Plasmodium vivax and Malaria

Plasmodium vivax and Malaria

Plasmodium vivax is a sporozoan protozoan parasite and one of the principal causative agents of malaria in India. Its life cycle is the most elaborate in this chapter because it is a digenetic parasite — it must pass through two hosts to complete its life cycle: humans (the intermediate host, in which only asexual multiplication occurs) and the female Anopheles mosquito (the definitive host, in which the sexual phase occurs).

The two cycles in man: hepatic and erythrocytic

When an infected female Anopheles mosquito bites a person, it injects thread-like, infective sporozoites from its salivary glands into the bloodstream.

Exo-erythrocytic (hepatic/pre-erythrocytic) cycle — sporozoites are carried by the blood to the liver, where they invade hepatocytes (liver cells). Inside a hepatocyte each sporozoite grows and undergoes multiple asexual fission (schizogony) to form a schizont packed with thousands of small daughter cells called cryptomerozoites. This liver-stage multiplication corresponds to the prepatent period — the interval between the infective mosquito bite and the first appearance of parasites in the peripheral blood. Some sporozoites of Plasmodium vivax do not develop immediately in the liver but remain dormant as resting forms called hypnozoites; these can reactivate weeks, months, or even years later and are the reason vivax malaria is notorious for causing relapses long after the original infection appears to have cleared. The clinical incubation period — the time from the infective bite to the first appearance of fever symptoms — is longer than the prepatent period and includes the time needed for the erythrocytic cycle to build up a symptom-producing parasite load.

Erythrocytic cycle — cryptomerozoites released from the liver invade red blood cells (RBCs). Inside an RBC, the parasite grows through a ring stage, then a feeding, ring-cytoplasm-actively-vacuolating trophozoite stage, digesting hemoglobin and depositing an insoluble, brownish-black pigment waste product called haemozoin (malaria pigment) inside the parasite and, later, the host cell. Infected RBCs also show fine stippling of the cell membrane known as Schuffner's dots, a classic microscopic hallmark of P. vivax infection. The growing trophozoite's nucleus then divides repeatedly by schizogony to form a schizont containing 12–24 daughter merozoites. The infected RBC ruptures, releasing the merozoites (along with haemozoin and cell debris) into the plasma — this synchronised rupture is what triggers the periodic bouts of chills and high fever. Released merozoites invade fresh RBCs and repeat the cycle. Because P. vivax completes one erythrocytic schizogonic cycle roughly every 48 hours, the resulting fever pattern is called benign tertian fever (fever recurring on the first and third day, i.e. every 48 hours) — distinguished from the malignant tertian fever caused by Plasmodium falciparum, which is far more dangerous.

Gametogony: formation of gametocytes

After several erythrocytic cycles, some merozoites that invade RBCs do not undergo schizogony but instead differentiate into sexual forms called gametocytes — larger, rounder cells of two kinds, a smaller microgametocyte (male) and a larger macrogametocyte (female). These gametocytes cause no further symptoms in humans and can develop no further unless taken up by a mosquito; they represent the only stage transmissible onward.

The sexual cycle in the mosquito: sporogony

When a female Anopheles mosquito bites an infected person, it ingests gametocytes along with the blood meal. Inside the mosquito's stomach:

• The microgametocyte undergoes exflagellation — its nucleus divides repeatedly and slender, motile, flagellum-like microgametes are thrown out from the cell surface, a dramatic and diagnostic process.
• The macrogametocyte matures directly into a single, non-motile macrogamete.
• A motile microgamete fuses with the macrogamete in anisogamy (fusion of dissimilar gametes) to form a diploid zygote.
• The zygote elongates and becomes motile, penetrating the stomach wall of the mosquito as a worm-like ookinete.
• The ookinete settles beneath the outer wall of the mosquito's stomach and rounds off into an oocyst.
• Inside the oocyst, the nucleus divides repeatedly by sporogony (a distinct process from schizogony, occurring only in the mosquito and producing sporozoites rather than merozoites), forming thousands of thread-like sporozoites. …