Plasmodium vivax
Ring form
The cytoplasmic ring of the typical P. vivax ring form measures approximately one third diameter of the red blood cell in which it resides. A single chromatin dot serves as the connecting point of this delicate ring. A vacuole is visible inside the ring. The parasite may be first visible as a crescent – shaped mass at the outer edge of the red blood cell .
Developing trophozoites
Although remnants of the cytoplasmic ring may be visible, the parasite takes on more of an irregularly shaped ameboid appearance. Asingle large chromatin dot is present among the cytoplasmic material.the vacuole remains visible and basically intact until the late stage of development. The presence of brown pigment, a remnant of the parasite feeding on RBC hemoglobin.
Immature schizonts
The immature schizont form of P. vivax is characterized by the presence of multiple chromatin bodies that emerge from progressive chromatin division.
Mature schizonts
The continuing division of chromatin results in 12 to 24 (average of 16) merozoites. The merozoites surrounded by cytoplasmic material occupy the majority of the RBCs.
Microgametocytes
The typical P. vivax microgametocyte consists of a large pink to purple chromatin mass , when giemsa stained, which is surrounded by a colorless to pale halo .
Macrogametocytes
The average P. vivax macrogametocyte is characterized by round to oval homogeneous cytoplasm and an eccentric chromatin mass, often located against the edge of the parasite.
Life cycle
Members of the mosquito genus Anopheles are responsible for the tranismission of malaria to humans via a blood meal. These vector transfers malarial sporozoites form its salivary gland into the human wound. Following entrance into the body, the sporozoites are carried through the preipheral blood to the parenchymal cells of the liver. It is here where schizogony occurs. This exoerythrocytic cycle of growth and reproduction lasts from 8 to 25 days, depending on the specific Plasmodium species involved. The infected liver cells eventually rupture and introduce merozoites into the circulating blood.
These migrating merozoites target age and – size – specific RBCs to invade, and upon doing so initiate the erythrocytic cycle of growth .it is in this asexual phase that the plasmodia feed on hemoglobin and pass through the numerous stages of growth, including the six morphologic forms previosly described. Upon formation of the merozoites, one of three paths may take place. Some of the RBCs infected with merozoites rupture, releasing these forms to target and infect new RBCs, and this part of the cycle repeats itself. Numerous erythrocytic cycles may occur. However, other infected RBCs containing merozoites develop into microgametocytes and macrogametocytes. Still others are destroyed by the immune system of otherwise healthy individual.
Transmission of malaria back into the vector occurs when the mosquito ingests mature male (micro) and female (macro) gametocytes during a blood meal, thus initiating the sexual cycle of growth. a male and female gametocyte unite in the mosquito’s stomach and form a zygote matures into an oocyst . upon complete maturation , the oocyst ruptures and releases numerous sporozoites , which migrate into salivary gland of the mosquito and are ready to infect another unsuspecting human . Thus the cycle repeats itself.
Clinical symptoms
Benign Tertian Malaria. Patients infected with P. vivax typically begin to develop symptoms following a 10 to 17 day incubation period following exposure . these vague symptoms mimic those usually seen in cases of the flu, including nausea, vomiting , headache , muscle pains, and photophobia .
As infected RBCs begin to rupture, the resulting merozoites, hemoglobin, and toxic cellular wast products initiate the first in a series of paroxysms. These paroxysms typically occur every 48 hours.
Laboratory diagnosis
All morphologic stages of P. vivax may be seen on thick and thin peripheral blood films. Although the best time to observe numerous infected RBCs is halfway between paroxysms, blood samples may be taken at any time during the illness.