Introduction
Background
Two species of hookworms commonly infect humans, Ancylostoma duodenale and Necator americanus. These species are found throughout the tropics and subtropics. The distribution of each species significantly overlaps that of the other.
Hookworms are estimated to infect more than 740 million people around the world, but most people who are infected are asymptomatic.1 These worms are much smaller than the large roundworm, Ascaris lumbricoides, and the complications of tissue migration and mechanical obstruction so frequently observed with roundworm infestation are less frequent in hookworm infestation. The most significant risk of hookworm infection is anemia secondary to loss of iron (and protein) into the gut.
Pathophysiology
A duodenale and N americanus are small, off-white worms. Males are 8-11 mm in length, and females are 10-13 mm. The sexes cannot be distinguished by the naked eye. Hookworm larvae emerge from passed eggs within 24 hours and molt once to an infective filariform larval stage in another 24 hours. After molting, larvae are able to penetrate intact skin. This rapid external portion of the hookworm life cycle is different from that of the roundworm, Ascaris, whose eggs require 3 weeks in the soil before becoming infective.
Walking barefoot in soil contaminated with feces (the source of hookworm eggs and larvae) is the most common method of exposure. After skin penetration, the venous circulation carries larvae to the pulmonary bed, where they lodge in pulmonary capillaries. Within 3-5 days, the larvae break through into alveoli and travel up the ciliary escalator from the lungs into the bronchi, the trachea, and the pharynx. Upon reaching the pharynx, larvae are swallowed and gain access to the GI tract. Once in the GI tract, worms attach to the wall of the intestine and begin to feed on the blood of the host. A N americanus adult worm consumes approximately 0.3 mL of blood per day, while the A duodenale consumes approximately 0.5 mL of blood each day. Chronic loss of blood and serum proteins leads to hookworm anemia and impaired nutrition.
Eggs begin to appear in the stool approximately 6-8 weeks after initial infection with N americanus and as long as 38 weeks after initial infection with A duodenale. The lifespan of the worm is up to one year for A duodenale and up to 5 years for N americanus.
Frequency
United States
Hookworm infection is rare in the United States.
International
The prevalence of infection is as high as 80% in lesser-developed countries with moist tropical climates but is only 10-20% in areas with drier climates.
Mortality/Morbidity
Hookworm infection is rarely fatal, but anemia can be significant in heavily infected individuals. Children and pregnant women with physiologically low iron reserves may suffer greater complications from hookworm anemia. Pulmonary complaints such as cough or wheezing are generally less common than in A lumbricoides infection.
Age
Children are infected more commonly and more heavily than adults. This is because children are more likely than adults to come in direct contact with fecally contaminated soil that contains infective larva.
Clinical
History
- Most infected individuals are asymptomatic.
- In the first 7-10 days following infection, patients may notice "ground itch" (ie, itchy papules where the skin was penetrated by larvae).
- Infected patients are not directly contagious because eggs require a brief period outside the body to hatch into their infective larval form.
- Patients may have a history of wearing open footwear or walking barefoot in endemic areas.
- Early symptoms - Larval migration phase (1-5 d)
- The onset of ground itch with localized maculopapular eruption ("ground itch") is noted.
- Low-grade fever may occur.
- Migration of worms through the lungs may produce a mild cough.
- Pulmonary inflammatory response can lead to wheezing.
- Late symptoms
- GI discomfort secondary to irritation may occur as the worms pass into the gastrointestinal tract.
- Hookworm anemia is usually due to iron deficiency and does not develop until iron reserves are depleted.
- About one half of the iron expelled by the worm is resorbed; the remainder is lost.
- A large worm burden and a history of poor iron intake increase the likelihood of significant anemia.
- The symptoms of anemia often include fatigue and dyspnea. In rare cases, anemia may provoke ischemic symptoms such as angina or claudication.
- In high prevalence areas, all anemic patients have hookworms; however, not all patients infected with hookworms have hookworm anemia.
Physical
Physical findings in the early (larval migration) and late (established GI infection) stages of the disease are different.
- Early infection (larval migration)
- Small, pruritic papules are observed at the larval skin entry site (usually feet).
- Wheezing may be triggered when the worms first break through from the venous circulation into the pulmonary air spaces.
- Established infection (adult/GI phase)
- Mild abdominal tenderness
- Pallor (hookworm anemia)
- Pale conjunctiva (hookworm anemia)
- Tachycardia (hookworm anemia)
Causes
The larvae enter through bare skin upon exposure to fecally contaminated soil that contains infective larva.
More on Hookworm |
 Overview: Hookworm |
| Differential Diagnoses & Workup: Hookworm |
| Treatment & Medication: Hookworm |
| Follow-up: Hookworm |
| References |
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References
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Freedman DO. Intestinal nematodes. In: Gorbach SL, Bartlett JG, Blacklow NR, eds. Infectious Diseases. WB Saunders Company; 1992:2003-8.
Gilles HM. Intestinal nematode infections. In: Hunter's Tropical Medicine. WB Saunders Company; 1984:620-46.
Hotez PJ, Brooker S, Bethony JM. Hookworm infection. N Engl J Med. Aug 19 2004;351(8):799-807. [Medline].
Hotez PJ, Pritchard DI. Hookworm infection. Sci Am. Jun 1995;272(6):68-74. [Medline].
Medical Letter, Inc. Drugs for parasitic infections. Med Lett Drugs Ther. The Medical Letter. Available at http://www.medletter.com. Accessed March 12, 2006.
Nawalinski TA, Schad GA. Arrested development in Ancylostoma duodenale: course of a self-induced infection in man. Am J Trop Med Hyg. Sep 1974;23(5):895-8. [Medline].
Rizzitelli G, Scarabelli G, Veraldi S. Albendazole: a new therapeutic regimen in cutaneous larva migrans. Int J Dermatol. Sep 1997;36(9):700-3. [Medline].
Sakti H, Nokes C, Hertanto WS. Evidence for an association between hookworm infection and cognitive function in Indonesian school children. Trop Med Int Health. May 1999;4(5):322-34. [Medline].
Further Reading
Keywords
hookworm infection, Ancylostoma duodenale, A duodenale, Necator americanus, N americanus, helminths, helminthic infection, hookworm, anemia, hookworm anemia, ground itch, iron deficiency, angina, claudication, tachycardia, abdominal tenderness, Strongyloides infection
