What is hookworm

Hookworms are small (less than 0.5 inches long) parasitic worms that can cause infections in the small intestines ¹. The major species of hookworms associated with infections in humans are Ancylostoma duodenale and Necator americanus. Hookworms get their name from the teeth (“hooks”) or cutting plates in their mouths by which they attach themselves to the intestinal wall.

Hookworm diseases are most common in tropical and subtropical climates. Hookworm infections develop after a person has contact with soil contaminated with human feces. Children are at high risk because they often play barefoot in areas with contaminated soil. In soil, hookworm eggs hatch and form larvae, which then burrow through the skin of a person’s foot and crawl into the blood. The blood carries the larvae to the lungs, where they enter into the air sacs. The hookworms then crawl up the breathing tubes to the throat, where they are swallowed. The larvae pass through the stomach and mature into adult worms in the bowel. Hookworm holds onto the bowel wall with hooks, which cause minor bleeding. Adult hookworms live in the bowel and lay eggs that pass out of the child with the stool.

Hookworm is the second most common human helminthic infection (after ascariasis or roundworm). Hookworm species are worldwide in distribution, mostly in areas with moist, warm climate. Both Necator americanus and Ancylostoma duodenale are found in Africa, Asia and the Americas. Necator americanus predominates in the Americas and Australia, while only Ancylostoma duodenale is found in the Middle East, North Africa and southern Europe. An estimated 576-740 million people in the world are infected with hookworm ¹. Hookworm was once widespread in the United States, particularly in the southeastern region, but improvements in living conditions have greatly reduced hookworm infections ¹. Hookworm, Ascaris, and whipworm are known as soil-transmitted helminths (parasitic worms). Together, they account for a major burden of disease worldwide.

Hookworms live in the small intestine. Hookworm eggs are passed in the feces of an infected person. If the infected person defecates outside (near bushes, in a garden, or field) of if the feces of an infected person are used as fertilizer, eggs are deposited on soil. They can then mature and hatch, releasing larvae (immature worms). The larvae mature into a form that can penetrate the skin of humans. Hookworm infection is mainly acquired by walking barefoot on contaminated soil. One kind of hookworm can also be transmitted through the ingestion of larvae.

Most people infected with hookworms have no symptoms. Some have gastrointestinal symptoms, especially persons who are infected for the first time. The most serious effects of hookworm infection are blood loss leading to anemia, in addition to protein loss.

Most children with hookworm infections have no signs or symptoms. However, especially when the hookworm infection is long term, it can cause iron deficiency and anemia (low red blood cells) because of bleeding from the bowel wall where the worm is attached. Other symptoms include mild diarrhea and stomach cramps. An itchy, red skin rash (ground itch) can appear on the feet where the larvae entered the body. Lung inflammation with cough, wheezing, and fever rarely occur while the larvae migrate through the lungs. Several weeks after exposure to this hookworm, a loss of appetite and weight loss may occur. Chronic infections can lead to poor nutrition.

Hookworm infections are treatable with medication prescribed by your health care provider. Hookworm infections are generally treated for 1-3 days with albendazole, mebendazole, or pyrantel pamoate prescribed by your health care provider. Commonly prescribed include a single dose of albendazole 400mg or 3 days of mebendazole or pyrantel. One to 2 weeks following treatment, your doctor may test another stool sample from you. The drug therapy should be repeated if the infection persists. These drugs are effective and appear to have few side effects. Iron supplements may be prescribed if you have anemia.

Reinfection is common. Wearing shoes prevents the larvae from entering the body through the feet. If your child goes barefoot, make sure he/she avoids contact with soil that may be contaminated with human feces.

Figure 1. Hookworms

How is hookworm spread?

Hookworm is a soil-transmitted helminth and is one of the most common roundworm of humans. Hookworm eggs are passed in the feces of an infected person. If an infected person defecates outside (near bushes, in a garden, or field) or if the feces from an infected person are used as fertilizer, eggs are deposited on soil. They can then mature and hatch, releasing larvae (immature worms). The larvae mature into a form that can penetrate the skin of humans. Hookworm infection is transmitted primarily by walking barefoot on contaminated soil. One kind of hookworm, Ancylostoma duodenale, can also be transmitted through the ingestion of larvae.

Who is at risk for infection?

People living in areas with warm and moist climates and where sanitation and hygiene are poor are at risk for hookworm infection if they walk barefoot or in other ways allow their skin to have direct contact with contaminated soil. Soil is contaminated by an infected person defecating outside or when human feces (“night soil”) are used as fertilizer. Children who play in contaminated soil may also be at risk.

