3.15 Study Guide 2: "What is the cause of malaria? Lifecycle of malaria parasites?"

SAQ: Explain how malaria is definitively diagnosed.

- A thick blood film is taken to diagnose malaria. - A thin film is commonly used to confirm the species of malaria parasite, as the morphology of the malaria parasite is more easily observed in individual erythrocytes which are seen in the thin film.

SAQ: Define the following terms: Exo-erythrocytic stage (aka pre-erythrocytic stage):

- Exo-erythrocytic stage (aka pre-erythrocytic stage): The parasite develops within hepatocytes, a process called tissue schizogony. - This may last 6 to 16 days, depending on the type of malaria. At the end of this stage, the infected hepatocytes rupture, releasing merozoites into the circulation. For P. falciparum, no hepatic parasites remain after this first stage. - For P. vivax, a dormant hepatic stage persists in the form of hypnozoites, responsible for recurrence of infection months or years later.

SAQ: Explain the terms thin film and thick film, in regard to the diagnosis of malaria. Explain the advantage of one vs the other.

- In a thick film of blood, the drop of blood is left as is, as a drop of blood on a slide, and let dry that way. - A thin film is prepared in the same way as a peripheral smear: the drop of blood is spread across the slide in a very thin layer (basically a single layer of erythrocytes). - A thick film allows for examination of a larger amount of blood at one time than does a thin film, which makes it easier to diagnose malaria in someone with a light parasitemia. It is more difficult on a thick film, however, to determine the species of malaria parasite, while a thin film makes this determination much easier. - Generally speaking, a thick film is for the diagnosis of malaria, while a thin film is for diagnosing the species. In busy health centres in malarious areas, the thin film is examined only if the thick film is positive.

SAQ: P. falciparum characteristically appears on a blood film only as a ring form trophozoite, and schizonts are rarely seen; whereas a blood film of someone with P. vivax will show all stages of the parasite, from ring forms to schizonts. Explain what a ring form is. Explain the reason for the difference.

- The ring form is an early developmental form of a trophozoite, usually characteristic of P.falciparum. - Erythrocytes infected with P. falciparum undergo cytoadhesion within the capillaries, which occurs as the parasite matures within the red cell. Thus, red cells with immature forms of the P. falciparum parasite are less likely to exhibit this "stickiness", and more likely to appear freely in the circulation and be caught during a blood draw. - P. vivax does not have this effect on RBCs, thus the parasite in all stages of maturity is likely to be seen on a blood film. (Although even here, if periodicity of symptoms has been established, most of the parasites should be at the same general stage of development).

SAQ: What is meant by asexual reproduction of the malaria parasite? What is meant by sexual reproduction of the malaria parasite?

Asexual reproduction occurs in the hepatocytes and in the erythrocytes, and is also known as schizogony. - Sexual reproduction occurs only in the stomach of the mosquito after ingestion of male and female gametocytes which are produced within the human erythrocyte during schizogony.

SAQ: Define the following terms: Erythrocytic Stage

Erythrocytic stage: This begins as the merozoites invade the erythrocytes, and begin schizogony, asexual reproduction of the malaria parasite within the erythrocyte. Schozonts are formed, which eventually rupture the red cell, releasing new merozoites.

SAQ: Define the following terms: Gametocyte and Oocyst

Gametocyte: Erythrocyte schizogony gives rise not only to new merozoites, but also to gametocytes, which are responsible for sexual reproduction of the malaria parasite. The mosquito obtains male and female gametocytes from an infected person during a blood meal, after which sexual reproduction of the parasite occurs in the mosquito. Oocyst: The structure formed on the outer surface of the mosquito stomach by a fertilized macrogamete. It is from the oocyst that sporozoites will emerge and travel to the salivary gland, to be injected into a new host during a subsequent blood meal.

SAQ: Define the following terms: Hypnozoite

Hypnozoite: Residual dormant hepatocyte form, characteristic of P. Vivax infection, which is responsible for recurrence of infection months or years after the primary infection.

SAQ: Define the following terms: Merozoite

Merozoite: The parasite form which emerges from the hepatocyte following tissue schizogony, and which emerges from the erythrocyte following erythrocyte schizogony. From either place, the merozoite infects new erythrocytes.

SAQ: Define the following terms: Schizont

Schizont: The result of cell division of the trophozoite, whereby the red cell is filled with new merezoites, the collection of which is called the schizont.

SAQ: Define the following terms: Sporozoite

Sporozoite: The sporozoite is the last stage of the parasite in the mosquito: oocysts in the wall of the stomach of the mosquito liberate sporozoites, which migrate through the body cavity to the salivary glands, from where they will be injected into the circulation of the host. - Following injection into the host, sporozoites will remain in the circulation for about 30 minutes, before entering the hepatocytes, where the exo-erythrocytic stage begins.

SAQ: Define the following terms: Trophozoite

Trophozoite: An early red cell form, characterised by one nucleated parasite.

SAQ: Apart from the Anopheles mosquito, how else may malaria be transmitted?

By blood transfusion, where the donor's RBCs are infected with the plasmodium parasite