Ancylostoma duodenale is a highly dangerous parasitic roundworm that causes serious infections in humans.
It is commonly known as the Old World hookworm.
This parasite is prevalent in tropical, subtropical, and temperate regions of Asia, Africa, Europe, the Pacific Islands, and the Southern States of America.
The name Ancylostoma duodenale is derived from Greek, where ankylos means “hooked” and stoma means “mouth.”
The worm was originally described by Dubini in 1843 in Italy.
The life cycle of Ancylostoma duodenale was elucidated by Looss in 1898 in Egypt.
Systematic Position of Ancylostoma duodenale
Kingdom: Animalia
Phylum: Nematoda
Class: Secernentea
Order: Strongiloidae
Family: Ancylostomatidae
Genus: Ancylostoma
Species:Ancylostoma duodenale
Habits and Habitat of Ancylostoma duodenale
Adult Ancylostoma duodenale worms are endoparasitic and inhabit the small intestines of infected humans, primarily in the jejunum, less commonly in the duodenum, and rarely in the ileum.
The infective juveniles enter the human host percutaneously from soil contaminated with human feces, where the larvae are abundant.
Transmission can also occur orally and possibly transplacentally.
The worms thrive in primitive conditions where people often go barefoot, modern sanitation is lacking, and human feces are deposited on the ground.
Morphology of Ancylostoma duodenale
Adult Worms
Small and cylindroidal in shape.
Pale pink or greyish-white, may appear reddish-brown due to ingested blood.
Body is curved: dorsal aspect concave, ventral aspect convex.
Anterior end is narrow and bent dorsally, following the general body curvature; this cervical curvature gives it the name “hookworm.”
Mouth is dorsally directed, not at the tip.
Prominent buccal capsule reinforced with a hard chitin-like substance, carrying 6 teeth: 4 hook-like teeth ventrally and 2 knob-like (triangular plates) or sharp lancets dorsally with a median cleft.
Male Worm
Smaller than female, 8–11 mm in length and 0.4 mm thick.
Posterior end expanded into an umbrella-like copulatory bursa surrounding the cloaca.
Copulatory bursa has 3 lobes: 1 dorsal and 2 lateral. Each lobe supported by 13 fleshy chitinous rays (5 in each lateral lobe, 3 in dorsal lobe: 1 dorsal and 2 extradorsal rays).
Dorsal ray partially divided at the tip; each division is tripartite.
Ray pattern helps distinguish different species.
Two long retractile bristle-like copulatory spicules, tips projecting from the bursa.
Female Worm
Larger than male, 10–13 mm long and 0.6 mm thick.
Hind end tapers bluntly into a short post-anal tail.
Vulva opens ventrally at the junction of the middle and posterior thirds of the body.
Vagina leads to two intricately coiled ovarian tubes occupying the hind and middle parts of the worm.
During copulation, the male attaches its copulatory bursa to the vulva, forming a Y-shaped appearance.
Sexes differentiated by size, posterior end shape, and genital opening position.
Eggs
Oval or elliptical, measuring 60 μm × 40 μm.
Colorless, not bile-stained.
Surrounded by a thin transparent hyaline shell membrane.
Float in saturated salt solution.
Contain an unsegmented ovum when released in the intestine.
Freshly excreted eggs contain a developing embryo in early cleavage stages (2–8 cells).
Clear space exists between the segmented ovum and the eggshell.
Life cycle of Ancylostoma duodenale
Monogenetic Life Cycle: The life cycle of Ancylostoma duodenale is completed in a single host, hence it is called monogenetic.
Definitive Host: Humans are the only natural host.
Intermediate Host: No intermediate host is required.
Infective Form: Third-stage filariform larva.
a. Copulation and Fertilization
Copulation occurs in the host’s intestine.
During copulation, the male’s copulatory bursa attaches to the female’s vulva and transfers sperm.
Fertilization occurs in the seminal receptacles of the female.
b. Egg Laying
The female lays approximately 28,000 eggs daily in the host’s intestine.
Eggs contain segmented ova with 4 blastomeres and are passed out in the feces.
Freshly passed eggs are not infective for humans.
c. Development in Soil
Eggs hatch in warm, moist, and shaded environments within about 48 hours, producing first-stage juveniles (rhabditiform larvae) ~250 μm long that feed on organic material and bacteria.
After ~7 days, larvae stop feeding and molt twice to become infective third-stage juveniles (filariform larvae).
Filariform larvae are 500–600 μm long with sharp-pointed tails, non-feeding, and can survive in soil for 5–6 weeks.
Larvae wait for hosts by waving their heads in the air or climbing vegetation, carried in thin water films.
Development from egg to filariform larva takes 8–10 days on average.
Direct sunlight, drying, or saltwater can kill the larvae.
d. Mode of Infection
Infection occurs when third-stage filariform larvae contact human skin.
Larvae have oral spears to penetrate the skin, usually hands, feet, arms, and legs, often via hair follicles or abraded skin.
