Ancylostomiasis (Hookworm)
Causal Agents:
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):
Eggs are passed in the stool
, 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 
, and after 5 to 10 days (and two molts) they become filariform (third-stage) larvae that are infective 
. 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 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 
. 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.
Life Cycle (cutaneous larval migrans):
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, 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 , and after 5 to 10 days (and two molts) they become filariform (third-stage) larvae that are infective . These infective larvae can survive 3 to 4 weeks in favorable environmental conditions. On contact with the animal 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 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 . Humans may also become infected when filariform larvae penetrate the skin 
. 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 Features:
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.
Laboratory Diagnosis:
Microscopic identification of eggs in the stool is the most common method for diagnosing hookworm infection. The recommended procedure is as follows:
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.
Diagnostic Findings
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 (L2) of hookworms and those of Strongyloides stercoralis.
Treatment:
In countries where hookworm is common and reinfection is likely, light infections are often not treated. In the United States, hookworm infections are generally treated with albendazole*. Mebendazole* or pyrantel pamoate* can also be used. For additional information, see the recommendations in The Medical Letter (Drugs for Parasitic Infections). Cutaneous larva migrans is a self-limiting infection but can be treated with albendazole or ivermectin, see recommendations in The Medical Letter (Drugs for Parasitic Infections). Treatment for more severe manifestations of zoonotic hookworm infection include albendazole and surgical removal of the parasite. There are also recommendations in The Medical Letter available for eosinophilic enteritis caused by A. caninum.
* This drug is approved by the FDA, but considered investigational for this purpose.
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):
Eggs are passed in the stool
, 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 , and after 5 to 10 days (and two molts) they become filariform (third-stage) larvae that are infective . 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 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 . 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.
Life Cycle (cutaneous larval migrans):
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
, 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 , and after 5 to 10 days (and two molts) they become filariform (third-stage) larvae that are infective . These infective larvae can survive 3 to 4 weeks in favorable environmental conditions. On contact with the animal 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 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 . Humans may also become infected when filariform larvae penetrate the skin . 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 Features:
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.
Laboratory Diagnosis:
Microscopic identification of eggs in the stool is the most common method for diagnosing hookworm infection. The recommended procedure is as follows:
- Collect a stool specimen.
- Fix the specimen in 10% formalin.
- Concentrate using the formalin–ethyl acetate sedimentation technique.
- Examine a wet mount of the sediment.
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.
Diagnostic Findings
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 (L2) of hookworms and those of Strongyloides stercoralis.
Treatment:
In countries where hookworm is common and reinfection is likely, light infections are often not treated. In the United States, hookworm infections are generally treated with albendazole*. Mebendazole* or pyrantel pamoate* can also be used. For additional information, see the recommendations in The Medical Letter (Drugs for Parasitic Infections). Cutaneous larva migrans is a self-limiting infection but can be treated with albendazole or ivermectin, see recommendations in The Medical Letter (Drugs for Parasitic Infections). Treatment for more severe manifestations of zoonotic hookworm infection include albendazole and surgical removal of the parasite. There are also recommendations in The Medical Letter available for eosinophilic enteritis caused by A. caninum.
* This drug is approved by the FDA, but considered investigational for this purpose.
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