Ankilostomiasis

01Pathophysiology

Ancylostomiasis follows a well-defined lifecycle that directly explains every clinical finding a vignette can present. Understanding the migration route is the key to linking symptoms to their underlying mechanism.

The infection begins when filariform larvae (L3, the infective stage) in warm, moist soil penetrate intact skin, most commonly the feet of individuals walking barefoot. This invasion triggers a local immune response, producing the characteristic "ground itch," a pruritic, erythematous, papulovesicular dermatitis at the entry site. This is a localized hypersensitivity reaction to larval antigens penetrating the epidermis and dermis.

Once through the skin, larvae enter the venous circulation and are carried to the pulmonary capillaries. They rupture through the alveolar walls into the airspaces, provoking an eosinophil-rich inflammatory infiltrate. This transient pulmonary phase can manifest as Loffler syndrome: dry cough, wheezing, and fleeting pulmonary infiltrates on chest radiograph with peripheral eosinophilia. The pulmonary phase is typically mild in hookworm compared to Ascaris but is still a testable concept.

From the alveoli, larvae ascend the tracheobronchial tree, reach the pharynx, and are swallowed. They arrive in the small intestine (primarily the jejunum) where they mature into adult worms. Ancylostoma duodenale attaches using teeth, while Necator americanus uses cutting plates to latch onto the intestinal mucosa. Both species feed on blood by secreting anticoagulant peptides that prevent clot formation at the attachment site. The worms actively ingest blood and, critically, the attachment sites continue to bleed even after the worm detaches and moves to a new location.

This chronic intestinal blood loss is the central pathogenic mechanism and directly causes iron deficiency anemia. Each adult A. duodenale worm causes approximately 0.1 to 0.3 mL of blood loss per day (roughly twice that of N. americanus). Over weeks to months, cumulative blood loss depletes iron stores, leading to a microcytic, hypochromic anemia with low ferritin, low serum iron, and elevated total iron-binding capacity. In children, chronic infection results in protein-energy malnutrition, growth retardation, and cognitive impairment. Hypoalbuminemia from intestinal protein loss can produce peripheral edema in heavy infections.

A notable difference between the two species: A. duodenale can also be transmitted orally (ingestion of L3 larvae) and through the transmammary route in breastfeeding, which is not a feature of N. americanus. This is occasionally tested as a distinguishing point.

02Classification and Clinical Manifestation

By Species

By Clinical Phase

Severity Classification by Worm Burden (WHO, based on eggs per gram of stool)

Heavy infections correlate most strongly with clinically significant anemia and are the primary target for mass drug administration programs.

03Diagnostic Workup

The best initial test when hookworm is suspected is a direct stool microscopy for ova and parasites. The hallmark finding is the hookworm egg: oval with a thin, smooth, colorless shell, containing a segmented morula (a cluster of developing cells) surrounded by a clear space between the morula and the shell wall. This morphology distinguishes hookworm eggs from those of most other helminths. Importantly, the eggs of N. americanus and A. duodenale are morphologically indistinguishable under routine microscopy; species identification requires larval culture, which is rarely needed clinically but is testable as a concept.

A CBC should be ordered concurrently. The combination of microcytic hypochromic anemia with eosinophilia in a patient from an endemic region with a history of barefoot soil contact is highly suggestive of hookworm. This pairing is a classic exam finding. Iron studies will show the expected pattern of iron deficiency: low ferritin, low serum iron, and elevated TIBC.

If initial direct smear is negative but clinical suspicion remains high (light infection), stool concentration techniques (formalin-ethyl acetate sedimentation) improve sensitivity by concentrating the eggs.

For epidemiologic or public health purposes, the Kato-Katz thick smear is the standard quantitative method. It provides an EPG (eggs per gram) count used to classify infection intensity according to WHO thresholds. This is more relevant in population-level screening than individual patient management, but it may appear in exam questions framed around community health interventions.

