The main mode of transmission of the disease is by bites from infected Anopheles mosquitoes that have previously had a blood meal from an individual with parasitemia. Less common routs of transmission are via infected blood transfusing, transplantation, infected needles, and from a mother to her fetus during pregnancy.
Plasmodium life cycle : Life cycle of plasmodium vivax is digenetic i.e. they complete their life cycle in two host :
1] Definitive host or Primary host : Female Anopheles mosquito is the primary host. The organisms which contains sexual phase of the parasite and is regarded as definitive host.
2] Intermediate host or secondary host : Human is the secondary host. Human contains asexual phse of the parasite and develops symptoms of of disease due to the presence of parasite and is termed as secondary host.
The infection begins when an infected female Anopheles mosquito bite a person and injects infected with sporozoites saliva into the blood circulation. That is the first life stage of plasmodium.{ stage of infection}
The next stage in malaria life cycle is the one of asexual reproduction that is divided into different phases : the pre - erythrocytic (exoerythrocytic) and the erythrocytic phase. Within only 30-60 minutes after the parasites inoculation , sporozoites find their way through blood circulation to their first target the liver. The sporozoites enter the liver cells and start dividing leading to schizonts creation in 6-7 days. Each schizont gives birth to thousand of merozoites(exoerythrocytic schizogony) that are then released into the blood stream marking the end of the exoerythrocytic phase of the asexual reproductive stage.
Merozites released into the blod stream, are directed towards their second target the red blood cells(RBC). As they invade into the cells, they mark the beginning of the erythrocytic phase. The first stage after invasion is a ring stage that evolves into a trophozoite. The trophzoites are not able to digest the globin that is used as a source of aminoacids for their reproduction. The next cellular stage is the erythrocytic schizont. Each mature schizont gives birth to new generation merozoits that after RBCs rupture are released in the blood stream in order to invade other RBCs. This is when parasitemia occurs and clinical manifestations. appear. The liver phase occurs only once while the erythrocytic phase undergoes multiple cycles ; the merozoites release after each cycle creates the febrile waves.
A second scenario into the RBCs is the parasite differentiation into male and female gametocytes that is a non pathogenic form of parasite. When a female anopheles mosquito bites an infected person, it takes up these gametocytes with the blood meal. The gametocytes, then, mature and become micro gametes (male) and macrogametes( female) during a process known as gametogenesis.
In mosquito gut , the microgamete nucleus divides three times producing eight nuclei; each nucleus fertilizes a macrogamete forming a zygote. The zygote after the fusion of nuclei and the fertilization, becomes the so called ookinete. The ookinete, then , penetrates the midgut wall of the mosquito, where it encysts into a formation called oocyst. Inside the oocyst, the ookinete nucleus divides to produce thousand of sporozoites. That is the end of the third stage. Sporogony lasts 8-15 days.
The oocyst ruptures and the sporozoites are released inside the mosquito cavity and find their way to its salivary glands but only few hundreds of sporozoites manage to enter. Thus, when the above mentioned infected mosquito takes a blood meal, it injects its infected saliva into the next victim marking the beginning of a new cycle.
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