Diagnosis of Plasmodium knowlesi infection. (A) Light microscopic images of parasites with characteristics of P. knowlesi from Giemsa-stained thin blood smears. (B) Agarose gel images of nested polymerase chain reaction (PCR) demonstrating P. knowlesi infection using species-specific primers with the expected PCR product size of 110 base pairs. f, v, m, o, and k denote the species of parasites targeted by each PCR, from Plasmodium falciparum, Plasmodium vivax, Plasmodium malariae, Plasmodium ovale, and P. knowlesi. Numbers on the left indicate size in base pairs. (C) Sequences of the nested PCR products. The sequences of all six cases are identical to those of the reference P. knowlesi 18S rRNA gene PKNH_0320900 and differ from all known 18S rRNA genes of other human malaria parasites. This figure appears in color at www.ajtmh.org.