Hookworm infections are common in the tropics and subtropics [1,2]. The prevalence of hookworm infection is highest in sub-Saharan Africa, followed by Asia, Latin America, and the Caribbean. Infection is rare in regions with less than 40 inches of rainfall annually.
There are two major species of hookworm that cause human infection: Ancylostoma duodenale (in Mediterranean countries, Iran, India, Pakistan, and the Far East) and Necator americanus (in North and South America, Central Africa, Indonesia, islands of the South Pacific, and parts of India). There are also case reports from Asia of zoonotic hookworm infections with Ancylostoma ceylanicum .
EPIDEMIOLOGY AND LIFE CYCLE
It is estimated that over 800 million people are infected with hookworms worldwide [2,4]. The prevalence of hookworm infection in rural areas of the southeastern United States in the early 20th century was high; extensive control efforts have diminished the prevalence. (See 'Prevention and control' below.)
Three conditions are important for transmission of hookworm infection: human fecal contamination of soil, favorable soil conditions for larval survival (moisture, warmth, shade) and contact of human skin with contaminated soil. Individuals who walk barefoot or with open footwear in fecally contaminated soil are at risk for infection; risk groups include native residents of endemic areas, tourists, and infantry troops [2,5].
The hookworm life cycle begins with passage of eggs from an adult host into the stool (figure 1). Hookworm eggs hatch in the soil to release rhabditiform larvae that mature into infective filariform larvae. Infection is transmitted by larval penetration into human skin; as few as three larvae are sufficient to produce infection . From the skin, larvae migrate into the blood vessels and are carried to the lungs. Approximately 8 to 21 days following infection, larvae penetrate into the pulmonary alveoli, ascend the bronchial tree to the pharynx, and are swallowed. In addition to percutaneous larval penetration (the principal mode of transmission), A. duodenale infection may also be transmitted by the oral route.