September 2020 — Clinical Microbiology, Hematology, Hematopathology, & Infectious Disease

The correct answer is Plasmodium vivax.

Plasmodium vivax is the most widely distributed of all of the species causing human malaria, being found in areas of the tropics and subtropics worldwide, as well as some temperate climates. The regions with highest prevalence are Latin America and Southeast Asia. While majority (60-70%) of malaria cases in Ethiopia are contributed to Plasmodium falciparum, the remaining 30-40% of infections is caused by P. vivax, and infections due to Plasmodium ovale and Plasmodium malariae are rare (1). Plasmodium knowlesi is endemic to Southeast Asia and does not fit this case due to travel history. P. vivax has a low degree of infectivity compared to P. falciparum, which can rapidly progress to life-threatening disease. While all malaria species have an initial liver stage, only P. vivax and P. ovale have a dormant liver stage, where hypnozoites can persist in the liver and cause relapses by invasion into the blood stream weeks to months after initial infection is cleared (2).

The various Plasmodium species can be differentiated based on their appearance on Giemsa and Wright-Giemsa stained thin blood films. P. vivax and P. ovale preferentially infect immature red blood cells (RBCs), and thus the infected cells are larger than many of the surrounding uninfected cells. Intracytoplasmic inclusions (i.e., Schüffner’s stippling or ‘dots’) are also characteristic when the stain is at a neutral pH (7.0 – 7.2). In comparison, the size of the infected RBCs is usually normal or small in P. falciparum and P. malariae infections, and Schüffner’s dots are not seen. Differentiating P. vivax from P. ovale can be more challenging. In general, P. vivax late stage trophozoites have an ameboid shape (i.e., having pseudopod-like projections) whereas P. ovale late stage trophozoites are more compact. Additionally, P. vivax infected RBCs exhibit increased flexibility, causing them to mold to the countours of the neighboring cells. This is not seen in P. ovale infected red blood cells, which instead take on an overall ovoid shape (in approximately 1/3 of infected RBCs) and may have jagged (i.e., fimbriated) edges. Characteristic features of P. vivax can be seen in both of the images in this case. In Image 1, a late-stage trophozoite (arrow) has an ameboid shape, and both infected RBCs are partially molded to neighboring cells. In the second image, stippling can be seen in the gametocyte, and the parasite fills nearly the entire RBC. The infected cell is also partially molded to the neighboring RBC.

References
1. https://wwwnc.cdc.gov/travel/yellowbook/2020/preparing-international-travelers/yellow-fever-vaccine-and-malaria-prophylaxis-information-by-country/ethiopia#5407
2. https://www.cdc.gov/dpdx/malaria/index.html