Table 1.
Morphological and clinical characters of P. knowlesi and the other four human malaria parasite species
| Characters | P. knowlesi | P. falciparum | P. malariae | P. ovale | P. vivax |
|---|---|---|---|---|---|
| Fever pattern/erythrocytic phase | Quotodian with 24 h cycle | Tertian, sub-tertian with 48 h cycle | Quartan with 72 h cycle | Tertian with 48 h cycle | Tertian with 48 h cycle |
| Relapse from persistent tissue stages | No | No | No, but blood forms can persist up to 30 years | Yes | Yes |
| Severe form in | 6–10% cases | Up to 24% cases | Very rare | Very rare | Up to 22% cases |
| Human erythrocyte | Not enlarged, rounded, not distorted | Not enlarged, rounded | Not enlarged, rounded, generally not distorted | Not enlarged, rounded, generally not distorted | Enlarged from trophozoite stage onwards, rounded, not distorted |
| Stages in peripheral blood | All | Rings and gametocytes | All | All | All |
| No. of parasites in RBC | Single or multiple (2 or 3) | Single or multiple | Single | Single | Single |
| Early trophozoite (ring form) | Dense cytoplasm with single or double chromatin, occasionally three chromatins. Chromatin lies inside the ring | Delicate cytoplasm, 1–2 small chromatin dots | Dense cytoplasm with large chromatin | Sturdy cytoplasm; large chromatin | Large cytoplasm with large single chromatin |
| Late trophozoite | Dense cytoplasm, cytoplasm slightly amoeboid and irregular, band forms, dark brown pigmentation | Rarely seen in peripheral blood. Compact cytoplasm, dark pigment | Compact cytoplasm, rounded large chromatin, occasional band forms, dark brown pigment | Compact cytoplasm with large chromatin, dark brown pigment | Large amoeboid cytoplasm, large chromatin, fine yellowish-brown pigment |
| Schizont and merozoites | Occupies whole erythrocyte. 10–16 merozoites scattered or grape-like cluster, pigment scattered or collects into a single mass | Seldom seen in peripheral blood. 8–24 small merozoites, dark pigment clumped in a mass | Occupies whole erythrocyte. 6–12 merozoites, usually 8–10 clustered around dark brown pigment forming rosette pattern | Slightly smaller than RBC, round to oval and fimbricated. 6–12 merozoites, usually 8 in a single ring, with large nuclei, clustered around mass of dark-brown pigment | Large may occupy full RBC. 12–24 merozoites, yellowish-brown coalesced pigment |
| Gametocyte | Occupies whole erythrocyte, round, compact, pigment scattered or collects into a single mass | RBC distorted by parasite, parasite is crescent or sausage shaped, dark pigment mass | Occupies whole erythrocyte, round to oval, scattered brown pigment | Round to oval, compact, may fill RBC, scattered brown pigment | Round to oval, compact, may fill RBC, scattered brown pigment |