Parasitology and Medical Entomology
Domain focus
Parasitology and medical entomology questions test malaria diagnosis and parasite counting, stool examination for helminths and protozoa, and vector identification relevant to dengue, malaria and lymphatic filariasis in Cambodia.
Core rules
- Use thick films for detection and thin films for species identification; examine at least 100 fields before reporting negative.
- Calculate malaria parasitaemia against white cell count; report species and stage.
- Stool examination should include direct wet mount and a concentration method; examine at least two samples for suspected helminths.
- Identify vectors by resting posture, wing pattern and breeding site: Anopheles for malaria, Aedes aegypti for dengue, Culex for Japanese encephalitis.
- RDTs detect antigen, not live parasites; a positive result can persist after treatment.
Worked example
A microscopist counts 200 asexual Plasmodium falciparum parasites per 200 white cells on a thick film; the patient's WBC is 8,000/microlitre. Parasitaemia = (200 / 200) x 8,000 = 8,000 parasites/microlitre. This is a high parasitaemia requiring urgent clinical review and, in Cambodia, artesunate-based therapy according to national guidelines.
Traps
- Reporting a negative malaria film after examining only a few fields.
- Confusing platelets or artefact with parasites.
- Using a thin film alone for a low-density infection.
- Identifying Aedes as Anopheles and giving wrong vector advice.