Stage of falciparum not seen in peripheral blood smear: March 2011
Ans. A: Schizont P. falciparum differs from the other forms of malarial parasites in that developing erythrocytic schizonts aggregate in the capillaries of the brain and other internal organs, so that only young ring forms are found in peripheral blood Life cycle of all Plasmodium species Infection in humans begins with the bite of an infected female Anopheline mosquito. Sporozoites are the infective form of the parasite, present in the salivary glands of the female anopheles mosquito. Sporozoites released from the salivary glands of the mosquito enter the bloodstream during feeding, quickly invading liver cells (hepatocytes). Sporozoites are cleared from the circulation within 30 minutes. During the next 14 days in the case of P. falciparum, the liver-stage parasites differentiate and undergo asexual multiplication resulting in tens of thousands of merozoites which burst from the hepatoctye. Individual merozoites invade red blood cells (erythrocytes) and undergo an additional round of multiplication producing 12-16 merozoites within a schizont. The length of this erythrocytic stage of the parasite life cycle depends on the parasite species: - Irregular cycle for P. falciparum, - 48 hours for P. vivax, and P. ovale and - 72 hours for P. malariae. The clinical manifestations of malaria, fever and chills, are associated with the synchronous rupture of the infected erythrocyte. The released merozoites go on to invade additional erythrocytes. Not all of the merozoites divide into schizonts, some differentiate into sexual forms, male and female gametocytes. These gametocytes are taken up by a female anophylean mosquito during a blood meal. Within the mosquito midgut, the male gametocyte undergoes a rapid nuclear division, producing 8 flagellated microgametes which feilize the female macrogamete. The resulting ookinete traverses the mosquito gut wall and encysts on the exterior of the gut wall as a oocyst. Soon the oocyst ruptures, releasing hundreds of sporozoites into the mosquito body cavity where they eventually migrate to the mosquito salivary gland. To diagnose malaria and determine which species of Plasmodium is causing the infection, blood film is examined. Each species has distinctive physical characteristics that are apparent under a microscope. In P. falciparum, only early (ring-form) trophozoites and gametocytes are seen in the peripheral blood. It is unusual to see mature trophozoites or schizonts in peripheral blood smears as these are usually sequestered in the tissues. The parasitised erythrocytes are not enlarged, and it is common to see cells with more than one parasite within them (multiply parasitised erythrocytes). On occasion, faint comma-shaped red dots called "Maurer's dots" are seen on the red cell surface. The comma-shaped dots can also appear as pear-shaped blotches. Plasmodium falciparum causes severe malaria a distinctive propey not shared by any other human malaria, that of sequestration. Within the 48-hour asexual blood stage cycle, the mature forms change the surface propeies of infected red blood cells causing them to stick to blood vessels (a process called cytoadherence). This leads to obstruction of the microcirculation and results in dysfunction of multiple organs, typically the brain in cerebral malaria.
Schizont
Gametocyte
Ring form
Double ring