Plasmodium Vivax Life Cycle

Plasmodium vivax, a malarial parasite, is an intracellular blood parasite that can affect humans and other vertebrates. It belongs to the class Sporozoa of the phylum Protozoa. Plasmodium vivax is an endoparasite and is well-adapted to the parasitic mode of life. The life cycle of Plasmodium vivax is complicated and is completed in two different types of hosts: definitive and intermediate.

Definitive and Intermediate Hosts

• A definitive host provides nutrition to the parasite. Humans act as definitive hosts where the asexual cycle occurs.
  An intermediate host helps in transference from one definitive host to the other. Mosquitoes (only female Anopheles) are the intermediate hosts where the sexual cycle occurs.
  The life cycle of Plasmodium vivax is divided into the following stages.

Pre-Erythrocytic Cycle

• Inoculation occurs when an infected female Anopheles mosquito bites a healthy person to feed on his blood, then spit in the place of the bite; numerous sporozoites enter into the blood stream along with the saliva.
  Pre-erythrocytic schizogony happens when the sporozoites first go to the parenchyma cells of the liver and remain within it for about seven days. During this period, each sporozoa develops into a schizont. Here they multiply for the first four days, and about 12,000 merozoites are formed.

Erythrocytic Cycle

• This cycle is divided into the following stages:
  Trophozoite Stage--After entering the blood stream, merozoites start invading the red blood cells (RBC).
  Signet Ring Stage--A non-contractile vacuole appears in its cytoplasm and grows by feeding the substance of the RBC. The nucleus is pushed on one side and parasite assumes a ring-like appearance.
  Amoeboid Stage--The vacuole disappears and the parasite assumes an amoeboid form to represent this stage.
  Schizont Stage--By thrusting its pseudopodia inside the cytoplasm of RBC, it feeds on the hemoglobin and increases its size and forms the schizont.
  Merozoite Stage--The parasite cannot grow in size, but undergoes asexual multiplication. The nucleus divides to form eight to 24 daughter individuals and form merozoites.

Post or Exo-Erythrocytic Cycle

• In this stage, they may continue to produce about 1,000 exo-erythrocytic merozoites within the liver without any symptom, and may live there for some years. After this dormant stage, they may again become infectious. These merozoites invade either fresh parenchyma cells or RBCs.

Sexual Cycle

• The female Anopheles mosquito takes a blood meal from the infected person and thus ingests both sexual and asexual forms. The asexual forms soon become digested and the sexual forms survive and develop. The sexual cycle is divided in the following stages:
  Gametogony, where some of the schizonts are modified for sexual reproduction and the resulting merozoites, after entering the RBC, do not form trophozoites but grow slowly and specialize themselves as gamonts or gametocytes. There are two forms of fully-grown gametocytes--female, or macrogametocyte, and male, or microgametocyte.
  Fertilization: The actively moving male gamete is attracted by the female gamete and unites to form a zygote.
  Sporogony: The actively moving zygote moves vibrantly inside the mid-gut, bores through the wall of the gut wall and comes to rest under epithelium and the outer wall of the stomach. The zygotes that fail to get shelter in the stomach wall of the mosquito develop into Ookinete. Ookinete secrete a thin membranous cyst wall; this is known as Oocyst. The nucleus divides repeatedly and ultimately a large number of sickle-shaped sporozoites are formed from a single Oocyst. The Oocyst ruptures about 10 days from its formation and sporozoites are liberated in the hemocoele of mosquito; later they migrate to the salivary gland and the female Anopheles becomes infected, enabling her to inoculate the parasite into the blood stream of healthy persons.