Hookworm species have a worldwide distribution, with two species known to infect humans: Ancylostoma duodenale and Necator americanus. They are the second most common helminthic infection reported in humans. Other soil-transmitted helminths, roundworm (Ascaris lumbricoides) and whipworm (Trichuris trichiura) are the first and second most prevalent helminthic infections.
The greatest number of hookworm cases occur in Asia, followed by sub-Saharan Africa. N. americanus is the most common hookworm worldwide, whereas A. duodenale is more geographically restricted. Iron-deficiency anemia and protein-energy malnutrition from blood loss are the two major manifestations of hookworm infections.
The eggs and rhabiditiform larvae of the two species are indistinguishable. Differentiation of the species is based on the morphology of the buccal capsule and the adult male copulatory bursa.
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Hookworms are found in areas with moist, warm soil capable of supporting the parasite’s life cycle. Soil becomes infectious around 5-10 days after contamination and remains so for 3-4 weeks, depending on conditions.
Hookworm infection is mainly acquired by walking barefoot on contaminated soil through direct skin penetration by filariform larvae. Ancylostoma duodenale can also be transmitted through the ingestion of larvae.
- Eggs are passed in the stool.
If there is a practice of open defecation (defecation in fields, bushes, near water bodies, or other open spaces) or if the feces of infected persons are used as fertilizer, eggs get deposited on soil.
- Under favorable conditions (moisture, warmth, shade), larvae hatch in 1 to 2 days and become free-living in contaminated soil. These released rhabditiform larvae grow in the feces and/or the soil.
- After 5 to 10 days (and two molts), the noninfective rhabdifiform larvae will mature into infective filariform larvae (third-stage larvae). These infective larvae can survive 3 to 4 weeks in favorable environmental conditions.
- On contact with the human host, typically bare feet, the infective filariform larvae penetrate the skin and enter the circulation.
Humans acquire hookworm when third-stage infective larvae in soil either penetrate the skin (as do both N. americanus and A. duodenale) or when they are ingested (A. duodenale only).
- Larvae exit circulation in the lungs coughed up and swallowed. Reach intestine, mature, and lay eggs.
The larvae are carried through the blood vessels to the heart. The larvae can break through the capillaries and enter the host’s lungs. They penetrate the pulmonary alveoli, migrate up the bronchial tree to the pharynx, and are swallowed.
Upon entering the digestive system, the larvae attach to the mucosa of the small intestine. Here they secrete anticoagulants, ingest blood as their source of nourishment, and mature into adults. Adult worms live in the lumen of the small intestine, typically the distal jejunum.
The worms mature, and eggs are passed in the feces. About 5 weeks after infection, the worms attain sexual maturity, fertilization occurs, and females lay eggs that are passed along with the feces. Each female hookworm can lay 15000 – 20000 eggs per day.
Most adult worms are eliminated in 1 to 2 years, but the longevity may reach several years.
Most people infected with hookworms have no symptoms, but some may develop epigastric pain, diarrhea, anorexia, and eosinophilia. People who are infected for the first time may have gastrointestinal symptoms. Repeated exposure to third-stage larvae of hookworm results in a local pruritic, erythematous, papular rash known as “ground itch” at the site of larval penetration.
Two severe consequences of hookworm infections are;
- Anemia: The most serious effects of hookworm infection are blood loss, anemia, and protein loss.
- Growth and Mental Retardation: When many worms continuously infect children, the loss of iron and protein can retard growth and mental development.
Major clinical features of hookworm infections are tabulated here:
|Dermal||Local erythema, macules, papules (ground itch) Creeping eruption||Cutaneous invasion and subcutaneous migration of larva|
|Pulmonary||Bronchitis, pneumonitis, and, sometimes, eosinophilia||Migration of larvae through lung, bronchi, and trachea|
|Gastrointestinal||Anorexia, epigastric pain, and gastrointestinal hemorrhage||Attachment of adult worms and injury to the upper intestinal mucosa|
|Hematologic||Iron deficiency, anemia, hypoproteinemia, edema, cardiac failure||Intestinal blood loss|
Sample: Stool, duodenal content, blood (to ascertain the nature of anemia and the presence of eosinophilia)
Hookworms are typically diagnosed by eggs or rhabditiform larvae found in stool specimens. Finding hookworm eggs on direct stool smear is problematic in light infections, so the concentration method is recommended. Ether concentration, FLOTAC, and Kato-Katz techniques are used to increase the diagnosis of hookworm infections. Species of hookworms cannot be distinguished by egg or larvae morphology. The eggs and larvae of the two species are indistinguishable.
The eggs are oval, thin-shelled, and contain a visible four-eight stage embryo. There is a characteristic clear space between the shell and the developing embryo.
The rhabditiform larvae are typically 250 to 300 μm with a long buccal capsule and an inconspicuous genital primordium. The larger filariform larvae are approximately 500 μm, with a pointed tail and an esophageal-to-intestinal ratio of 1:4. The rhabiditiform and filariform larvae must be differentiated from S. stercoralis.
Adult female hookworms measure up to 15 mm and males up to 10 mm in length. The anterior end is bent slightly dorsally, hence called hookworm. The mouth is large, and buccal capsule is equipped with four teeth on the ventral surface and a pair of smaller teeth on its dorsal surface
- Male: the posterior end is expanded into a copulatory bursa containing two copulatory spicules supported by fleshy rays. The expanded umbrella-like pattern of the posterior end is the characteristic feature of the male worm.
- Female: the hind end is coiled with a subterminal anus situated ventrally
Rearing filariform larvae in a fecal smear on a moist filter paper strip for 5 to 7 days) to identify the species of nematode involved.
Anthelmintic agents, including albendazole and mebendazole, are indicated. Iron supplements may also be required in severely anemic patients.
Avoid contaminated soil (places with open defecation) and beaches. When traveling to a potentially contaminated area, wear appropriate footwear, such as enclosed shoes, and avoid other skin-to-soil contact forms. Communities can be protected by implementing effective sewage disposal systems. Currently, no preventive vaccine exists.
Types of Hookworms
Necator americanus and Ancylostoma duodenale are the significant hookworm species responsible for >90% of cases. Other species that can also cause infections are Ancylostoma ceylanicum, Ancylostoma braziliense, and Ancylostoma caninum.
Ancylostoma duodenale, Old World hookworm, is prevalent in southern Europe, Northern Africa, Southeast Asia, and South America. They attach to the intestinal mucosa by well-developed mouthparts, especially teeth.
Necatar americanus, New World hookworm, is prevalent in Africa, Southeast Asia, South and Central America, and the southeastern United States. They attach to the intestinal mucosa by well-developed cutting plates.
Differences between Ancylostoma duodenale and Necator Americanus
|Ancylostoma duodenale||Necator americanus|
|Blood loss||10.20 mL/day||0.03 mL/day|
|Life Span||1-2 years||3-5 years or more|
|Egg production||10,000 to 25000 /day||5000 to 10,000/day|
|Mode of Transmission||Skin penetration and ingestion||Skin penetration only|
|Buccal capsule||Ancylostoma contains sharp teeth.||Necator contains cutting plates.|
References and further readings
- Sastry A.S. & Bhat S. (2014) Essentials of Medical Parasitology. Jaypee Brothers Medical Publishers (P) Ltd
- Gracia, L.S. (2016). Diagnostic Medical Parasitology. ASM Press.