What Causes Malaria In Humans?
Malaria is a protozoan disease which occurs due to invasion by a human blood parasite known as Plasmodium. The transmission of the infective agent is vector-borne or you can say disease is spread by female Anopheles mosquito. Patients have periodic fever, chills and malaise as common symptoms. Treatment in most of the cases is promising and economic too, but in some cases, it may prove fatal too.
According to estimates (December 2015) more than 214 million cases were observed during the year with 438,000 deaths approximated caused by Plasmodium infection. Though the risk of infection among people has declined by 37% all over the world during the period of 2000 and 2015 yet, some parts of the world like Sub-Saharan Africa still carries huge burden of disease in a disproportionate ratio with 80 percent of total malaria incidence and 90% of deaths due to the disease.
What is Plasmodium?
Plasmodium is a genus of human parasite which comprises of four clinically significant species as they are known to cause malaria in human beings. They are-
- Plasmodium vivax
- Plasmodium ovale
- Plasmodium falciparum
- Plasmodium malariae
P. vivax shows the widest distribution of the global area. Next is P. falciparum with 4 to 8 percent, it is due to the fact that this species shows mixed infections. Rest of the cases are of P. malariae which shows a restricted geographical incidence and accounts for less than 1 percent of the cases. Interestingly, P. ovale is rare parasite to human and was confined to tropics of Africa until incidence has been reported in Vietnam too. However, severity of the disease is directly related to the species of the protozoan.
Life History of Plasmodium
The development phase of the parasite has two main cycle i.e., asexual cycle in humans and sexual cycle in it’s vector mosquito.
This process begins with the bite of already infected mosquito who releases sporozoites along with anti-coagulatory saliva into the human blood stream. Now these sporozoites undergo four different phases of development in the human body which includes-
Hepatic Phase: It roughly takes an hour to clear these sporozoites from peripheral blood as defence mechanism of body responds and they are destroyed or engulfed by the phagocytes. Still some sporozoites manage to escape and reach hepatocytes in the liver where they undergo development for 1 to 2 weeks depending on type of species before they are released into the blood stream as schizonts. However, not all the hepatic forms are released, but some remain dormant in the liver cells which later on results in relapse of the disease, commonly seen in vivax and ovale infection. From here they invade the erythrocytes or red blood cells.
Erythrocytic Phase: Inside red blood cells, they further go into developmental phase and get released by bursting the cells which also release hemozoin (degraded hemoglobin) and prostaglandins resulting in the fever and. They may attain the form of merozoites or trophozoites. On one hand, trophozoites further divide and form merozoites and more trophozoites, while merozoites reinfect other red blood cells and continue the cycle. Important fact is duration of this cycle tends to be constant around 48 hours for falciparum and 72 hours for other species.
Gametogony: Some trophozoites develop into male and female sexual forms and migrate towards peripheral blood stream to get sucked by a healthy mosquito during feeding and complete sexual cycle inside the vector.
The mosquito cycle also known as sporogony initiates with ingestion of gametocytes by feeding on an active infected person from where they reach gut mucosa of the insect by exflagellation and further develop into micro- and macro-gametes which fertilize to form a motile form called “ookinite.” After some divisions, it bursts to release sporozoites which migrate to salivary glands of the vector and get released into another healthy human to repeat the whole cycle.
Thus, we can say human acts as secondary host to the Plasmodium and naturally tends to develop malaria if gets infected.
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