Hookworm life cycle

The human hookworms include the nematode species, Ancylostoma duodenale and Necator americanus. A larger group of hookworms infecting animals can invade and parasitize humans (Ancylostoma ceylanicum) or can penetrate the human skin (causing cutaneous larva migrans), but do not develop any further (Ancylostoma braziliense, Ancylostoma caninum, Uncinaria stenocephala). Occasionally Ancylostoma caninum larvae may migrate to the human intestine, causing eosinophilic enteritis. Ancylostoma caninum larvae have also been implicated as a cause of diffuse unilateral subacute neuroretinitis.

Figure 2. Hookworm life cycle – Intestinal Hookworm Infection

Footnote: Eggs are passed in the stool (1), and under favorable conditions (moisture, warmth, shade), larvae hatch in 1 to 2 days. The released rhabditiform larvae grow in the feces and/or the soil (2) and after 5 to 10 days (and two molts) they become filariform (third-stage) larvae that are infective (3). These infective larvae can survive 3 to 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 (4). 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 (5). Most adult worms are eliminated in 1 to 2 years, but the longevity may reach several years.

Some Ancylostoma duodenale larvae, following penetration of the host skin, can become dormant (in the intestine or muscle). In addition, infection by Ancylostoma duodenale may probably also occur by the oral and transmammary route. Necator americanus, however, requires a transpulmonary migration phase.

Figure 3. Hookworm life cycle – Cutaneous Larval Migrans

Footnote: Cutaneous larval migrans (also known as creeping eruption) is a zoonotic infection with hookworm species that do not use humans as a definitive host, the most common being Ancylostoma braziliense and Ancylostoma caninum. The normal definitive hosts for these species are dogs and cats. The cycle in the definitive host is very similar to the cycle for the human species. Eggs are passed in the stool (1), and under favorable conditions (moisture, warmth, shade), larvae hatch in 1 to 2 days. The released rhabditiform larvae grow in the feces and/or the soil (2) and after 5 to 10 days (and two molts) they become filariform (third-stage) larvae that are infective (3). These infective larvae can survive 3 to 4 weeks in favorable environmental conditions. On contact with the animal host (4), 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. Some larvae become arrested in the tissues, and serve as source of infection for pups via transmammary (and possibly transplacental) routes (5). Humans may also become infected when filariform larvae penetrate the skin (6). With most species, the larvae cannot mature further in the human host, and migrate aimlessly within the epidermis, sometimes as much as several centimeters a day. Some larvae may persist in deeper tissue after finishing their skin migration.

Figure 4. Hookworm rash (cutaneous larva migrans)

Footnote: Grossly inflamed and superinfected hookworm larval track on the thigh of a 8-year-old boy.

[Source ²]

Figure 5. Hookworm rash (cutaneous larva migrans)

Footnote: A 31-year-old previously healthy woman presented to our outpatient clinic with pruritic cutaneous lesions, which had been progressing for 3 days, after a vacation at the southeast beaches of Brazil (Ubatuba City). The dermatological exam showed a cutaneous eruption composed of vesicles and papules disposed in a linear and serpiginous pattern on plantar regions of the left foot (Figure A) and on the dorsal aspect of right foot (Figure B).

[Source ³]

Hookworm symptoms

Itching and a localized rash also known as “ground itch” are often the first signs of hookworm infection. These symptoms occur when the hookworm larvae penetrate the skin and where the migrating filariform larvae (cutaneous larva migrans) cause an intensely pruritic serpiginous track in the upper dermis. Less commonly, hookworm larvae may migrate to the bowel lumen and cause an eosinophilic enteritis. Respiratory symptoms can be observed during pulmonary migration of the larvae. In some cases of diffuse unilateral subacute retinitis, single larvae compatible in size to Ancylostoma caninum have been visualized in the affected eye.

A person with a light infection may have no symptoms. A person with a heavy infection may experience abdominal pain, diarrhea, loss of appetite, weight loss, fatigue and anemia. The physical and cognitive growth of children can be affected.

Iron deficiency anemia (caused by blood loss at the site of intestinal attachment of the adult worms) is the most common symptom of hookworm infection, and can be accompanied by cardiac complications. Gastrointestinal and nutritional/metabolic symptoms can also occur.

The most serious effects of hookworm infection are the development of anemia and protein deficiency caused by blood loss at the site of the intestinal attachment of the adult worms. When children are continuously infected by many worms, the loss of iron and protein can retard growth and mental development.

Cutaneous larva migrans

Cutaneous larva migrans is a parasitic skin infection caused by hookworm larvae that usually infest cats, dogs and other animals. Humans can be infected with the larvae by walking barefoot on sandy beaches or contacting moist soft soil that have been contaminated with animal feces. It is also known as creeping eruption as once infected, the larvae migrate under the skin’s surface and cause itchy red lines or tracks.

