Protozoa - Plasmodia (Malaria)

Plasmodia are protozoa; specifically, they are a type of Apicomplexa. Life cycles have a sexually reproductive stage, which occurs in mosquitoes, and, an asexually reproductive stage, which occurs in humans. Malaria is an infection of the red blood cells, and is characterized by cyclical episodes of acute fever, chills, headache, and other flu-like symptoms. Pregnant women, young children, the elderly, and immunosuppressed patients are most vulnerable to complications from malaria, which can be fatal. Transmission Female Anopheles mosquitoes, which are found in tropical and subtropical areas, transmits Plasmodia parasites to humans during feeding. Transmission can also occur during organ transplant, from a mother to her fetus, or via infected needles; these routes are less common. Prevention Relies upon reduction in mosquito populations and the consistent use of mosquito repellant and barriers; prophylactic drugs can also be administered. Additionally, it seems that some blood cell polymorphisms, such as those that alter the surface antigens or shape of the red blood cells, may offer some protection against some malarial parasites. Treatment Patients with malaria can be treated with antimalarial drugs, and severe cases require combination therapies. We'll draw a very generalized diagram, with a focus on infection of the human host. For details, click here. Human infection is characterized by asexual reproduction in two specific anatomical sites: – The exo-erythrocytic phase, which occurs in the liver. – The erythrocytic phase, which occurs in the red blood cells. Infection of the human host is initiated when a mosquito transfers the parasites from her saliva during a blood meal. The parasites quickly travel to the liver, where they invade hepatocytes. Following asexual reproduction in the liver, the parasites are released into the circulation and invade red blood cells. Within the red blood cells, the erythrocytic phase begins, and asexual reproduction occurs again. Replicating parasites cause the red blood cell to rupture and spill the parasites, as well as toxins and other substances, into the circulation. The presence of these materials in the blood causes the clinical symptoms of malaria. The newly released parasites invade new red blood cells and undergo additional rounds of asexual reproduction. With every round of asexual reproduction, new blood cells rupture and produce the cyclical malarial episodes. Some parasites leave the asexual reproduction cycle and enter the sexual reproduction pathway. These give rise to gametocytes which travel freely in the blood. Gametocytes are precursors to gametes, which are the male and female sex cells. Then, when a mosquito feeds upon the infected blood, she'll ingest the gametocytes. In the mosquito, the gametocytes will complete the sexual reproductive phase to produce new parasites that can be transferred to a new human host. Important differences between the species include geographical distributions, the types of red blood cells they infect, and their histological appearances. To see additional images of Plasmodia species, click here Plasmodium falciparum is a significant cause of malaria, and is the predominant species in Africa. It causes the most severe form of malaria, and is associated with the highest mortality rates. This is because, unlike other Plasmodium species, Plasmodium falciparum can invade any and all red blood cells. It has a short reproductive cycle, so it multiplies rapidly and causes significant parasitemia. Significant red blood cell destruction causes anemia, and the toxins and other substances that are released cause severe clinical symptoms. Cells infected by Plasmodium falciparum tend to adhere to the walls of the microvasculature, which obstructs blood flow and causes organ damage. For example, when this occurs in the brain, cerebral malaria can be fatal; damage to the kidneys or lungs are also serious complications. Mature forms of Plasmodium falciparum are sequestered in the microvasculature, so look for the immature "ring stage" in peripheral blood smears. Multiple parasites may be found in a single cell; their rings are often seen on the edges of red blood cells (called the "appliqué position" because they appear to be "stuck" on the edges). Maurer's clefts may also be visible. Little to no red blood cell distortion occurs. Mature gametocytes, which travel freely in the blood, are crescent shaped. Plasmodium vivax is also a significant cause of malaria, especially in Asia, Latin America, and some parts of Africa. Plasmodium vivax can produce relapsing malaria, because it can create dormant forms that reside in the liver. Plasmodium vivax can only invade young immature red blood cells; notice that this contrasts with Plasmodium falciparum, which can invade red blood cells of any age. Plasmodium vivax can be seen in the peripheral blood during any stage of its development. The mature trophozoite has an amoeboid shape. Infected red blood cells become enlarged and distorted. With Giemsa staining, Schüffer's dots may be visible. Be aware that that it was thought that individuals whose red blood cells lacked the Duffy antigen were protected against Plasmodium vivax, however, reports of infections in Duffy-negative patients paints a more complicated picture. Plasmodium ovale is found in Sub-Saharan Africa and the western Pacific Islands. Similar to Plasmodium vivax in the following ways: causes relapsing malaria via dormant forms in the liver; infects and distorts immature red blood cells; and Schüffer's dots are seen with Giemsa staining. These dots can give cells infected by Plasmodium ovale a characteristically "stippled" look. Infected cells may have jagged edges, called fimbriae. The trophozoite form is relatively compact. Multiple ring forms may be present within a single cell (like Plasmodium falciparum). Plasmodium malariae is found worldwide, but is a less common cause of malaria. Plasmodium malariae invades senescent red blood cells, which are not distorted due to their rigid cell membranes. In the blood, Plasmodium malariae schizonts can have a characteristic rosette pattern, and their trohpozites tend to appear as "bands" across the red blood cell. Plasmodium knowlesi is a more recently recognized cause of malaria in humans in Southeast Asia; it is the most common cause of malaria in Malaysia. Often mistaken for Plasmodium falciparum or Plasmodium vivax.