Malaria is a serious parasitic infection caused by a few Plasmodium species which are unicellular protozoans and transmitted by the Anophilus mosquito. Once Plasmodium enters the bloodstream, it infects and destroys mainly hepatocytes and red blood cells, which causes a variety of symptoms or even death. Commonly affected groups include young children, pregnant women, immunocompromised hosts like AIDS patients, and travelers. Malaria is a global health problem predominant in South America, Africa, South Asia and Southeast Asia.
Although many Plasmodium species have been identified, only five of them cause diseases in human. They are Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale, Plasmodium malariae and Plasmodium nalesi. And Plasmodium nalesi does not have a mosquito vector. It is transmitted by infected monkeys. Malaria starts when a plasmodium-infected female anopheles mosquito hunts for a blood meal in the evening and through the night.
Mosquitoes are attracted to exhaled carbon dioxide and foot odor. At this time, plasmodium is in its developing stage, called sporozoite. Reside within the mosquito's salivary gland. When the mosquito draws blood from a normal person, the sporozoites can be spilled into the blood. Within minutes, sporozoites reach the liver and enter into the hepatic parenchymal cells.
where they undergo asexual reproduction. Also known as schizogony. Over the next one to two weeks, sporozoites mature into merozoites. While host hepatic parenchymal cells die. However, sometimes Plasmodium vivax and Plasmodium ovale sporozoites become hypnozoites and stay dormant over months to years.
These hypnozoites can cause latent infection, causing a long delay between the initial infection and symptoms from the disease. This phase of plasmodium in the liver is called exoerythocytic phase. And it is generally asymptomatic.
Merozoites formed inside the liver cells are released into the blood. In the blood, merozoites bind surface receptors on the red blood cells and invade them. Plasmodium falciparum and plasmodium ovale infect all ages of red blood cells.
Plasmodium vivax prefer reticulocytes. And plasmodium malariae and plasmodium nalesi invade older red blood cells. Once inside the red blood cell, merozoites undergo asexual reproduction. This phase is known as erythrocytic phase, generally lasts 2-3 days.
In the first stage of the erythrocytic phase, the merozoite looks like a tiny ring within the erythrocyte. Known as the early trophozoite or ring form. In the second stage, the ring form trophozoite grows.
And it is known as the late trophozoite. In the third and final stage, the parasite grows more by digesting hemoglobin and becomes a schizont. This is the actual replicative stage in which the parasite undergo mitosis and gives rise to a lot of merozoites, which can get released into the blood. Instead of getting into the erythrocytic phase again, some of the merozoites undergo sexual reproduction, also known as gametogony, where they divide and give rise to gametocytes. These gametocytes will remain inside the red blood cell.
When another Anopheles mosquito takes a blood meal from this infected person, the gametocytes can reach the mosquito gut and give rise to gametes. This phase of plasmodium life cycle is called spirogyny. Gametes fuse together to form a zygote.
The zygote becomes an eukenete and then an oocyst, which ruptures inside the mosquito gut and give rise to thousands of sporozoites. The sporozoites then reach the mosquito's salivary gland. The clinical presentation of malaria may vary, depending on the Plasmodium species.
Major symptom is fever with choles and rigors. Others include splenomegaly anemia especially in falciparum malaria. Fever corresponds to the rupture of red blood cells.
For Plasmodium malariae, fever occurs in every 72 hours. For Plasmodium vivax, Plasmodium ovale and Plasmodium falciparum, fever occurs every 48 hours. And for Plasmodium nalesi.
Fever occurs every 24 hours. Most Plasmodium species cause mild symptoms, known as uncomplicated malaria. Out of all, Plasmodium falciparum causes more severe disease.
And in most severe cases, patient can get cerebral malaria, acute respiratory distress syndrome, acute renal failure and severe anemia. This is known as complicated malaria. There are two main diagnostic methods. Microscopy and antigen detection. Microscopy is the gold standard method in diagnosing malaria.
In this method, three samples of thin and thick blood smears are looked under the microscope. By the thick film, we can identify the parasite sitting in the red blood cell. And by the thin film, we can directly identify the species. This identification is based on specific morphological patterns of Plasmodium trophozoites. For example, Plasmodium falciparum have earphone-shaped rings.
And Plasmodium vivax and Plasmodium ovale infected red blood cells resemble an oval shape. In Plasmodium malariae, there is a large chromatin dot in the center of the ring. Microscopy cannot do in the field and need experienced workers to get the results.
In antigen detection, we look for parasite-derived products such as parasitic lactate dehydrogenous. Dipstick methods are used in antigen detection, and it can be done in the field. However, it has a very low sensitivity compared to microscopy.
Treatment depends on the stage of the infection. Prophylactic treatment for non-infected people to prevent getting infected. Anti-malarial drugs are used to kill merozoites and gametocytes.
Type of the drug depends on the severity of the disease, plasmodium species, age and anti-malarial resistance status of the parasite. Radical treatment includes killing hypnozoites. There are three main preventive methods. Prevention of man-vector contact by sleeping under insecticide bed nets, full-body clothing and using mosquito repellents.
Reduce vector density by destroying breeding sites and using insecticide sprays. And finally, we can reduce parasite population by early detection and treatment.