Hookworm — Ancylostoma duodenale vs Necator americanus, Life Cycle, Anaemia and Deworming

Hookworms are small blood-feeding nematodes (roundworms) transmitted through contaminated soil. Two species cause most human infection: *Ancylostoma duodenale* and *Necator americanus*. StatPearls estimates that N. americanus causes about 79% of hookworm infections worldwide, while A. duodenale remains endemic around the Mediterranean, the Middle East, northern India and northern China.

The disease is one of the major soil-transmitted helminthiases — a neglected tropical disease of poverty. Risk factors are walking barefoot, poor sanitation and open defecation, a warm moist climate and poor hygiene; children and pregnant women carry the highest risk because their iron needs are high.

1. Eggs pass in the stool onto warm, moist, shaded soil.
2. Rhabditiform (L1) larvae hatch in 1–2 days and feed in the soil.
3. After 5–10 days and two moults they become filariform (L3) larvae — the infective stage, about 0.5–0.6 mm long, surviving 3–4 weeks in favourable conditions.
4. Filariform larvae penetrate the skin (usually bare feet or hands) and are carried in the blood to the right heart and lungs.
5. They break into the alveoli, ascend the bronchial tree to the pharynx and are swallowed.
6. In the small intestine they moult twice and mature into adults within 4–6 weeks; they attach to the mucosa and feed on blood.
7. After mating, a female lays up to about 30,000 eggs a day. Most adults are cleared within 1–2 years, though some live for several years.

Adult worms grip the mucosa with their buccal capsule and keep the wound bleeding using metalloproteases and anticoagulant peptides. Most of the blood loss is leakage around the attachment site, not blood the worm swallows. Experimental studies with labelled red cells estimate daily loss at 0.03–0.30 mL per worm, with A. duodenale losing 2–10 times more per worm than N. americanus. In heavy infections, total loss can reach about 9 mL a day.

• Anaemia appears when daily iron loss exceeds intake — so worm burden and diet decide severity. Children can become anaemic at lower worm loads.
• Blood film: microcytic hypochromic red cells, low ferritin; eosinophilia is common but non-specific.
• Protein loss causes hypoalbuminaemia, oedema and even anasarca, worsening malnutrition.
• Severe anaemia can cause pica (eating soil), which perpetuates reinfection.
• In pregnancy, schoolchildren and adults the consequences are fatigue, poor growth, lower school performance and reduced work capacity — morbidity more than mortality.

For the red-cell picture, compare with megaloblastic anaemia — a heavy, long-standing fish-tapeworm (Dibothriocephalus, formerly Diphyllobothrium) infection can cause B12-deficiency megaloblastic anaemia, while hookworm gives microcytosis.

Cutaneous larva migrans ('creeping eruption') is caused by animal hookworms, mainly *Ancylostoma braziliense* and A. caninum (dog and cat hookworms). Their larvae cannot penetrate beyond the stratum corneum in humans, so they wander in the epidermis, producing itchy serpiginous tracks 1–5 cm long, usually on the feet or hands. It is self-limiting but responds to albendazole or ivermectin.

• Stool microscopy for eggs — direct wet mount or formol-ether concentration (sedimentation).
• Kato-Katz thick smear — counts eggs per gram of stool, an indirect measure of worm burden used in surveys and deworming programmes; it underestimates light infections.
• PCR (including duplex real-time PCR) separates N. americanus from A. duodenale and is highly sensitive.
• Blood: microcytic hypochromic anaemia, low ferritin, eosinophilia (detectable during larval migration, before eggs appear).
• Capsule endoscopy can show worms when stool tests are negative in obscure GI bleeding.

• Treat the anaemia too: iron supplementation alongside deworming improves haemoglobin more than either alone.
• Transfusion for severe anaemia (Hb < 7 g/dL) or haemodynamic instability.
• Pregnancy: WHO recommends preventive single-dose albendazole 400 mg or mebendazole 500 mg after the first trimester where soil-transmitted helminth prevalence exceeds 20%.
• Follow-up: reinfection is common (about a quarter by 4 months in endemic settings), so repeat stool tests and haemoglobin checks are sensible.
• Prevention: footwear, latrines and an end to open defecation, hand hygiene, and periodic mass deworming. There is no licensed vaccine yet; candidates such as Na-GST-1 and Na-APR-1 are in trials.

StatPearls estimates that about 406–480 million people carry hookworms (the Global Burden of Disease 2021 study reports 642.72 million cases), costing roughly 1.38–4 million disability-adjusted life years a year. The disease mainly causes illness rather than death, but anaemia lowers work capacity in adults and harms growth and learning in schoolchildren.

Drug resistance is a growing worry after repeated mass drug administration, although definitive proof of benzimidazole resistance in human hookworms is still limited. Reduced cure rates can also reflect meal timing, infection intensity and species, so surveillance and routine efficacy testing are recommended.

India launched the fixed-day, school- and Anganwadi-based National Deworming Day (NDD) in 2015 to deworm all children aged 1–19 years — enrolled and non-enrolled. It is held in two rounds a year: 10 February and 10 August, with a mop-up day for children who missed the main day. NHM notes that about 22 crore Indian children aged 1–14 are at risk of worm infection.

Deworming sits under 'specific protection' in the levels of prevention; sanitation and safe water (purification of water) are the long-term fix.