Skin penetration may cause severe dermatitis called “ground itch,” with ulceration at the wound site.
Rarely, infection occurs orally via contaminated fruits or vegetables; larvae can penetrate buccal mucosa to enter circulation and migrate via the lungs.
Transmammary and transplacental transmission has also been reported.
e. Larval Migration
After skin penetration, larvae enter subcutaneous venules and lymphatics to reach the host’s circulation.
They enter pulmonary capillaries, penetrate alveolar spaces, ascend the bronchial tree to the trachea, pass the epiglottis into the pharynx, and are swallowed into the gastrointestinal tract.
During migration or upon reaching the esophagus, larvae undergo the third molt.
In the small intestine, they feed, grow, undergo a fourth and final molt, develop the buccal capsule, attach to the intestinal wall, and mature into adults.
Adult worms vary in size; females are generally larger than males.
Sexual maturity is reached in 3–4 weeks, completing the cycle.
The normal lifespan of adult worms in the human intestine is generally 3–4 years.
Pathogenicity of Ancylostoma duodenale
Pathogenicity affects the skin, lungs, and small intestine.
Infection may be asymptomatic in some individuals, while others exhibit mild to severe symptoms.
The disease progresses through three phases: invasion, migration, and establishment in the intestine.
Invasion Phase
Filariform larvae penetrate the skin, causing a hypersensitivity reaction.
Local symptoms include intense itchiness, irritation, and vesicular rash lesions known as ground itch.
Migration Phase
1–2 weeks after skin penetration, larvae travel through the bloodstream to the lungs.
Pulmonary involvement can occasionally cause pneumonitis.
Pulmonary symptoms are usually mild and transient, such as dry cough, sore throat, wheezing, and slight fever.
Oral ingestion of larvae can lead to Wakana syndrome, characterized by nausea, vomiting, pharyngeal irritation, cough, dyspnea, and hoarseness.
Establishment in the Intestine
Eosinophilia develops with the maturation of adult worms in the intestine.
Major pathology arises from intestinal blood loss due to adult worms attaching to the mucosa and submucosa of the small intestine.
Clinical manifestations usually occur in moderate to high-intensity infections, most severe in children, but low-intensity infections may cause dyspepsia, nausea, and epigastric discomfort.
Infection may lead to acute enteritis with uncontrollable diarrhea and foul stools lasting indefinitely.
Chronic infection results in iron-deficiency anemia, loss of appetite, abdominal discomfort, and malnutrition due to protein deficiency, which can cause physical and cognitive impairment.
Rarely, patients may experience constipation or diarrhea with occult blood or frank melena, especially in children, and pica (urge to eat soil).
Severe or overwhelming infections can lead to listlessness, coma, and even death, particularly in infants under one year of age.
Diagnosis of Ancylostoma duodenale
Direct Methods
Fecal Examination: Demonstration of characteristic oval, segmented eggs in feces via direct wet microscopy or formalin-ether concentration methods is the best diagnostic approach.
Larvae in Stool: If stool samples are examined 24 hours or more after collection, eggs may have hatched into rhabditiform larvae, which must be distinguished from Strongyloides larvae.
Egg Counts: Quantitative estimation of eggs indicates infection intensity; the modified Kato-Katz smear technique is commonly used.
Stool Cultures: Fecal smears on moistened filter paper in a closed tube (Harada-Mori technique) demonstrate third-stage filariform larvae.
Identification:A. duodenale larvae have long buccal cavities and smaller genital primordia, while Strongyloides larvae have short buccal cavities.
Indirect Methods
Blood Examination: Reveals microcytic, hypochromic anemia and eosinophilia.
Stool Examination: May show occult blood and Charcot-Leyden crystals.
Chest Radiography: Usually negative during pulmonary larval migration; sputum examination may occasionally show erythrocytes, eosinophils, and rare migrating larvae.
Radiographic Findings: May include intestinal hypermotility, proximal jejunal dilatation, and coarsening of mucosal folds.
Immunoserological Tests: Detect host antibodies against A. duodenale antigens, but cannot reliably distinguish current versus past infections.
Molecular Methods: DNA-based tools, such as PCR assays, are increasingly used for accurate diagnosis, specific hookworm identification, and genetic analysis, as hookworm eggs often resemble other parasitic eggs.
Treatments and prophylaxis of Ancylostoma duodenale
Effective treatment of Ancylostoma duodenale includes a single dose of albendazole (400 mg) or mebendazole (500 mg), or a 3-day course of albendazole, mebendazole, or pyrantel pamoate (11 mg/kg × 3 days).
For children, pregnant women, or individuals who develop anemia due to hookworm infection, health care providers often prescribe iron supplements to restore hemoglobin levels.
Prevention of soil contamination with feces and the proper use of sanitary latrines are important measures.
Wearing footwear prevents larval entry through the skin of the feet, and gloves provide similar protection for the hands of farmworkers.
Simultaneous treatment of patients and carriers is recommended to reduce the source of infection.
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