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Hookworm

[Ancylostoma braziliense] [Ancylostoma caninum] [Ancylostoma duodenale] [Necator americanus]

Causal Agent

The human hookworms include the nematode species, Ancylostoma duodenale and Necator americanus. A larger group of hookworms infecting animals can invade and parasitize humans (A. ceylanicum) or can penetrate the human skin (causing cutaneous larva migrans), but do not develop any further (A. braziliense, A. caninum, Uncinaria stenocephala). Occasionally A. 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.


Life Cycle

Intestinal Hookworm Infection

lifecycle

Eggs are passed in the stool The number 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 The number 2, and after 5 to 10 days (and two molts) they become filariform (third-stage) larvae that are infective The number 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 The number 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 The number 5. Most adult worms are eliminated in 1 to 2 years, but the longevity may reach several years.

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

Cutaneous Larval Migrans

lifecycle

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 A. braziliense and A. 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 The number 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 The number 2, and after 5 to 10 days (and two molts) they become filariform (third-stage) larvae that are infective The number 3. These infective larvae can survive 3 to 4 weeks in favorable environmental conditions. On contact with the animal host The number 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 The number 5. Humans may also become infected when filariform larvae penetrate the skin The number 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.

Geographic Distribution

Hookworm is the second most common human helminthic infection (after ascariasis). Hookworm species are worldwide in distribution, mostly in areas with moist, warm climate. Both N. americanus and A. duodenale are found in Africa, Asia and the Americas. Necator americanus predominates in the Americas and Australia, while only A. duodenale is found in the Middle East, North Africa and southern Europe.

Clinical Presentation

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. In addition, local skin manifestations ('ground itch') can occur during penetration by the filariform (L3) larvae, and respiratory symptoms can be observed during pulmonary migration of the larvae.

The most common manifestation of zoonotic infection with animal hookworm species is cutaneous larva migrans, also known as ground itch, where migrating larvae cause an intensely pruritic serpiginous track in the upper dermis. Less commonly, larvae may migrate to the bowel lumen and cause an eosinophilic enteritis. In some cases of diffuse unilateral subacute retinitis, single larvae compatible in size to A. caninum have been visualized in the affected eye.

Hookworm eggs.

 

The eggs of Ancylostoma and Necator cannot be differentiated microscopically. The eggs are thin-shelled, colorless and measure 60-75 µm by 35-40 µm.
Figure A

Figure A: Hookworm egg in an unstained wet mount, taken at 400x magnification.

Figure B

Figure B: Hookworm egg in an unstained wet mount, taken at 400x magnification.

Figure C

Figure C: Hookworm egg in an unstained wet mount.

Figure D

Figure D: Hookworm egg in an unstained wet mount.

Figure E

Figure E: Hookworm egg in a wet mount.

Figure F

Figure F: Hookworm egg in a wet mount under UV fluorescence microscopy; image taken at 200x magnification.

Hookworm rhabditiform larva.

 

Rhabditiform (L1) larvae that hatch from eggs are 250-300 µm long and approximately 15-20 µm wide. They have a long buccal canal and an inconspicuous genital primordium. Rhabditiform larvae are usually not found in stool, but may be found there is a delay in processing the stool specimen. If larvae are seen in stool, they must be differentiated from the L1 larvae of Strongyloides stercoralis.
Figure A

Figure A: Hookworm rhabditiform larva (wet preparation).

Figure B

Figure B: Hookworm rhabditiform larva (wet preparation).

Hookworm filariform larva.

 

Infective, third-stage (L3), filariform larvae are 500-600 µm long. They have a pointed tail and a striated sheath. These L3 are found in the environment and infect the human host by penetration of the skin.
Figure A

Figure A: Filariform (L3) hookworm larva.

Figure B

Figure B: Filariform (L3) hookworm larva.

Figure C

Figure C: Filariform (L3) hookworm larva in a wet mount..

Figure D

Figure D: Close-up of the posterior end of a filariform (L3) hookworm larva.

Adult hookworms.

 

Adult hookworms reside in the small intestine of their hosts. Males measure approximately 8-12 mm long and are bursate, with two spicules that do not fuse at their distal ends. Females measure approximately 10-15 mm long.  Adults of both sexes have a buccal capsule containing sharp teeth.
Figure A

Figure A: Adult worm of Ancylostoma duodenale. Anterior end is depicted showing cutting teeth.

Figure B

Figure B: Adult worm of Necator americanus. Anterior end showing mouth parts with cutting plates.

Figure A

Figure C: Anterior end of an adult of Ancylostoma caninum, a dog parasite that has been found to produce a rare human infection known as eosinophilic enteritis.

Figure D

Figure D: Anterior end of an adult female Ancylostoma sp.

Figure E

Figure E: Posterior end of the worm seen in Figure D.

Hookworms in tissue, stained with hematoxylin and eosin (H&E).

 

Adult hookworms in tissue, stained with hematoxylin and eosin (H&E).
Figure A

Figure A: Longitudinal section of an adult hookworm worm in a bowel biopsy, stained with H&E. Note the oral cavity (OC) and strong, muscled esophagus (ES).

Figure B

Figure B: Cross-section of an adult hookworm from the same specimen in Figure A. Shown here are the platymyarian musculature (MU), intestine with brush border (IN), excretory ducts (ED), and coiled ovaries (OV).

Figure C

Figure C: Another-cross section of the specimen in Figures A and B.

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.

More on: Morphologic comparison with other intestinal parasites

Examination of the eggs cannot distinguish between N. americanus and A. duodenale. Larvae can be used to differentiate between N. americanus and A. duodenale, by rearing filariform larvae in a fecal smear on a moist filter paper strip for 5 to 7 days (Harada-Mori). Occasionally, it may be necessary to distinguish between the rhabditiform larvae (L1) of hookworms and those of Strongyloides stercoralis.

Treatment Information

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

Albendazole

Oral albendazole is available for human use in the United States.

Albendazole is pregnancy category C. 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.

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.

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

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

Mebendazole is available in the United States only through compounding pharmacies.

Mebendazole is in pregnancy category C. 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.

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.

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.

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

Pyrantel pamoate is available for human use in the United States.

Pyrantel pamoate is in pregnancy category C. 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.

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.

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.

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.

 

 

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DPDx is an education resource designed for health professionals and laboratory scientists. For an overview including prevention and control visit www.cdc.gov/parasites/.

  • Page last reviewed: May 3, 2016
  • Page last updated: May 3, 2016
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