Author: Daniel Jun Yi Wong, Medical Student, University of Melbourne, Australia, 2013.
Hookworms are nematodes, parasites that live in the small intestine of the host. There are two species that commonly infect humans: Acylostoma duodenale and Necator americanus. Hookworm infections are most common in the tropics and subtropics.
Acylostoma duodenale can be found in Mediterranean countries, the Far East, Iran, India, and Pakistan. Necator americanus tends to be found in North and South America, Central Africa, South Pacific islands, Indonesia, and certain parts of India .
There are four phases of hookworm infection:
Skin penetration by hookworm larvae produces a focal, itchy area at the site of infection (also called ‘ground itch’). This may appear as a localized area of redness and swelling, or papules. Uncommonly, the larvae may migrate within the skin, producing a snake-like track similar to cutaneous larva migrans, which is due to infection by the hookworms that infect cats and dogs.
As the hookworm larvae travel via the bloodstream to the lungs, they penetrate into the lung air sacs and ascend the windpipe to the pharynx (the back of the mouth) and are swallowed. In a minority of patients this phase is associated with generalised urticaria. Infected individuals are often unaware of the larvae travelling through the lungs but they may have a mild cough and throat discomfort.
Once the larvae have been swallowed, they travel to the small intestine where they mature into adult worms and attach to the intestinal wall. At this stage, the infected individual may experience gastrointestinal symptoms such as nausea, diarrhoea, vomiting, and abdominal pain.
The hookworms feed off the host's blood and grow larger. Chronic blood loss leads to anaemia and tiredness. Malnutrition may be severe. Hookworm infection in a pregnant woman leads to low birthweight babies, and infested children may fail to grow normally.
The hookworms eventually reproduce and the female worms lay eggs in the bowel.
Three conditions are necessary for the spread of hookworm:
Individuals who walk barefoot or with open footwear are at particular risk of hookworm infection.
Hookworm may be diagnosed clinically if there is a characteristic history, such as residence in or travel to an endemic area, walking barefoot in contaminated soil, and symptoms of hookworm infection as described above.
Infection can be confirmed by examining the stool for eggs. The eggs are typically detectable in faeces six to eight weeks following infection with N. americanus. Eggs take up to 38 weeks to be detectable following A. duodenale infection. Under the microscope, the eggs of these two species are indistinguishable.
The blood count may show raised eosinophils, a nonspecific indication of parasitic infection, and iron deficiency anaemia. Gross or occult blood may be present in the faeces.
Protective footwear must be worn in areas endemic to hookworm infestation. Other preventative measures include drinking safe water, hygienic food preparation and cooking, and adequate hand washing.
Treatment is based on the parasite involved and the phase of disease the patient is in.
Both species of hookworm are susceptible to the antihelminthics albendazole, mebendazole or pyrantel pamoate. Ivermectin is not effective. Gastrointestinal symptoms improve following treatment.
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