Answer to Case 158

Answer: Plasmodium falciparum

Anonymous said it well:
"Rings galore, cells with multiple infections, applique forms, double chromatin dots - classic P. falciparum.

Also important in the diagnosis is the fact that the infected red blood cells (RBCs) are not enlarged in comparison to the neighboring uninfected cells and that no stippling is present. I've highlighted the important diagnostic features of P. falcipaum in the image below:



To answer LR's question on how to approach Plasmodium speciation:

The size of the infected cells and presence/absence of stippling are the first 2 things I tend to use to formulate my diagnosis. Enlarged RBCs (+/- stippling) supports the diagnosis of P. ovale/P. vivax. Normal sized or small RBCs with no stippling (possible clefts/dots) supports the diagnosis of P. falciparum/P. malariae/P. knowlesi. I then look at other features, such as size/shape of trophozoites, classic forms (e.g. "band" form of P. malariae or "head phone" form of P. falciparum), number of merozoites in schizonts, and presence of later stage forms (typically not seen in P. falciparum infection). The "classic forms" are characteristic but not definitive for the various species, and storage/transport conditions and anti-malarial treatment may cause variations in morphology; therefore, the diagnosis must be made taking the entire morphologic picture into consideration. Rapid antigen tests and PCR may be helpful with difficult cases.

Thanks to everyone who wrote in with comments on this case!