General Characteristics
P. knowlesi invades all ages of RBCs, and the number of infected cells can be significantly more than seen in P. vivax, P. ovale, and P. malariae. P. knowlesi infection should be considered in patients with a travel history to forested areas of Southeast Asia, especially if P. malariae is diagnosed, unusual forms are seen with microscopy, or if a mixed infection with P. falciparum/P. malariae is diagnosed. Because the disease is potentially fatal, proper identification to the species level is critical.
The early blood stages of P. knowlesi resemble those of P. falciparum, whereas the mature blood stages and gametocytes resemble those of P. malariae (see Tables 1 to 3, Figures 1 to 3). Unfortunately, these infections are often misdiagnosed as the relatively benign P. malariae; however, infections with P. knowlesi can be fatal. The RBCs are all sizes, there is no true stippling (fine, granular, blue stippling in RBCs stained with Wright’s stain or red when using eosin hematoxylin as seen in Figure 2 (P. vivax photo, third from the top), often there are multiple rings per RBC (there may be 2 to 3 rings), the rings are delicate and often have 2 to 3 dots of chromatin, band forms are typically seen with the developing trophozoites, and the mature schizont contains 16 merozoites. The early stages mimic P. falciparum, whereas the later stages mimic P. malariae.
Table1. Plasmodium spp.: Clinical Characteristics of the Five Human Infections
Table2. Plasmodia in Giemsa-Stained Thin Blood Smears
Table2. Plasmodia in Giemsa-Stained Thin Blood Smears—cont’d
Table3. Malaria Characteristics with Fresh Blood or Blood Collected Using EDTA with No Extended Lag Time *
Fig1. The morphology of malaria parasites. Plasmodium vivax: 1, Early trophozoite (ring form). 2, Late trophozoite with Schüffner’s dots (note enlarged red blood cell). 3, Late trophozoite with ameboid cytoplasm (very typical of P. vivax). 4, Late trophozoite with ameboid cytoplasm. 5, Mature schizont with merozoites (18) and clumped pigment. 6, Microgametocyte with dispersed chromatin. 7, Macrogametocyte with compact chromatin. Plasmodium malariae: 1, Early trophozoite (ring form). 2, Early trophozoite with thick cytoplasm. 3, Early trophozoite (band form). 4, Late trophozoite (band form) with heavy pigment. 5, Mature schizont with merozoites (9) arranged in rosette. 6, Microgametocyte with dispersed chromatin. 7, Macrogametocyte with compact chromatin. Plasmodium ovale: 1, Early trophozoite (ring form) with Schüffner’s dots. 2, Early trophozoite (note enlarged red blood cell). 3, Late trophozoite in red blood cell with fimbriated edges. 4, Developing schizont with irregularly shaped red blood cell. 5, Mature schizont with merozoites (8) arranged irregularly. 6, Microgametocyte with dispersed chromatin. 7, Macrogametocyte with compact chromatin. Plasmodium falciparum: 1, Early trophozoite (accolé or appliqué form). 2, Early trophozoite (one ring is in headphone configuration/double chromatin dots). 3, Early trophozoite with Maurer’s dots. 4, Late trophozoite with larger ring and Maurer’s dots. 5, Mature schizont with merozoites (24). 6, Microgametocyte with dispersed chromatin. 7, Macro gametocyte with compact chromatin. Note: Without the appliqué form, Schüffner’s dots, multiple rings/cell, and other developing stages, differentiation among the species can be difficult. It is obvious that the early rings of all four species can mimic one another very easily. Remember: One set of negative blood films cannot rule out a malarial infection. (Reprinted by permission of the publisher from Garcia LS: Diagnostic medical parasitology, ed 5, Washington, DC, 2007, Copyright by American Society for Microbiology.)
Fig2. Morphology of malaria parasites. Column 1 (left to right): Plasmodium vivax (note enlarged infected RBCs). (1) Early trophozoite (ring form) (note one RBC contains 2 rings—not that uncommon); (2) older ring, note ameboid nature of rings; (3) late trophozoite with Schüffner’s dots (note enlarged RBC); (4) developing schizont; (5) mature schizont with 18 merozoites and clumped pigment; (6) microgametocyte with dispersed chromatin. Column 2: Plasmodium ovale (note enlarged infected RBCs). (1) Early trophozoite (ring form) with Schüffner’s dots (RBC has fimbriated edges); (2) early trophozoite (note enlarged RBC, Schüffner’s dots, and RBC oval in shape); (3) late trophozoite in RBC with fimbriated edges; (4) developing schizont with irregular-shaped RBC; (5) mature schizont with 8 merozoites arranged irregularly; (6) microgametocyte with dispersed chromatin. Column 3: Plasmodium malariae (note normal or smaller than normal infected RBCs). (1) Early trophozoite (ring form); (2) early trophozoite with thick cytoplasm; (3) late trophozoite (band form); (4) developing schizont; (5) mature schizont with 9 merozoites arranged in a rosette; (6) microgametocyte with compact chromatin. Column 4: Plasmodium falciparum. (1) Early trophozoites (the rings are in the headphone configuration with double chromatin dots); (2) early trophozoite (accolé or appliqué form); (3) early trophozoites (note the multiple rings/cell); (4) late trophozoite with larger ring (accolé or appliqué form); (5) crescent shaped gametocyte; (6) crescent-shaped gametocyte. Column 5: Plasmodium knowlesi—with the exception of image 5, these were pho tographed at a higher magnification (note normal or smaller than normal infected RBCs). (1) Early trophozoite (ring form); (2) early trophozoite with slim band form; (3) late trophozoite (band form); (4) developing schizont; (5) mature schizont with merozoites arranged in a rosette; (6) microgametocyte with dispersed chromatin. Note: Without the appliqué form, Schüffner’s dots, multiple rings per cell, and other devel oping stages, differentiation among the species can be very difficult. It is obvious that the early rings of all five species can mimic one another very easily. Remember: One set of negative blood films cannot rule out a malaria infection. (From Garcia LS: Malaria Clin Lab Med 30:93 129, 2010,with permission. Column 5 courtesy CDC.)
Fig3. Plasmodium falciparum. A, Ring forms; B, oocyte; and C, sporozoites. (Courtesy Dr. Henry Travers, Sioux Falls, S.D.)
Because of different levels of parasitemia, low organ ism densities, and confusion among various morphologic criteria for identification, detection of mixed infections can be quite difficult. Even if a mixed infection is suspected, identification to the species level may not be possible using routine microscopy methods. However, using polymerase chain reaction (PCR) methods, it is likely that higher detection and identification rates of chronic and mixed malarial infections will be possible.
Pathogenesis and Spectrum of Disease
Patients exhibit chills, minor headaches, and daily low grade fever. Patients who have been diagnosed with high numbers of P. malariae organisms by microscopy should receive intensive management as appropriate for severe P. falciparum malaria, assuming the infection is actually caused by P. knowlesi. Overall, these infections can be as severe as those caused by P. falciparum, with fatal outcomes.
