HOOKWORM

Hookworm infections are caused by intestinal nematodes (roundworms), with most human infections being caused by either Necator americanus or Ancylostoma duodenale. Less frequently, human hookworm infections may be caused by A. ceylanicum or A. caninum. Occasionally, animal hookworms such as A. brasiliensis cause disease in humans. Hookworm infection is widely distributed throughout tropical and subtropical regions of the world. Favorable conditions for the spread of infection include warm temperatures, high humidity, shade, and contamination of soil with human feces.

It is estimated that up to one-fifth of the world's population is infected with hookworms. Humans harbor the adult worms in their small intestine. Eggs are passed in stool and will hatch within twenty-four to forty-eight hours if conditions are favorable. After hatching, the larvae molt twice in soil and, in approximately ten days, become infective filariform larvae. When a barefoot human walks in contaminated soil these filariform larvae penetrate the skin, migrate through the blood stream to the lungs, break through pulmonary capillaries into the alveoli, and then travel up the tracheobronchial tree to the epiglottis. The larvae are then swallowed and reach the small intestine where they mature into adult worms. Eggs will appear in the stool within one to three months. Female worms will lay between 5,000 and 10,000 eggs a day for up to ten years; the average adult lifespan is four to six years.

Larval penetration of skin may cause local itching and a papulovesicular rash, sometimes called "ground itch." Nonhuman hookworm infections classically produce a very pruritic migratory linear rash. As human hookworm larvae migrate through the lungs there may be mild pulmonary symptoms, usually a cough or wheezing. There may also be gastrointestinal symptoms such as nausea, abdominal pain (peptic ulcer-like symptoms), or diarrhea during the intestinal phase; however, the infection is usually asymptomatic. The major problems attributable to hookworms are iron deficiency, anemia, and protein deficiency, which are frequently seen in malnourished individuals who are infected with a large number of worms. Each hookworm consumes approximately0.03 to 0.15 milliliters of blood each day. A large number of worms can remove enough blood to cause an anemia severe enough to result in growth retardation in children. Anemia does not usually occur in those with normal iron intake.

Diagnosis is made by finding the characteristic eggs in stool. Treatment involves the elimination of the worms and correction of anemia and protein deficiency, if present. Medications that are effective against hookworm include albendazole, mebendazole, and pyrantel pamoate.

Humans are the only reservoir of infection for N. americanus and A. duodenale. Prevention of hookworm infection involves good sanitation systems and public education regarding the risks of contamination of soil with human feces and the importance of wearing shoes in endemic areas.

Elimination of hookworm infection in a population would reduce levels of anemia and malnutrition and result in improved physical and intellectual development. All members of a community are at risk of infection, but certain groups are more vulnerable to the effects of chronic infection, including preschoolers, school-age children, and women of child-bearing age. Community control programs are often targeted specifically at these groups.

Humans do not develop an immunity to hookworm infection and can be reinfected with subsequent exposures. For this reason, eradication programs such as mass treatments of a population need to be repeated on a regular basis.

Martha Fulford

Jay Keystone

Bibliography

Bundy, D. A. P., and de Silva, N. R. (1998). "Can We Deworm This Wormy World?" British Medical Bulletin 54(2):421–432.

Chin, J., ed. (2000). Control of Communicable Diseases, 17th edition. Washington, DC: American Public Health Association.

Grencis, R. K., and Cooper, E. S. (1996). "Enterobius, Trichuris, Capillaria, and Hookworm, Including Ancylostoma Caninum." Gastroenterology Clinics of North America 25(3):579–597.

Hotez, P., and Pritchard, D. (1995). "Hookworm Infection." Scientific American, June.

World Health Organization (1996). Report of the WHO Informal Consultation on Hookworm Infection and Anemia in Girls and Women. Geneva: Author.

hookworm any of a number of bloodsucking nematodes in the phylum Nematoda , order Strongiloidae that live as parasites in humans and other mammals and attach themselves to the host's intestines by means of hooks. Hookworm infection in humans is caused by infestation with Ancylostoma duodenale (the European species) or with Necator americanus (the American species). It is found in tropical and subtropical climates, especially where the inhabitants do not wear shoes or stockings and where the soil is contaminated by human excrement. The larva of the hookworm, living in moist soil or mud, easily penetrates the exposed skin, usually the sole of the foot, and is then carried by the blood to the lungs. An early sign of hookworm infestation is a dermatitis at the site of entry, known as ground itch. As the larva passes through the lungs, it causes episodes of coughing with bloody sputum. Raised with the mucus into the mouth, the larva is then swallowed. It may also be swallowed with polluted drinking water or with unclean vegetables eaten raw. By means of its hooks the larva attaches itself to the upper portion of the small intestine, where it nourishes itself on the blood of its host. The larva matures and the female produces eggs, as many as 30,000 per day, that are passed from the intestine with the feces, usually to contaminate the soil still further. The drain on the blood of the host results in anemia. This, together with the resulting abdominal pain and diarrhea, causes general debility. Hookworm is treated with drugs, notably tetrachloroethylene, that loosen and destroy the parasite, as well as with specifics for the anemia and abdominal symptoms. Incidence of this disease, which was once seriously prevalent, has been much reduced by improved sanitation and the wearing of shoes.

Hookworm

What Is Hookworm?

How Do Hookworms Grow?

How Is Hookworm Diagnosed and Treated?

Can Hookworm Be Prevented?

Resources

Hookworm is a parasitic roundworm that burrows through the skin, moves through the bloodstream to the lungs, and finally moves into the intestinal tract.

KEYWORDS

for searching the Internet and other reference sources

Infestations

Necator americanus

Nematodes

Parasitic infections

What Is Hookworm?

