The hookworm is a parasitic nematode worm that lives in the small intestine of its host, e.g. dog, cat or human. Two species of hookworms commonly infect humans, Ancylostoma duodenale and Necator americanus. Other hookworms that mainly infect animals can also be parasites of humans (Ancylostoma ceylanicum) or can cause cutaneous larva migrans (Ancylostoma braziliense, Ancylostoma braziliense caninum, Uncinaria stenocephala). A. caninum larvae can occasionally migrate to the human intestine, causing eosinophilic enteritis. A. caninum larvae have also been implicated as a cause of diffuse unilateral subacute neuroretinitis.¹ This article covers intestinal hookworm infection. Cutaneous larva migrans is discussed in a separate article (see link above).

Life cycle¹

• Eggs are passed in the stool and (with favourable conditions of moisture, warmth and shade), larvae hatch in 1 to 2 days.
• The released larvae grow in the faeces and/or the soil, and after 5 to 10 days they become filariform (third-stage) larvae that are infective and can survive for 3 to 4 weeks in favourable 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 then swallowed.
• The larvae reach the small intestine where they mature into adults. Adult worms live in the lumen of the small intestine, where they attach to the intestinal wall, causing intestinal blood loss. Most adult worms are eliminated within 1 to 2 years.
• Some A. duodenale larvae, following penetration of the host skin, can become dormant in the intestine or muscle. Infection by A. duodenale can probably also occur by the oral route, but N. americanus requires a transpulmonary migration phase.

Epidemiology¹

• Hookworm is the second most common human helminthic infection (after ascariasis). Hookworm infections are thought to affect approximately 1 in 4 of the world's population.²
• Distribution is worldwide but mostly in areas with a moist, warm climate. Both N. americanus and A. duodenale are found in Africa, Asia and the Americas.
• N. americanus predominates in the Americas and Australia.
• A. duodenale predominates in the Middle East, North Africa and Southern Europe.
• Infection is usually acquired by walking, handling, or lying in contaminated soil.³

Presentation

• Most individuals with hookworm infection are asymptomatic.
• Symptoms are due to inflammation in the bowel (e.g. nausea, abdominal pain and intermittent diarrhoea) and the clinical manifestations of iron-deficiency anaemia.
• Local skin manifestations ('ground itch') can occur during penetration by the filariform larvae, and respiratory symptoms may occur during pulmonary migration of the larvae.
• The blood loss in the stools is occult and not visibly apparent.
• Chronic protein loss can result in hypoproteinaemia and anasarca (widespread swelling of the skin due to effusion of fluid into the extracellular space).
• Although the most common manifestation of hookworm infection is cutaneous larva migrans, larvae may also occasionally migrate to the bowel lumen and cause an eosinophilic enteritis.¹

Investigations

• FBC: eosinophilia (the appearance of eosinophilia coincides with the development of adult hookworms in the intestine); microcytic (iron deficiency) anaemia.
• Diagnosis depends on finding characteristic worm eggs on microscopic examination of the stools, although this is not possible in early infection.
• CXR may show diffuse alveolar infiltrates during the migration of the worms through the lung in severe infection.³
• There are no serological tests for hookworm infections.

Management

• In countries where hookworm is common and reinfection is likely, mild infections are often not treated.
• Hookworm can be treated with local cryotherapy when it is still in the skin.
• Albendazole is the most effective medication;⁴ albendazole or mebendazole are effective both in the intestinal stage and during the stage the parasite is still migrating under the skin.
• In case of anaemia, iron supplementation (folic acid or vitamin B12 may also be required as red blood cells are replenished).
• Treatment for more severe infections may also include surgical removal of the parasites.

Complications

• Rapid hookworm reinfection is common in endemic areas.³
• Hookworm is a leading cause of maternal and child morbidity in developing countries.
• Hookworm infections contribute to anaemia, malnutrition, developmental delay and poor growth in children and adolescents in the developing world.⁵
• Hookworms also cause intrauterine growth restriction, prematurity, and low birthweight in newborns born to infected mothers.
• Hookworm infection is rarely fatal, but anaemia can be significant if there is heavy infection.

Prevention

Community control is difficult unless socioeconomic conditions, sanitation, education, and the availability of proper footwear significantly improve. Current World Health Organization (WHO) recommendations for hookworm infection include periodic mass therapy with albendazole to lower the overall worm burden.³ Hookworm vaccines are being developed but are not yet available.⁶ Advice for individuals includes:

• Prevent skin/soil contact: do not walk barefoot.
• Do not defecate outside latrines, toilets etc.
• Do not use human excrement or raw sewage as manure or fertiliser in agriculture.

Document references

1. Hookworm; DPDx
2. Dhawan VK et al; Ancylostoma Infection, eMedicine, Jul 2010
3. Haburchak DR; Hookworms, eMedicine, Jul 2008
4. Keiser J, Utzinger J; Efficacy of current drugs against soil-transmitted helminth infections: systematic review and meta-analysis. JAMA. 2008 Apr 23;299(16):1937-48. [abstract]
5. Di Pentima C; Burden of non-sexually transmitted infections on adolescent growth and Adolesc Med State Art Rev. 2009 Dec;20(3):930-48, x. [abstract]
6. Diemert DJ, Bethony JM, Hotez PJ; Hookworm vaccines. Clin Infect Dis. 2008 Jan 15;46(2):282-8. [abstract]

Internet and further reading

• A-Z Index of Parasitic Diseases, Centers for Disease Control and Prevention

Acknowledgements