What causes cutaneous larva migrans?

Many types of hookworm can cause cutaneous larva migrans. Common causes are:

• Ankylostoma braziliense: hookworm of wild and domestic dogs and cats found in central and southern US, Central and South America, and the Caribbean
• Ankylostoma caninum: dog hookworm found in Australia
• Uncinaria stenocephala: dog hookworm found in Europe
• Bunostomum phlebotomum: cattle hookworm

Who is at risk of cutaneous larva migrans?

People of all ages, sex and race can be affected by cutaneous larva migrans if they have been exposed to hookworm larvae. It is most commonly found in tropical or subtropical geographic locations. Groups at risk include those with occupations or hobbies that bring them into contact with warm, moist, sandy soil. These may include:

• Barefoot beachcombers and sunbathers
• Children in sandpits
• Farmers
• Gardeners
• Plumbers
• Hunters
• Electricians
• Carpenters
• Pest exterminators

Most larva migrans seen in the US arises during overseas holidays, but it has rarely been reported in those who have never been out of the country.

Hookworm prevention

Do not walk barefoot in areas where hookworm is common and where there may be fecal contamination of the soil. Avoid other skin-to-soil contact and avoid ingesting such soil. Fecal contamination occurs when people defecate outdoors or use human feces as fertilizer.

The infection of others can be prevented by not defecating outdoors or using human feces as fertilizer, and by effective sewage disposal systems.

Hookworm diagnosis

Health care providers can diagnose hookworm by taking a stool sample and using a microscope to look for the presence of hookworm eggs.

Laboratory diagnostic findings

Microscopic identification of eggs in the stool is the most common method for diagnosing hookworm infection. The recommended procedure is as follows:

1. Collect a stool specimen.
2. Fix the specimen in 10% formalin.
3. Concentrate using the formalin–ethyl acetate sedimentation technique.
4. Examine a wet mount of the sediment.

Where concentration procedures are not available, a direct wet mount examination of the specimen is adequate for detecting moderate to heavy infections. For quantitative assessments of infection, various methods such as the Kato-Katz can be used.

Cutaneous larval migrans is usually diagnosed clinically, as there are no serologic tests for zoonotic hookworm infections. Larvae may be seen in stained tissue sections, but this procedure is usually not recommended as the parasites are usually not found in the visible track.

Examination of the eggs cannot distinguish between N. americanus and A. duodenale. Larvae can be used to differentiate between Necator americanus and Ancylostoma duodenale, by rearing filariform larvae in a fecal smear on a moist filter paper strip for 5 to 7 days. Occasionally, it may be necessary to distinguish between the rhabditiform larvae of hookworms and those of Strongyloides stercoralis.

Hookworm treatment

Hookworm infection is treated with albendazole, mebendazole, or pyrantel pamoate. Dosage is the same for children as for adults. Albendazole should be taken with food. Albendazole is not FDA-approved for treating hookworm infection.

Drug	Dosage for adults and children
Albendazole	400 mg orally once
Mebendazole	100 mg orally twice a day for 3 days or 500 mg orally once
Pyrantel pamoate	11 mg/kg (up to a maximum of 1 g) orally daily for 3 days

• Oral albendazole is available for human use in the United States.
• Mebendazole is available in the United States only through compounding pharmacies.
• Pyrantel pamoate is available for human use in the United States.

Albendazole

Treatment in Pregnancy

Albendazole is pregnancy category C.

• Pregnancy Category C: Either studies in animals have revealed adverse effects on the fetus (teratogenic or embryocidal, or other) and there are no controlled studies in women or studies in women and animals are not available. Drugs should be given only if the potential benefit justifies the potential risk to the fetus.

Data on the use of albendazole in pregnant women are limited, though the available evidence suggests no difference in congenital abnormalities in the children of women who were accidentally treated with albendazole during mass prevention campaigns compared with those who were not. In mass prevention campaigns for which the World Health Organization (WHO) has determined that the benefit of treatment outweighs the risk, WHO allows use of albendazole in the 2nd and 3rd trimesters of pregnancy. However, the risk of treatment in pregnant women who are known to have an infection needs to be balanced with the risk of disease progression in the absence of treatment.

Treatment During Breastfeeding

It is not known whether albendazole is excreted in human milk. Albendazole should be used with caution in breastfeeding women.

