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Travel-related Infections Quiz
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Which investigation should be arranged urgently to look directly for malaria parasites and assess their species and burden in a patient with a compatible illness and travel history?
- A Serum antibody testing for Plasmodium species You picked ✕
- B Urine antigen testing for Plasmodium lactate dehydrogenase You picked ✕
- C Microscopy of thick and thin peripheral blood films You picked ✓
- D Blood culture for protozoal organisms You picked ✕
- E Examination of a stained bone marrow aspirate for malarial pigment You picked ✕
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The answer is C: Microscopy of thick and thin peripheral blood films
Explanation
Why C is correct
See Malaria. Option C is correct. Malaria parasites live inside red blood cells, so the most direct test is to look at the blood under a microscope. A thick film is a drop of blood spread so that many cells are packed together, and it is the more sensitive test for finding parasites. A thin film is a single layer of cells that allows the parasites to be identified by their shape, so the laboratory can name the species and calculate the parasitaemia (the percentage of red cells that are infected). Knowing the species and the parasite density guides treatment, because Plasmodium falciparum causes the most severe disease. A single negative film does not exclude malaria, so films are repeated at 12 to 24 hour intervals until three are negative. Rapid diagnostic tests, which detect parasite antigens in blood, can be used alongside microscopy. Anyone with fever who has returned from an area where malaria occurs needs urgent testing.
- Best test: a blood film under the microscope, because the parasites live inside red cells.
- Thick film: the more sensitive test for finding parasites.
- Thin film: shows the parasite shape, so the species and the parasitaemia can be worked out.
- Repeat: a single negative film does not exclude malaria, so films are repeated until three are negative.
Why the other options are incorrect
- A. Serum antibody testing for Plasmodium species: Antibody tests show that someone has been exposed to the parasite at some point, which may reflect an old infection. They are not useful for diagnosing acute malaria and they cannot show current parasite burden. Your answer
- B. Urine antigen testing for Plasmodium lactate dehydrogenase: A urine antigen test for Plasmodium lactate dehydrogenase is not a validated way to diagnose malaria or measure parasite burden, so it would not provide the required information. Your answer
- D. Blood culture for protozoal organisms: Plasmodium parasites live inside red blood cells and are not grown in routine blood culture, which is designed for bacteria and some fungi. It would miss malaria and delay the right test. Your answer
- E. Examination of a stained bone marrow aspirate for malarial pigment: Bone marrow examination can show malaria pigment but it is invasive, slow and not used for urgent species identification or parasitaemia, so it is not the correct first step. Your answer
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1
How is typhoid fever most commonly transmitted?
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2
A 22-year-old woman returns from Southeast Asia with sudden fever, severe headache, muscle aches and a widespread red rash after mosquito bites. Dengue is diagnosed. Which additional finding would most strongly suggest progression to dengue haemorrhagic fever and the need for urgent reassessment?
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