Question
A 41-year-old man attends hospital with recurrent attacks of high fever, sweating and rigors occurring roughly every 48 hours. He returned to the UK 3 weeks ago after spending several months in the Indian subcontinent. During his trip he was diagnosed with malaria and received chloroquine, after which his symptoms initially resolved.
Examination now shows a temperature of 38.8°C and splenomegaly. A peripheral blood film again demonstrates malaria parasites.
Which organism best explains this recurrence?
A. Babesia microti
B. Trypanosoma cruzi
C. Plasmodium knowlesi
D. Plasmodium malariae
E. Plasmodium vivax
Answer
E. Plasmodium vivax
Explanation
The key clue is recurrent malaria after apparently successful treatment.
P. vivax and P. ovale can form dormant liver stages called hypnozoites. These can remain in hepatocytes after the circulating blood-stage parasites have been cleared and later reactivate, producing a relapse.
Chloroquine primarily clears the erythrocytic blood-stage parasites and therefore does not eradicate hypnozoites. Radical cure of P. vivax requires treatment directed against these dormant hepatic forms, classically primaquine, after checking G6PD status because primaquine can cause severe haemolysis in G6PD deficiency.
The geographical clue also favors P. vivax, which is an important cause of malaria in the Indian subcontinent. P. ovale can also relapse but is classically more associated with sub-Saharan Africa.
The fever pattern helps:
- P. vivax / P. ovale → 48-hour cycle (“tertian” malaria)
- P. malariae → 72-hour cycle (“quartan” malaria)
P. malariae does not have a hypnozoite stage and therefore does not produce true relapse from dormant hepatic parasites. It is also classically associated with nephrotic syndrome.
High-yield distinction
Relapse = P. vivax or P. ovale → hypnozoites in the liver.
This differs from recrudescence, where residual blood-stage parasites persist because treatment has not completely eradicated them.
Summary for quick exam revision
Plasmodium vivax is a major cause of non-falciparum malaria, particularly in the Indian subcontinent. It typically produces fever attacks approximately every 48 hours. Plasmodium ovale can produce a similar 48-hour fever pattern but is classically more common in Africa. Both P. vivax and P. ovale form dormant hepatic parasites called hypnozoites. Hypnozoites can reactivate weeks, months or occasionally much later after the initial infection. This produces relapsing malaria even after the initial blood-stage infection has been successfully treated. Chloroquine acts predominantly against blood-stage parasites and does not reliably eliminate hypnozoites. Therefore, recurrence following chloroquine treatment strongly suggests P. vivax or P. ovale. Travel to India makes P. vivax the more likely exam answer. Radical cure requires eradication of the liver hypnozoites, classically using primaquine. G6PD status must be checked before primaquine because of the risk of haemolysis. P. malariae produces fever approximately every 72 hours and does not form hypnozoites. P. malariae is also classically associated with nephrotic syndrome. P. falciparum and P. knowlesi do not cause relapse through dormant hepatic hypnozoites. The key MRCP association is therefore 48-hour fever + previous malaria treatment + recurrence + Indian subcontinent = Plasmodium vivax.