Hookworms are one of the most important neglected tropical diseases globally due to the high human disease burden and the fact that this pathogen is the leading cause of anemia in Africa and Oceania.
PATHOGENESIS AND EPIDEMIOLOGY
The hookworm life cycle begins with the excretion of fertilized eggs within the feces of an infected individual. The eggs hatch to release first-stage (L1) larvae, which undergo 2 subsequent molts to the infective third stage (L3). These L3 hookworm larvae migrate along moisture and temperature gradients within the soil until they encounter a permissive host. When larvae contact the skin, they quickly penetrate the epidermis and dermis, ultimately invading small blood vessels and entering the venous circulation. They are then carried passively to the heart and lungs, where they lodge in the pulmonary capillaries and break through to the alveolar space. Larvae then migrate up the respiratory tree, are swallowed, and undergo their final developmental molts to the adult stage when they reach the small intestine. Once in the proximal small bowel, the adult worms attach to the mucosal surface and begin to feed (Figs. 324-1 and 324-2). Adult hookworms secrete anticoagulants, platelet inhibitors, and hemoglobin-degrading proteases that facilitate blood feeding and digestion of red blood cells. When the plug of intestinal mucosa at the site of attachment has been digested, the worm releases and reattaches at a new site. Male and female worms mate, and the female releases 10,000 to 30,000 eggs per day into the intestinal lumen. It takes approximately 6 weeks for eggs to appear in the feces of an infected individual.
Photomicrograph of an adult hookworm attached to the intestine. Note area of hemorrhage adjacent to site of attachment. (Used with permission from R. Bungiro.)
Cross-section of adult hookworm showing host red blood cells in the intestine. (Used with permission from R. Bungiro.)
Two important features of the life cycle distinguish Ancylostoma hookworms from Necator. First, Ancylostoma duodenale can cause infection when ingested, whereas Necator americanus can only complete its life cycle in humans following skin penetration. Second, there is epidemiologic evidence to suggest that third-stage larvae of A duodenale may arrest within various tissues of their host, ultimately resuming development and completing their life cycle months to years later. Reports of severe disease manifestations in young infants raise the possibility that A duodenale may also be transmissible through breast milk.
Hookworm infection remains a major health burden in developing countries. In 2010, it was estimated that 438.9 million people globally were infected with hookworms, leading to 3,230,800 years lived with disability. Infections with A duodenale occur in focal regions of Africa, Asia, and South America, whereas N americanus is ...