Treatment in Pediatric Patients

The safety of albendazole in children less than 6 years old is not certain. Studies of the use of albendazole in children as young as one year old suggest that its use is safe. According to WHO guidelines for mass prevention campaigns, albendazole can be used in children as young as 1 year old. Many children less than 6 years old have been treated in these campaigns with albendazole, albeit at a reduced dose.

Mebendazole

Treatment in Pregnancy

Mebendazole is in pregnancy category C.

• Pregnancy Category C: Either studies in animals have revealed adverse effects on the fetus (teratogenic or embryocidal, or other) and there are no controlled studies in women or studies in women and animals are not available. Drugs should be given only if the potential benefit justifies the potential risk to the fetus.

Data on the use of mebendazole in pregnant women are limited. The available evidence suggests no difference in congenital anomalies in the children of women who were treated with mebendazole during mass treatment programs compared with those who were not. In mass treatment programs for which the World Health Organization (WHO) has determined that the benefit of treatment outweighs the risk, WHO allows use of mebendazole in the 2nd and 3rd trimesters of pregnancy. The risk of treatment in pregnant women who are known to have an infection needs to be balanced with the risk of disease progression in the absence of treatment.

Treatment During Breastfeeding

It is not known whether mebendazole is excreted in breast milk. The WHO classifies mebendazole as compatible with breastfeeding and allows the use of mebendazole in lactating women.

Treatment in Pediatric Patients

The safety of mebendazole in children has not been established. There is limited data in children age 2 years and younger. Mebendazole is listed as an intestinal antihelminthic medicine on the WHO Model List of Essential Medicines for Children, intended for the use of children up to 12 years of age.

Pyrantel Pamoate

Treatment in Pregnancy

Pyrantel pamoate is in pregnancy category C.

• Pregnancy Category C: Either studies in animals have revealed adverse effects on the fetus (teratogenic or embryocidal, or other) and there are no controlled studies in women or studies in women and animals are not available. Drugs should be given only if the potential benefit justifies the potential risk to the fetus.

Data on the use of pyrantel pamoate in pregnant women are limited. In mass treatment programs for which the World Health Organization (WHO) has determined that the benefit of treatment outweighs the risk, WHO allows use of pyrantel pamoate in the 2nd and 3rd trimesters of pregnancy, acknowledging that the effects of pyrantel on birth outcome are not certain. The risk of treatment in pregnant women who are known to have an infection needs to be balanced with the risk of disease progression in the absence of treatment.

Treatment During Breastfeeding

It is not known whether pyrantel pamoate is excreted in breast milk. The WHO classifies pyrantel pamoate as compatible with breastfeeding, although data on the use of pyrantel pamoate during lactation are limited.

Treatment in Pediatric Patients

The safety of pyrantel pamoate in children has not been established. According to WHO guidance on preventive chemotherapy, pyrantel may be used in children age 1 year and older during mass treatment programs without diagnosis. Pyrantel pamoate is listed as an intestinal antihelminthic medicine on the WHO Model List of Essential Medicines for Children, intended for the use of children up to 12 years of age.

What is preventive treatment?

In developing countries, groups at higher risk for soil-transmitted helminth infections (hookworm, Ascaris, and whipworm) are often treated without a prior stool examination. Treating in this way is called preventive treatment (or “preventive chemotherapy”). The high-risk groups identified by the World Health Organization are preschool and school-age children, women of childbearing age (including pregnant women in the 2nd and 3rd trimesters and lactating women) and adults in occupations where there is a high risk of heavy infections. School-age children are often treated through school-health programs and preschool children and pregnant women at visits to health clinics.

What is mass drug administration?

The soil-transmitted helminths (hookworm, Ascaris, and whipworm) and four other “neglected tropical diseases” (river blindness, lymphatic filariasis, schistosomiasis and trachoma) are sometimes treated through mass drug administrations. Since the drugs used are safe and inexpensive or donated, entire risk groups are offered preventive treatment. Mass drug administrations are conducted periodically (often annually), commonly with drug distributors who go door-to-door. Multiple neglected tropical diseases are often treated simultaneously using mass drug administrations.

1. Parasites – Hookworm. https://www.cdc.gov/parasites/hookworm/[↩][↩][↩]
2. Life Quality Impairment Caused by Hookworm-Related Cutaneous Larva Migrans in Resource-Poor Communities in Manaus, Brazil. PLOS Neglected Tropical Diseases 5(11): e1355. https://doi.org/10.1371/journal.pntd.0001355[↩]
3. Criado, P. R, Jr, W. B, Vasconcellos, C., & Silva, C. S. (2012). Cutaneous Larva Migrans: A bad souvenir from the vacation. Dermatology Online Journal, 18(6). Retrieved from https://escholarship.org/uc/item/8s76836w[↩]