Hookworm is a type of parasitic* roundworm, known as a nematode (NEM-a-tode), that has hooked mouthparts. The worm uses these mouthparts to fasten itself onto the intestinal walls of various hosts*, including humans. Hookworm larvae (the stage between egg and adult) often enter the body through the skin between the toes in people who go barefoot. Hookworm affects about a quarter of the world’s population, most often in warm, moist areas and in places with poor sanitation.

* parasitic

(par-a-SIT-ik) refers to creatures that live in and feed on the bodies of other organisms.

* hosts

are the animals or plants that harbor a parasite.

“The Germ of Laziness”

Charles Stiles started the National Hookworm Eradication Program in the early 1900s. At the time, hookworm was a serious problem in the southern United States because of the warm, moist climate and poor sanitation conditions, combined with the habit of going barefoot. Hookworm was called the “germ of laziness.”

In 1914, oil magnate John D. Rockefeller gave Stiles a $1 million grant to target sanitation in the public schools. Stiles’s program became a model used in hookworm eradication programs worldwide.

The U.S. and the World

Approximately 3.5 billion people worldwide have some form of infection from intestinal parasites, with about 450 million people in rich and poor countries alike experiencing symptoms of illness.

• Hookworms, roundworms, and pin-worms are among the most common intestinal parasites and are a major public health problem.
• Children living in poor regions with tropical climates are especially at risk for hookworm and roundworm infections.
• Hookworm and roundworm infections also occur in countries like the United States, especially from contact with infected pets or their waste or from poor personal hygiene.
• Hookworms are responsible for 60,000 deaths annually. The large roundworm Ascaris lumbricoides (which causes ascariasis) is responsible for another 65,000 deaths worldwide each year.

How Do Hookworms Grow?

Adult hookworms live inside the small intestine of humans and other animals, where they attach to the intestinal wall and feed on the host’s blood. Eggs are passed in the host’s stools. The eggs hatch into larvae in 1 to 2 days under warm, moist conditions out of direct sunlight. The larvae grow in the stools and/or soil for 5 to 10 days until they become filariform (fi-LARE-i-form), or threadlike. It is during this stage that hookworm can cause infection.

Filariform larvae can survive for up to 3 or 4 weeks. When they make contact with the bare skin of a human or another animal, they burrow into the skin and enter the host’s bloodstream. They then are carried through the bloodstream to the lungs, where they burrow through the thin walls. Once inside the lungs, they move up the airways

to the throat, where they are swallowed. They then pass down the digestive tract to the small intestine. Inside the small intestine, they attach to the intestinal wall, and the cycle begins again.

How Is Hookworm Diagnosed and Treated?

The most common symptom of hookworm is anemia* caused by a loss of iron due to blood loss. Other symptoms include lack of appetite, weight loss, diarrhea*, and vague abdominal pain. When the larvae are in the lungs, there may be a dry cough and a mild fever. When the larvae burrow into the skin, there may be mild itching and a rash. It is not uncommon, however, for mild infections to show no symptoms at all.

* anemia

(a-NEE-mee-a) results when people have too few red blood cells and hemoglobin to carry oxygen in the blood.

* diarrhea

(di-a-REE-a) refers to large, frequent watery stools (bowel movements).

An infection is confirmed in the laboratory by observing eggs in stool samples under a microscope. Medications can be prescribed to treat the infection. Anemia is treated with iron supplements.

Can Hookworm Be Prevented?

The most effective method of prevention is limiting direct contact between infected soil and skin. In areas where hookworm is common, people should avoid walking barefoot or touching soil with their bare hands.

See also

Anemia

Ascariasis

Diarrhea

Parasitic Diseases

Pinworm (Enterobiasis)

Worms

Resources

U.S. Centers for Disease Control and Prevention (CDC), 1600 Clifton Road, Atlanta, GA 30333. CDC has a Division of Parasitic Diseases that posts a fact sheet about hookworm infection at its website. Telephone 800-311-3435 http://www.cdc.gov/ncidod/dpd/hookworm.htm

U.S. National Institute of Allergy and Infectious Diseases (NIAID). NIAID posts a fact sheet about hookworm and other parasitic roundworm diseases at its website. http://www.niaid.nih.gov/factsheets/roundwor.htm

hook·worm / ˈhoŏkˌwərm/ • n. a parasitic nematode (Ancylostoma, Uncinaria, Necator, and other genera, class Phasmida) that inhabits the intestines of humans and other animals. It has hooklike mouthparts with which it attaches itself to the wall of the gut, puncturing the blood vessels and feeding on the blood. ∎  a disease caused by an infestation of hookworms, often resulting in severe anemia.

hookworm (huuk-werm) n. either of two nematode worms, Necator americanus or Ancylostoma duodenale, which live as parasites in the human intestine. h. disease a condition resulting from an infestation of the small intestine by hookworms. Heavy hookworm infections may cause considerable damage to the wall of the intestine, leading to severe anaemia. Mebendazole is used in treatment.

hookworm Two species (Necator americanus and Ancyclostoma duodenale) of roundworm. A human parasite, hookworm larvae usually enter the host through the skin of the feet and legs and attach to the wall of the small intestine. Symptoms can include anaemia and constipation. Phylum Nematoda.

hookworm •affirm, berm, confirm, firm, germ, herm, midterm, perm, sperm, squirm, term, therm, worm •pachyderm • echinoderm •wheatgerm • endosperm •gymnosperm • isogeotherm •ragworm • flatworm • threadworm •tapeworm •eelworm, mealworm •silkworm • ringworm • inchworm •blindworm • lobworm • roundworm •slow-worm • screw worm •woodworm •bookworm, hookworm •bloodworm • lugworm • lungworm •earthworm