The life cycle of Plasmodium vivax is completed in two vectors or hosts. Man and Mishki. Asexual reproduction occurs in the human liver and red blood cells, and sexual reproduction occurs in the mouse crop. So man is the main host and the mouse is the secondary host.
Asexual cycle of malaria parasite in human body
Hepatic zygogy occurs in human liver cells and erythrocytic zygogy occurs in red blood cells. Two phases are discussed.
Hepatic schizogony or liver schizogony
Asexual reproduction by multiplicity of malaria parasites in human liver cells is called hepatic zygogy. In 1948 scientists Shortt and Garnham described the hepatic schizogony of malaria parasites in the human liver. This cycle takes place through two phases. Pre-erythrocytic schizogony and Ex-erythrocytic schizogony.
1. Pre-erythrocytic schizophrenia
(i) Sporozoite: Sporozoites are mobile, very small, slightly curved and both ends of the body are smooth. Their bodies are covered with elastic membranes. When an Anopheles mosquito, which carries malaria, bites a healthy person, the sporozoites enter the human body through the saliva of the mosquito. Sporozoites penetrate human liver cells within 30-45 minutes. Due to chemotaxis, it is carried by the blood and enters the liver and grows.
(ii) Cryptozoite: The sporozoites take food and become swollen and spherical. It is called cryptozoite.
(iii) Schizont: The nucleus of the cryptozoite divides repeatedly to form numerous (about 1200) tiny nuclei. This multinucleated condition is called zygote.
(iv) Cryptomerozoite: Cytoplasm accumulates around each nucleus of the zygote to form new cells. These are called cryptomerozoites. Cryptomerozoites break through the liver cell wall and come out and lodge in the sinusoids. It then invades the new liver cells causing recurrence of hepatic schizophrenia. A total of 8000-20,000 merozoites are produced.
2. X-erythrocytic schizophrenia
(i) Schizont: Cryptomerozoites invade new liver cells. Grows in size by consuming food and divides repeatedly to form numerous nuclei. This multinucleated condition is called zygote.
(ii) Metacryptomerozoite: Cytoplasm accumulates around each nucleus of the zygote to form new cells. The cells are called metacryptomerozites. Metacryptomerozites are of two types. Micrometacryptomerozites and macrometacryptomerozites. Micrometacryptomerozotes are small in size and have large nuclei. Macrometacryptomerozotes are large in size and have small nuclei. Macrometacryptomerozites invade new liver cells. Micrometacryptomerozoites enter the bloodstream and invade red blood cells. From sporozoite to metacryptomerozote takes 7-10 days. Merozoites can undergo zygogy in liver cells for years without showing any symptoms of malaria.
Erythrocytic schizogony
Asexual reproduction of malaria parasites in human red blood cells is called erythrocytic zygogy. After the sporozoite stage, it takes 7-8 days for the malaria parasite to manifest itself in human blood. This period is called pre-patent period. Often the sporozoite remains dormant for months or years after entering the liver cells. Such dormant state of sporozoite is called hypnozoite. The stages of erythrocytic schizophrenia are as follows-
1. Trophozoites: The merozoites enter the red blood cells and become swollen, large and spherical after taking food. This stage is called trophozoite. The trophozoite is 2.5-3.0 µm in diameter. It is a temporary condition.
2. Signet ring: A cavity formed inside the trophozoite. Its cytoplasm moves to the periphery and the nucleus is positioned to one side. In this condition, the germ looks like a stone ring. It is called a signet ring.
3. Amoeboid trophozoites: The cavity of the signet ring disappears within 8 hours. The parasite is irregular in shape, amoeba-like, and motile. This stage is called amoeboid trophozoite. Amoeboid trophozoites consume the protein component of hemoglobin as food. Hematin of hemoglobin is converted to toxic hemogene. At this time the red blood cell is enlarged in size and tiny granules are seen in its cytoplasm. These grains are called Schiffner’s dots. Malaria is diagnosed by the presence of softeners granules in blood cells. Schiffner’s dots are named after the inventor Wilhelm Schiffner (1904).
