Hookworm infestation is a soil-transmitted helminthic infection that is considered one of the neglected tropical diseases by the World Health Organization (WHO). It is caused by 2 nematode parasites in humans: Ancylostoma duodenale (Old World hookworm, mainly in Mediterranean countries, the Middle East including Iran, Pakistan, India, East Asia, and Southeast Asia) and Necator americanus (New World hookworm, found in North, Central and South America, Central Africa, and Indonesia). The infection is uncommon in countries where annual rainfall is less than 40 inches.
Human hookworm infestation is one of the most prevalent parasitic infections globally, affecting around 10% of the world's population. Most infections present in the poor and underdeveloped communities of the subtropics and tropics. Inadequate sanitation and warm climates promote the transmission of these parasites. Regions of sub-Saharan Africa, Latin America, the Caribbean, China, and Southeast Asia have the highest prevalence rates. It was once a concern in the Southern United States but is no longer so, since the implementation of adequate control programs.
Humans are the definitive host, and no intermediate host is required for completion of the life cycle of these parasites. Persons living in endemic regions who walk barefoot on soil contaminated with human feces are at high risk of acquiring infection.
The adult hookworms live in the small intestine (particularly in the jejunum). When the eggs are passed in human feces, they hatch into rhabditiform larva under optimal weather conditions. This form of larva then develops into the filariform larva that is infective to humans. It enters the human body by penetration into the skin (generally on the feet), which is the principle mode of transmission. It is carried by the venous circulation into the lung. There, it penetrates the alveoli and migrates up to the trachea and is swallowed to reach the small intestine. The adult worms become sexually mature in the intestine, and the cycle resumes after the eggs are excreted in human feces. Necator americanus is only transmitted to humans by penetration of the skin, while A. duodenale can also be transmitted by oral route.
Most infections with hookworms are asymptomatic, but a variety of symptoms may develop. When the larva penetrates the skin, some individuals may develop a local dermatitis, known as "ground itch." When the larva migrates through the lungs, it may evoke an eosinophilic inflammatory response (pneumonitis with pulmonary infiltrates) resulting in wheezing, cough, and shortness of breath. When the adult worm enters the small intestine, it binds to the gut mucosa by its buccal armature, resulting in abdominal pain, anorexia, and nausea. The most serious consequence is iron deficiency anemia from these "blood-sucking" worms. Necator americanus consumes 0.03-0.3 mL/day, whereas A. duodenale ingests 0.2-0.5 mL/day. In persons with heavy hookworm infection, anemia could be profound enough that patients may present with fatigue, pallor, dyspnea, and cardiac murmurs. Children may manifest with protein-energy malnutrition and growth retardation. Severe iron deficiency anemia may precipitate cardiac congestive failure and cause death, especially in children and pregnant individuals.
The larvae of hookworms that primarily infect animals (Ancylostoma braziliense) may seldom burrow into human skin but do not enter the venous circulation. They remain confined to the superficial layers of skin and cause erythematous, serpiginous, and pruritic tracks on the skin. This entity is called cutaneous larva migrans.
Other soil-transmitted helminthic infestations, such as ascariasis and trichuriasis, may present with abdominal symptoms and remain in the differential diagnosis for hookworm infection. As with hookworm, Ascaris and Strongyloides species are the 2 parasites associated with pneumonitis and peripheral eosinophilia when their larvae migrate through pulmonary circulation. Stool examination will differentiate the eggs of these parasites and aid in identification of the parasite.
Another parasite infestation that is associated with abdominal symptoms and anemia is diphyllobothriasis. The anemia, however, with hookworm infestation is that of iron deficiency (microcytic-hypochromic), in contrast to macrocytic-hyperchromic anemia seen with diphyllobothriasis. The thicker shell and the operculum of the fish Diphyllobothrium tapeworm egg help distinguish it from the ovum of hookworm.
Other common parasitic causes of cutaneous larva migrans include:
Ancylostoma braziliense (cat and dog hookworm) can be found in the Central and Southern United States.
Ancylostoma caninum (dog hookworm) is geographically limited to Australia. It also causes secondary eosinophilic enteritis in humans, but without causing blood loss and anemia.
Iron deficiency anemia in endemic regions should not be treated solely with iron supplementation. An underlying cause, especially hookworm infection, should be considered and treated.