MBBS viva · Infectious Diseases / Tropical Medicine / Parasitology
Helminth infection — stool microscopy, life cycles, and anthelmintic choice viva
A final-prof viva on interpreting a stool OCP photomicrograph and a returned-traveller eosinophilia panel, framing the differential, naming the worm by egg morphology, and justifying the species-specific anthelmintic. Examiner expects life-cycle reasoning and dose-level detail, not labels.
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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]
- 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.
- Life cycle — skin penetration by filariform larvae, venous migration to the lungs, alveolar rupture, ascent, swallowing, and maturation to adults in the duodenum.
- Treatment — single-dose albendazole 400 mg is the dose used in mass drug administration; add iron for hookworm anaemia.[1]
- 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).
- The dangerous non-STH — Strongyloides stercoralis (skin + auto-infection; treat with ivermectin, screen before immunosuppression; hyperinfection has more than 60 percent case fatality).[9]
- 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]
- 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]
- 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
- Jourdan PM, et al. Soil-transmitted helminth infections. Lancet 2018.[1]
- Buonfrate D, et al. Human strongyloidiasis: complexities and pathways forward. Clin Microbiol Rev 2023.[9]
- Garcia HH, et al. Neurocysticercosis. Lancet Neurol 2014.[4]
- Colley DG, et al. Human schistosomiasis. Lancet 2014.[3]
References4ShowHide
- [1]Jourdan PM, Lamberton PHL, Fenwick A, et al. Soil-transmitted helminth infections. Lancet, 2018.PMID 28882382
- [9]Buonfrate D, Bradbury RS, Watts MR, et al. Human strongyloidiasis: complexities and pathways forward. Clinical Microbiology Reviews, 2023.PMID 37937980
- [4]Garcia HH, Nash TE, Del Brutto OH. Clinical symptoms, diagnosis, and treatment of neurocysticercosis. Lancet Neurology, 2014.PMID 25453460
- [3]Colley DG, Bustinduy AL, Secor WE, et al. Human schistosomiasis. Lancet, 2014.PMID 24698483