Mosquitoes Do Not Cause Malaria.
- bioworld090
- Jul 13, 2022
- 4 min read
Photo by icon0.com
Malaria is one of the deadliest diseases known to humans. Despite having better treatments as compared to earlier times, it still infects and kills a large number of people. According to cdc.gov, 241 million cases of malaria infection and 627,000 deaths were reported in 2020.
We are advised to avoid mosquito bites to prevent malarial infection, which leads many of us to think that mosquitoes cause malaria. People get infected by malaria through mosquito bites but mosquitoes are not the actual cause of it.
There are two terms people should be familiar with to understand this better. These terms are "causative organism" and "vector organism". A causative organism is an organism that causes an infection or disease, while a vector organism is an organism that spreads the causative organism from an infected individual to a healthy individual.
Now, the statement "Mosquitoes do not cause malaria" might make more sense. In the case of malaria, the female Anopheles mosquito is not the causative organism but just a vector or medium that carries and spreads the causative organism. The actual cause of malaria is a parasite called plasmodium. So,
what exactly is a plasmodium?
Plasmodium is a single-celled obligate parasite(a parasite that requires a suitable host to complete its life cycle) belonging to the phylum Apicomplexa. There are over 200 species of Plasmodium but only 5 are known to infect humans regularly. These are P. vivax, P. malariae, P. falciparum, P. knowlesi, and P. ovale.
HOW DOES PLASMODIUM CAUSES MALARIA?
Image Source: cdc.gov
Plasmodium requires two hosts to complete its life cycle. The primary host of plasmodium is a mosquito and the intermediate host is a human.
In humans, the life cycle of plasmodium can be divided into 2 phases: an exoerythrocytic or pre-erythrocytic phase that occurs outside the erythrocytes(red blood cells) and an erythrocytic phase that takes place inside the erythrocytes.
LIFE STAGES OF PLASMODIUM IN HUMANS(INTERMEDIATE HOST)
Sporozoites → entry into liver → Schizont → Merozoites → entry into RBCs →trophozoites→ Schizont → Merozoites → either enter RBCs or turn into gametocytes
Exoerythrocytic or Pre-erythrocytic Phase
It all begins when an infected female Anopheles mosquito bites a human. The infective sporozoites(a stage in the life cycle of the plasmodium) reside in the salivary glands of the infected mosquitoes. When a mosquito bites, it also releases its saliva into the person's blood to prevent blood clotting or coagulation. Along with the saliva, around 100 sporozoites are also released into the dermis(middle layer of skin) and after some time, they reach the liver through the bloodstream. While the majority of the sporozoites reach the hepatocytes(cells that make up most of the liver), some are killed.
The sporozoites invade the hepatocytes or hepatic cells and circumsporozoite protein(protein present on the surface of sporozoites) plays a key role in this invasion. Then, the sporozoites undergo nuclear divisions and form a multinucleated schizont. The schizont after a certain time releases merozoites. The process of formation of merozoites is called merogony.
Some plasmodium species such as P. vivax and P. ovale can enter a state of dormancy rather than forming a schizont. The dormant stage is called hypnozoite and they can stay dormant in the hepatocytes for many days, months or even years.
The exoerythrocytic phase or the liver infection produces no symptoms and hence, is an asymptomatic phase.
Erythrocytic Phase
Image Credit: sciencedirect
Merozoites exit the hepatocytes and enter the erythrocytes(red blood cells) by recognising them with the help of invasion organelles. Here, the merozoites lose their invasion organelles,i.e, apical complex and surface coat and turn into trophozoites which reside in the erythrocytes in a parasitophorous vacuole.
The trophozoites multiply and transform into schizont which then ruptures to release merozoites. The merozoites again enter the erythrocytes, turn into trophozoites which then release merozoites in the bloodstream after undergoing schizogony and hence, this cycle continues. However, some merozoites undergo a sexual stage rather than going through the above cycle again and again. These merozoites form gametocytes of two types: the male gametocyte, microgametocyte and the female gametocyte, macrogametocyte.
Image Credit: sciencedirect
Trophozoites feed on the haemoglobin and turn it into hemozoin(a granular pigment) by biocrystallization. The process of haemoglobin degradation takes place inside the digestive vacuole of the parasite. Two main proteases(enzymes that break down proteins), plasmepsin and falcipain are involved in the process. Read about malarial hemozoin in detail here.
LIFE STAGES OF PLASMODIUM IN MOSQUITOES(PRIMARY HOST)
Gametocytes → Gametes → Zygote → Ookinete → Oocyst → Sporozoites
When a female Anopheles mosquito bites an infected human, the gametocytes also enter the body of the mosquito and turn into gametes. The microgamete nucleus divides three times and produces eight nuclei. Each nucleus fertilizes a macrogamete and forms a zygote. After some time, the zygote differentiates into a motile form called ookinete. The ookinete then passes through the peritrophic matrix and epithelial cells to enter the midgut of the mosquito. Once, in the midgut, the ookinete forms a cyst and is now known as an oocyst. The oocyst divides to form numerous sporozoites and ruptures to release sporozoites. The process of the formation of sporozoites is called sporogony. From here, the sporozoites travel to the mosquito's salivary glands and are transferred to another person during a mosquito bite.
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