Plasmodium ovale protozoan inside red blood cells at the ring-form trophozoite stage, computer illustration. P. ovale is the causative agent of benign tertian malaria, also known as ovale malaria. It is transmitted to humans by the bite of the female Anopheles mosquito. The illustration shows the typical morphological features of P. ovale in the ring form (early trophozoite) stage. It resembles the ring form of P. vivax, but is larger in size, thick and has a somewhat amoeboid appearance. The red blood cells are oval and enlarged, with distorted and ragged cell walls.