A traveler who recently returned from Africa experiences recurring fevers; a peripheral smear at 1000x magnification shows a malaria parasite. Which Plasmodium species is most likely?

• A. Plasmodium knowlesi
• B. Plasmodium vivax
• C. Plasmodium malariae
• D. Plasmodium ovale

Answer: (B)

Relapsing malaria is driven by dormant liver forms called hypnozoites that can reactivate after the initial infection, causing recurrent febrile episodes. Plasmodium vivax and Plasmodium ovale are the species known to form these hypnozoites, which explains why fevers can recur in a traveler who recently returned from Africa. Among the two, vivax is the more commonly encountered cause of relapsing malaria in many settings, making it the best match for a traveler with recurring fevers and a blood smear demonstrating malaria parasites. In contrast, malariae tends to cause a quartan fever pattern and does not form hypnozoites, falciparum typically presents with severe disease and does not classically relapse in the same way, and knowlesi is a Southeast Asian zoonosis. Thus, the recurring fever pattern points toward vivax.