17 This hypothesis was developed based upon several observations, including the predominance of A. duodenale over N. americanus in ...

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