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35-22: Hookworm Disease

Philip J. Rosenthal

ESSENTIALS OF DIAGNOSIS

Transient pruritic skin rash and lung symptoms.
Anorexia, diarrhea, abdominal discomfort.
Iron deficiency anemia.
Characteristic eggs and occult blood in the stool.

GENERAL CONSIDERATIONS

Infection with the hookworms Ancylostoma duodenale and Necator americanus is very common, especially in most tropical and subtropical regions. Both worms are broadly distributed. Prevalence is estimated at about 500 million, causing approximately 65,000 deaths each year. When eggs are deposited on warm moist soil they hatch, releasing larvae that remain infective for up to a week. With contact, the larvae penetrate skin and migrate in the bloodstream to the pulmonary capillaries. In the lungs, the larvae penetrate into alveoli and then are carried by ciliary action upward to the bronchi, trachea, and mouth. After being swallowed, they reach and attach to the mucosa of the upper small bowel, where they mature to adult worms (eFigure 35–35). Ancylostoma infection can also be acquired by ingestion of the larvae in food or water. Hookworms attach to the intestinal mucosa and suck blood. Blood loss is proportionate to the worm burden.

eFigure 35–35.

Life cycles of Ancylostoma duodenale and Necator americanus (human hookworms). Eggs are passed in the stool

, and under favorable conditions (moisture, warmth, shade), larvae hatch in 1–2 days. The released rhabditiform larvae grow in the feces and/or the soil

, and after 5–10 days (and two molts) they become filariform (third-stage) larvae that are infective

. These infective larvae can survive 3–4 weeks in favorable environmental conditions. On contact with the human host, the larvae penetrate the skin and are carried through the blood vessels to the heart and then to the lungs. They penetrate into the pulmonary alveoli, ascend the bronchial tree to the pharynx, and are swallowed

. The larvae reach the small intestine, where they reside and mature into adults. Adult worms live in the lumen of the small intestine, where they attach to the intestinal wall with resultant blood loss by the host

. Most adult worms are eliminated in 1–2 years, but the longevity may reach several years.

Some A duodenale larvae, following penetration of the host skin, can become dormant (in the intestine or muscle). In addition, infection by A duodenale may probably also occur by the oral and transmammary route. N americanus, however, requires a transpulmonary migration phase. (From Global Health, Division of Parasitic Diseases and Malaria, CDC.)

A flowchart of the life cycle of intestinal hookworm.

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CLINICAL FINDINGS

A. Symptoms and Signs

Most infected persons are asymptomatic. A pruritic maculopapular rash (ground itch) may occur at the site of larval penetration, usually in previously sensitized persons. Pulmonary symptoms may be seen during larval migration through the lungs, with dry cough, wheezing, and low-grade fever, but these symptoms are less common than with ascariasis. About 1 month after infection, as maturing worms attach ...

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