4. Syzoant: The amoeboid trophozoite has a morphozoite and the parasite is spherical or elongated. Its nucleus divides to form 12-24 daughter nuclei. This multinucleated condition is called zygote. After 36-40 hours it takes up all the space in red blood cells. At this time waste material called hemogene accumulates in the cytoplasm of the parasite.
5. Merozoite: After 45 hours, 12-18 tiny cells are seen in the cytoplasm of the zygote. A nucleus enters each segment. They are arranged in two layers like petals. This condition is called rosette. Each part, including the nucleus, becomes a merozoite. Merozoites invade new red blood cells. During this time, a large amount of chemical substances called pyrogen accumulates in the blood and due to its effect, fever occurs in the body.
6. Gametocytes: Some merozoites develop into gametocytes after their release from the liver cells. Gametocytes are of two types. Microgametocyte and Macrogametocyte.
(i) Microgametocytes: Microgametocytes are small in size. Its diameter is 9-10 µm and the cytoplasm is light blue. At its center is a large pink nucleus. Its nucleus is small, dense and peripheral.
(ii) Macrogametocytes: Macrogametocytes are large in size. Its diameter is 10-12 µm and the cytoplasm is dark blue.
It takes 96 hours to develop mature gametocytes. It takes 48-72 hours to complete this cycle.
The reproductive cycle of malaria parasites in the mosquito
When an Anopheles mosquito bites a person with malaria, gametocytes are released into the mosquito’s saliva. The sexual cycle of the malaria parasite in the mosquito is completed in two stages. Gametogony and sporogony.
1. Gametogony
The process of gamete formation is called gametogony. The stages of gametogeny are:
(i) Formation of Pungametes: The process of formation of pungametes or spermatozoa is called spermatogenesis. In the process of exflagellation, the nucleus of each microgametocyte divides to produce 4-8 nuclei. At this time 4-8 projections are formed in the cytoplasm. This is called cytoplasmic projection. One nucleus enters each projection. Each projection with a nucleus develops into a pungamete or microgamete. Pungamites swim for fertilization.
(ii) Formation of strygametes: The process of formation of strygametes or ovules is called oogenesis. Each macrogametocyte gives rise to one strigamete or macrogamete. One end of the macrogamete swells to form the fertilization cone. The cone region is called the Fertilization Cone or Reception Cone. The nucleus of the egg is located near the cone.
(iii) Fertilization: The gametes move individually towards the fertilization cone of the ovule. One sperm enters each egg. Later their nuclei fuse to form a zygote. This process is called fertilization. Fertilization is completed within 20-22 hours.
(iv) Wookinet/Vermicule: After 12-14 hours the quiescent spherical zygote becomes elongated and motile. It is called wokkinet or vermicule. Uokinetes are 18-24 microns in length and 3-5 microns in width.
(v) Oocysts: Within 24 hours the ookinetes break through the wall of the mosquito crop and come out and take spherical shape. Then within 40 hours it is covered with a thin film. It is called a oocyst. The nucleus of the oocyst divides first by meiosis and then by mitosis. Meiosis of oocyst is called postzygotic meiosis. A mosquito crop may contain 50-500 oocysts. The resulting oocysts appear spongy and 4-5 times larger in size (50-60 µ).
2. Sporogony
The process of sporozoite formation is called sporogony. The oocyst grows by consuming food. Its nucleus divides repeatedly by first mitosis and then meiosis to form many haploid nuclei. Cytoplasm accumulates around each nucleus to form spherical cells. Later round cells transform into sickle or macule shaped sporozoites. About 10,000 sporozoites are produced from each oocyst. The sporozoites reside in the hemocele of the mosquito. At this time, if the mosquito bites a healthy person, the sporozoite enters the person’s body. About 10% of sporozoites enter the human body during a single bite. A mosquito’s salivary gland can contain up to 326,000 sporozoites. The sporozoites remain in the salivary glands of the mosquito for about 60 days. It takes about 10-20 days from stinging to sporozoite formation.