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Ancylostoma (Hookworm)

Taxonomy and Species

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Epidemiology

• Over 700 million people are infected with hookworms globally; approximately 460 million cases by some estimates
• Prevalence is highest in sub-Saharan Africa and Asia
• Endemic in tropical/subtropical zones between 45°N and 30°S of the equator - the larval stage cannot withstand freezing temperatures
• A. duodenale predominates in northern Africa, the Middle East, southern Europe, and parts of Asia ("Old World hookworm")
• N. americanus has a more widespread distribution in the Americas, tropical Africa, and Southeast Asia ("New World hookworm")
• Approximately 44 million pregnant women are infected worldwide
• Risk factors: poor sanitation, barefoot walking, use of human excrement as fertilizer
• Yamada's Textbook of Gastroenterology, p. 3117; Park's Textbook of Preventive and Social Medicine, p. 284

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Life Cycle

1. Eggs in feces (diagnostic stage) - Adult female worms in the small intestine produce thousands of eggs daily: A. duodenale ~10,000-30,000 eggs/day; N. americanus ~5,000-10,000 eggs/day. Eggs pass with stool into soil.
2. Rhabditiform larva hatches - In warm, moist soil, eggs embryonate and hatch within 1-2 days, releasing non-infective rhabditiform larvae.
3. Filariform larva - The rhabditiform larva molts twice over 5-10 days in soil to become the infective third-stage filariform larva. These can survive in shaded, moist soil for up to 1 month.
4. Skin penetration (infective stage) - Filariform larvae penetrate exposed skin (usually the feet), enter the venous circulation, and travel to the pulmonary vasculature. (A. duodenale can also be transmitted orally.)
5. Lung migration - Larvae penetrate alveoli, ascend the bronchial tree to the trachea, are coughed up and swallowed.
6. Adult worms in small intestine - Larvae mature in the small intestine over 5-9 weeks. Adults inhabit the proximal small intestine (mainly the jejunum), attaching to intestinal villi. A. duodenale survives ~1 year; N. americanus survives 3-5 years.

• Goldman-Cecil Medicine, p. 3477

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Morphology and Pathobiology

• Adult worms measure 7-13 mm in length; males slightly smaller than females
• Anterior end is dorsally curved (hence "hookworm")
• Attachment mechanism: A. duodenale has cutting teeth; N. americanus has a rounded cutting plate
• Each worm aspirates a plug of intestinal villus tissue, secretes anticoagulant and anti-platelet enzymes, and ingests blood - moving to fresh villi several times per minute
• Blood loss per worm per day: N. americanus = 0.03-0.1 mL; A. duodenale = 0.15-0.26 mL (A. duodenale causes significantly more blood loss)
• Hemoglobinases within the worm's digestive canal degrade host hemoglobin as a nutrient source
• The major pathology is gastrointestinal blood loss → iron deficiency anemia
• Goldman-Cecil Medicine, p. 3477; Yamada's Textbook of Gastroenterology, p. 3117

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Clinical Manifestations

1. Ground Itch (Skin entry)

• Pruritic, erythematous, papular rash - typically on hands/feet - representing a hypersensitivity reaction at the site of larval skin penetration

2. Cutaneous Larva Migrans (CLM)

• Caused by zoonotic hookworms (A. caninum, A. braziliense) that penetrate human skin but cannot complete the life cycle
• Intensely pruritic, erythematous, serpiginous (snake-like) tracks on feet, legs, buttocks
• Tracks advance a few mm to cm per day; vesiculobullous lesions may form; self-resolves within weeks to months

3. Pulmonary Phase (Larval migration)

• Dry cough, sore throat, wheezing, low-grade fever
• Wakana syndrome (oral ingestion of A. duodenale larvae) - pharyngeal irritation, cough, dyspnea, hoarseness, nausea, vomiting

4. Intestinal Phase - Hookworm Disease

• Heavy infections (≥2000 eggs/gram of feces) cause iron deficiency anemia: weakness, palpitation, dizziness, dyspnea, fatigue, headache
• Hypoproteinemia and anasarca with very heavy worm burdens (protein loss)
• Gastrointestinal symptoms per se are uncommon; the main disease is anemia
• Endoscopy shows punctate mucosal erosions and pooling of blood in the small bowel lumen
• Children are most vulnerable to chronic sequelae: stunted growth, cognitive impairment, malnutrition
• Goldman-Cecil Medicine, p. 3477-3478

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Diagnosis

• Stool microscopy - identification of characteristic thin-shelled, oval eggs containing segmented larvae (4-8 cells at the time of passage); this is the gold standard
• Quantitative stool examination (e.g., Kato-Katz technique) to estimate worm burden
• PCR - highly sensitive and specific; currently available only in specialized/research laboratories
• Peripheral eosinophilia is typically present
• CBC may reveal hypochromic microcytic anemia
• Serum albumin may be low in heavy infections
• No need to differentiate between hookworm species clinically
• Yamada's Textbook of Gastroenterology, p. 3118; Goldman-Cecil Medicine, p. 3478

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Treatment

• Iron supplementation (with folic acid) is essential for anemia treatment
• Treat hypoproteinemia and hypereosinophilia as needed
• Goldman-Cecil Medicine, p. 3479 (Table 327-1); Park's Textbook, p. 287

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Prevention and Control

• Proper sanitary disposal of human excreta
• Provision of safe water and food hygiene
• Wearing protective footwear (the single most effective individual measure)
• Health education on use of sanitary latrines

• Periodic deworming of at-risk groups without individual diagnosis in endemic areas:
  • Preschool children, school-aged children
  • Women of childbearing age (including pregnant women in 2nd/3rd trimester)
  • Adults in high-risk occupations (tea-pickers, miners)
• Once yearly if community prevalence >20%; twice yearly if >50%
• Drugs used: albendazole (400 mg single dose) or mebendazole (500 mg single dose)
• Park's Textbook of Preventive and Social Medicine, p. 287

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Key Comparison: A. duodenale vs N. americanus

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