If the stool sample is not examined promptly, hookworm eggs can hatch and release rhabditiform (L1) larvae. These larvae must be distinguished from those of Strongyloides stercoralis, which also shed rhabditiform larvae in stool. The key distinguishing feature is the buccal cavity: hookworm rhabditiform larvae have a long buccal cavity, while Strongyloides larvae have a short buccal cavity. This is a frequently tested morphologic detail.

04Management and Treatment

Acute treatment centers on anthelmintic therapy. Albendazole 400 mg as a single oral dose is the preferred agent due to its simplicity, efficacy, and tolerability. It works by inhibiting tubulin polymerization, which disrupts the worm's glucose uptake and energy metabolism. Mebendazole at 100 mg twice daily for three days is an acceptable alternative and works through the same mechanism. Both are benzimidazole class drugs.

The critical teaching point for the exam: killing the worms alone does not resolve the anemia. The "next best step" after prescribing the anthelmintic is to correct the iron deficiency. Oral iron supplementation with ferrous sulfate (325 mg, containing approximately 65 mg of elemental iron, two to three times daily) should be continued until hemoglobin normalizes and iron stores are fully repleted, which typically takes three to six months. Monitoring ferritin levels guides the duration of supplementation.

In patients with severe anemia (hemoglobin below 7 g/dL or hemodynamic compromise), blood transfusion may be necessary as an initial stabilization measure before or alongside anthelmintic therapy.

Pregnancy introduces an important contraindication scenario. Benzimidazoles are category C and are generally avoided in the first trimester due to teratogenic risk demonstrated in animal studies. However, the WHO supports administration of albendazole in the second and third trimesters in endemic areas where the risk of severe anemia outweighs the theoretical teratogenic risk. Iron supplementation should begin immediately regardless of trimester.

Follow-up: A repeat stool examination should be performed two to four weeks after treatment to confirm eradication. If eggs persist, a second course of albendazole can be administered. In endemic settings, reinfection is common, and preventive measures (wearing shoes, improved sanitation, periodic mass drug administration) are essential components of long-term control.

For children in endemic areas, the WHO recommends periodic deworming with albendazole or mebendazole one to two times per year as part of school-based or community-based mass drug administration programs.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The most common way students miss hookworm questions is by failing to connect eosinophilia with iron deficiency anemia in a patient with soil exposure. When a vignette describes a patient from a tropical region with fatigue, pallor, and a microcytic anemia, many students jump to nutritional iron deficiency or GI bleeding and overlook the peripheral eosinophil count buried in the lab panel. The presence of eosinophilia transforms a routine iron deficiency case into a parasitic infection question. Always scan the CBC differential for eosinophils when anemia is the presenting problem.

A second trap involves confusing hookworm with Strongyloides. Both penetrate skin, both migrate through the lungs, and both cause eosinophilia. The critical discriminator is what you find in the stool: hookworm produces eggs, while Strongyloides produces larvae. Additionally, Strongyloides carries the unique risk of hyperinfection in immunocompromised hosts (particularly those receiving corticosteroids), a feature hookworm does not share.

Students also sometimes confuse the skin findings. "Ground itch" (a localized papulovesicular rash at the entry point, usually the foot) is hookworm. A serpiginous, creeping, migratory track in the skin is cutaneous larva migrans caused by animal hookworms that cannot complete the human lifecycle. The distinction matters because cutaneous larva migrans does not cause systemic disease or anemia.

Finally, remember the treatment logic: the exam commonly tests whether you know that anthelmintic therapy alone is insufficient. The "next best step" after deworming is iron supplementation, because the depleted stores will not replenish without exogenous iron. A question may present a treated patient whose hemoglobin fails to improve and ask you to identify the missing intervention.

The core competency being tested across hookworm questions is the ability to recognize a parasitic cause of iron deficiency anemia based on epidemiologic clues and eosinophilia, correctly identify the organism through stool findings, and manage both the infection and its hematologic consequence.