Life Cycle of Plasmodium vivax

Life Cycle of malaria causing parasite (Plasmodium vivax)


Life cycle of malarial parasites completes in two hosts ie. Man and Female anopheles mosquito. In man the parasites develop asexually whereas in female anopheles mosquito it develops sexually.

Asexual Cycle
Malaria infection in the human host starts when the sporozoites are injected into the blood stream during a blood meal by an infectious mosquito. Although it is assumed than one single
sporozoite is capable of initiating the infection in men, the number of sporozoites injected by a mosquito bite is supposed to vary from dozens to thousands. It is likely that this number strongly affects the clinical picture: the greatest the sporozoite load, the shortest the incubation period and the most serious the symptoms. The sporozoites remain into the circulation for a short period, calculated as 60 minutes at maximum, before they actively enter the liver of the host. The Kuppfer cells in the liver may be invaded but the sporozoites are not able to develop in those cells and die shortly after invasion. Most parasites however invade the hepatocytes (Cells of the liver) and start the asexual exo-erythrocitic schizogonic cycle.
Sporozoites inside the liver cell rapidly grow in size by absorbing nourishment. Nucleus of sporozoites divides into several small pieces and piece is enclosed by cytoplasm. Thus, about 80-1000 merozoites of 1µ are produced. (The schizont formed in liver cell is called by some authors as cryptozoites and its products are called cryptomerozoites, some author call them simply merozoites)
Merozoites are liberated by the bursting of liver cells due to the pressure on liver cells. Merozoites thus liberated reach the fine blood sinusoids of liver from where they reach the general blood circulation. Whereas,some of the merozoites again attack the fresh liver cells. (In the exoerythrocytic cycle, there is no effect of various drugs as the parasite is intracellular.)

Erythrocytic or Schizogonic Cycle
Merozoites penetrate the RBC. It burrows into red blood cells and slowly enters the resting stage. This is the trophozoite stage in which parasite is converted into a rounded body with a single nucleus and grows at the expense of protoplasm of RBC. The trophozoite attains its maximum size. Soon, a vacuole is formed in the middle of cytoplasm, which pushes the nucleus to one side. This trophozoite stage is called a signet ring stage. Soon, the vacuole disappears and parasite changes to amoeboid form. This stage is called ameboid stage. In this stage parasite feeds on the haemoglobin and other contents of RBC.
After feeding, amoeboid form increase in size and becomes round. The stage is called schizont. At the mean time, some small granules called Schuffer’s granules (dots) appear in the RBC cytoplasm. This is the toxic released by parasites and are called haemozoin which are responsible for causing the fever. The nucleus of schizont multiplies by multiple fission to form 12-24 daughter nuclei
At the end of this phase the schizogonic cycle is completed, the erythrocyte ruptures releasing the merozoites into the blood stream and the merozoites discharged into the circulation invade new erythrocytes to repeat the schizogonic cycle. While some merozoites will develop into two types of gametocytes ie. Microgametocytes and megagametocytes. The further development of gametocytes takes place in female Anaopheles mosquito. The life span of gametocytes is only few hours. If these gametocytes are not taken by mosquito, then die soon.

Sporogony or Sexual Cycle
When a female anopheles mosquito bites a malarial patient and sucks the blood having gametocytes for its food by piercing into the skin, further development of the gametocytes is only possible in the stomach of female anopheles and not any other type of mosquito. If mosquito is other type, the gametocytes are destroyed in its stomach.

Maturation of Gametocytes
a) Microgametocytes:- The microgametocytes become very active and their nuclei divide into several small nuclei. A number of six to eight flagella like microgametes are formed. A nucleus move to each flagellum. Flagella start lashing movements and break off from the cytoplasm. These structures are called microgametes, which can be compared to spermatozoa and this process is called exflagellation.
b) Macrogametocytes:- The mega or macrogametocytes develop into female gametes or ova. The megagametocytes show little change. It enlarges in size and develops a reception cone. This structure is called female gamete or ova.

Fertilization and sporozoites formation
Microgametes make active lashing movements and approach megagamete, one of the microgamete reaches to protoplasmic projection of megagamete. The nuclei of mega and microgamete fused resulting in the formataion of zygote after fertilization. Zygote remains inactive for sometime but later on becomes elongated or worm like ookinete or vermicule. It moves into the stomach of the host. There it passes into the epithelium and finally reaches in between the epithelial and subepithelial tissues. Zygote becomes enclosed into a cyst called oocyst. Afterwards, oocyst attains maximum size and the nucleus of the oocyst divides into several nuclei to form hundred of spindle-shaped nuclei. This process is called sporulation.
Cyst brust and sporozoites are liberated into the body cavity or haemocoel of mosquito. Sporozoites are motile and majority of them reach the salivary glands of the mosquito. Eventually, sporozoites enter the salivary duct along with the saliva. When a mosquito in this condition pierces the skin of man, some sporozoites are injected with the saliva into the puncture caused by the mosquito. Sporozoites then reach liver cell and cycle is repeated.


Pathogenesis
The disease caused by plasmodium is malaria. The basic symptom is intermittent fevers with chills and shivering. The general symptom of malaria includes three successive stages. In cold stage which lasts for 15 to 60 minutes, the patient feels intense cold and uncontrolled shivering. It is followed by the hot stage which lasts for 2-6 hours, the patient experiences intently hot. The fever reaches to 41 degree centigrade or higher. Severe headache, vomiting and nausea are common. Afterwards come the sweating stage, when the patient sweats much, then the temperature drops rapidly and patient usually falls into deep sleep. The complete symptom usually begins in early afternoon and lasts for 8-12 hours.

Preventive Measures
Destruction of Vectors
Prevention of infection
Treatment of infected person
Avoid mosquito bites etc