Helminth infection — stool microscopy, life cycles, and anthelmintic choice viva

Interpretation

The examiner presents a stool OCP photomicrograph showing thin-shelled hookworm eggs on a direct faecal film, alongside a peripheral eosinophil count of 1.3 x10^9/L and haemoglobin of 7.5 g/dL (microcytic, hypochromic), and asks: "What do you see, what is the worm, how did the patient acquire it, what is the clinical picture, and how would you treat?"^[1]

• Egg morphology: characteristic hookworm (Necator americanus or Ancylostoma duodenale) egg.
• Acquisition: filariform larvae penetrate the bare skin (usually the feet) of a person walking on faecally contaminated soil in a warm, moist tropical region.
• Clinical picture: the larval skin penetration produces transient 'ground itch'; the hepatopulmonary migration may produce a mild Loeffler pneumonitis and eosinophilia; the adults inhabit the small intestine and consume host blood — producing iron-deficiency anaemia and hypoproteinaemia (the dominant clinical problem), and in children, growth and cognitive impairment.
• Laboratory signature: peripheral eosinophilia (tissue migration) and iron-deficiency anaemia.^[1]

Key points

The examiner will probe each of these; be ready to defend them at viva depth:^[1]

1. Definition and classification — hookworm is one of the three soil-transmitted helminths (STH) alongside Ascaris lumbricoides and Trichuris trichiura. Necator americanus dominates the Americas, sub-Saharan Africa, and Southeast Asia; Ancylostoma duodenale dominates the Mediterranean, Middle East, and northern India.
2. Life cycle — skin penetration by filariform larvae, venous migration to the lungs, alveolar rupture, ascent, swallowing, and maturation to adults in the duodenum.
3. Treatment — single-dose albendazole 400 mg is the dose used in mass drug administration; add iron for hookworm anaemia.^[1]
4. The other STH and their drugs — Ascaris (albendazole 400 mg is the MDA dose; obstruction is usually managed conservatively, with surgery if peritonitis or perforation); Trichuris (mebendazole is the drug of choice; multiple doses are needed for complete parasitological cure).
5. The dangerous non-STH — Strongyloides stercoralis (skin + auto-infection; treat with ivermectin, screen before immunosuppression; hyperinfection has more than 60 percent case fatality).^[9]
6. The tissue helminths to distinguish — Schistosoma (S. haematobium: terminal-spined eggs in urine, bladder cancer; S. mansoni: lateral-spined eggs in stool, periportal fibrosis; praziquantel 40 mg/kg (60 mg/kg is also listed for S. japonicum)); Taenia solium (eggs cause neurocysticercosis; seizures, 'hole-with-dot' scolex; albendazole + steroids + antiepileptics).^[3][4]
7. The cardinal reflex — screen with Strongyloides serology and treat with ivermectin BEFORE any immunosuppression (corticosteroids, transplant, biologicals, chemotherapy) in any patient from an endemic region; the single most preventable fatal error in tropical medicine.^[9]
8. Prevention — sanitation, safe water, footwear, thorough cooking of meat and fish, and annual or twice-annual mass drug administration (albendazole 400 mg) targeting children and women of reproductive age (the WHO preventive chemotherapy strategy).^[1]

References

1. Jourdan PM, et al. Soil-transmitted helminth infections. Lancet 2018.^[1]
2. Buonfrate D, et al. Human strongyloidiasis: complexities and pathways forward. Clin Microbiol Rev 2023.^[9]
3. Garcia HH, et al. Neurocysticercosis. Lancet Neurol 2014.^[4]
4. Colley DG, et al. Human schistosomiasis. Lancet 2014.^[3]

References4ShowHide

1. [1]Jourdan PM, Lamberton PHL, Fenwick A, et al. Soil-transmitted helminth infections. Lancet, 2018.PMID 28882382
2. [9]Buonfrate D, Bradbury RS, Watts MR, et al. Human strongyloidiasis: complexities and pathways forward. Clinical Microbiology Reviews, 2023.PMID 37937980
3. [4]Garcia HH, Nash TE, Del Brutto OH. Clinical symptoms, diagnosis, and treatment of neurocysticercosis. Lancet Neurology, 2014.PMID 25453460
4. [3]Colley DG, Bustinduy AL, Secor WE, et al. Human schistosomiasis. Lancet, 2014.PMID 24698483