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Case Study 1, Question 1: Correct Answer

Plasmodium vivax

This is the most probable diagnosis. The reported microscopic findings are compatible with P. vivax: some infected red cells are enlarged and deformed, and the "other forms with four nuclei" are compatible with the presence of schizonts. Plasmodium vivax does occur in Pakistan, where it is found in slightly more than 50% of malaria cases.

The history suggests a relapse of P. vivax malaria, following an earlier episode five weeks ago. The earlier treatment apparently did not include primaquine, thus allowing the persistence of hypnozoites which caused this relapse.

An alternate explanation would be that the earlier infection was caused by chloroquine-resistant P. vivax (which has been reported in Pakistan), with recrudescence of blood-stage parasites occurring after an unsuccessful earlier treatment (if indeed the earlier treatment included chloroquine). However, recrudescences usually occur within 28 days of the intial episode, rather than at five weeks as described here.

The other species are less likely:

• While P. falciparum does occur in Pakistan (slightly less than 50% of malaria cases), this patient reportedly did not develop symptoms until 10 months after departure from the exposure area: most cases of P. falciparum would have become symptomatic earlier
• P. ovale occurs mainly in Africa and has been found only occasionally in Asia (in the western Pacific)
• P. malariae occurs worldwide, but its distribution is spotty, and its frequency in Pakistan is low to negligible. The diagnosis of P. malariae made during the earlier episode is not consistent with the current microscopic findings: red cells infected with P. malariae would not be enlarged and deformed, as described here.
• Babesia would not fit with the microscopic description; in addition, babesiosis has not been reported in Pakistan, although admittedly the disease might have escaped detection