The plasmodium parasite develops in the female anopheles mosquitoes and humans. Infection in humans is associated with the presence of multiple (thousands) of antigens against the parasite in the human body. Throughout the life stages, the parasite is known to change which leads to the production of different antigens. Lately, the parasite had developed different strategies to confuse the human immune system.
The life cycle of the malaria parasite takes place in two main stages (hosts): The female Anopheles mosquitoes and humans. In humans it strives in the liver and spreads infection in the red blood cells. The parasites destroy red blood cells and release daughter parasites known as merozoites. The cycle continues by invading more red blood cells and affecting major organs of the body if not treated early.
The plasmodium parasite constitutes of more than 5000 genes due to which it can thrive in multiple cell types, both in invertebrate and vertebrate environments. They also contain specialized proteins that help invade the hosts and other host-related immune responses.
Based on the host environment, the parasite goes through a series of developmental stages which are as follows: Sporozoites (The infectious form of the parasite released or injected by the mosquito), Merozoites (Stage that includes invasion of the red blood cells), Trophozoites (Stage involved in the multiplication of red blood cells), and Gametocytes also known as the sexual stage (Multiplication in the red blood cells). The parasites have their own shape and size at each stage. The parasites display unique surface proteins at each stage that would help in invading the host immune response. Thus, malaria treatment is complex and thus the development of a vaccine is all the more difficult.
The Life Cycle of Malaria Parasite
The malaria life cycle begins with the infection of an individual by an infected female Anopheles mosquito. The parasite is released into the bloodstream as Sporozoites. The sporozoites are passed into the liver. In the next 7 to 10 days, they multiply by sexual means in the liver cells.
On the other hand, the parasites in the animal host form merozoites that are released in the liver cells and go to the lung capillaries in through the heart. The merozoites disintegrate and enter the blood stream. Once they enter the bloodstream, the merozoites multiply and spread across major organs of the body. The cycle repeats each time the cells burst it causes fever.
Most of the blood cells are bound to replication and develop into sexual forms of the parasite which leads to gametocytes. These cells are known to circulate into the bloodstream. Other blood cells lead to the asexual from of multiplication.
Once a mosquito bites an infected human it ingests the sexual form of the parasites known as gametocytes which further develop into gametes. In the female mosquito, the gametes develop into ookinetes that enter the midgut. On the surface of the midgut the ookinetes form oocysts. Within the oocysts thousands of sporozoites develop. These oocysts burst into the body cavity that eventually reach the salivary glands. The human cycle of infection commences when the mosquito bites an individual. Two of the most common forms of parasite that cause malaria are the Plasmodium ovale and Plasmodium vivax. They are known to cause relapses and infection even after months and years after the initial infection. These parasites remain within the liver cells and burst into the bloodstream once multiplication has reached its peak.
One of the best treatment available for malaria is primaquine which affects the parasites in the liver cells. However, this treatment cannot be used in individuals with G6P-D deficiency.
Figure 1. Malaria Life Cycle
The only species of mosquito that spreads the plasmodium parasite is the female Anopheles. However, the parasite cannot be spread through other species of mosquitoes. In Africa, Plasmodium Falciparum is known to cause malaria. It also causes malignant malaria.
In summary, the parasite undergoes three3 stages of asexual development known as exoerythrocytic schizogony, blood stage schizogony, and sporogony. These are known to form merozoites and sporozoites. Sexual reproduction is known to occur in human host while asexual reproduction is known to occur in animal host. Thus, the malaria life cycle is a complex process and takes placed in both human and animal hosts.