Malaria and the life cycle of Plasmodium · Biology… | StemPy Lab
Course 25.1·Lesson 2 of 3
Malaria and the life cycle of Plasmodium
Malaria is caused by a protozoan, Plasmodium, and spread by the female Anopheles mosquito. The parasite lives in two hosts. It multiplies in the liver and in the red blood cells of a person, and the sexual stages unite in the gut of the mosquito. The fever comes when many red cells burst together.
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A disease with a long history
Malaria is one of the oldest and most serious diseases of humans. It is caused by Plasmodium, a protozoan. A protozoan is a single celled organism with a nucleus.
NCERT names three species that infect humans: Plasmodium vivax, Plasmodium malariae and Plasmodium falciparum. Of these, P. falciparum causes malignant malaria, the most serious kind, and it can be fatal. The WHO lists two more species, P. ovale and P. knowlesi.
The vector is the female Anopheles mosquito. Only the female sucks blood, which she needs for her eggs. The male feeds on plant juices.
The life cycle in two hosts
The parasite needs two hosts. The human is the intermediate host, where the parasite multiplies by asexual division. The female mosquito is the definitive host, where the sexual stages unite.
A drawing of the life cycle of the malarial parasite in two panels. The left panel, in the human, has four boxes. Box 1: sporozoites enter the blood in the saliva of the mosquito. An arrow leads down to box 2: liver cells, where the parasite multiplies. An arrow leads right to box 3: red blood cells, where merozoites invade and multiply. A note says that the red cells burst, releasing new merozoites and haemozoin, which cause chill and fever, and that the cycle repeats. An arrow leads down to box 4: some parasites form gametocytes in red cells. The right panel, in the female Anopheles mosquito, has four boxes. An arrow marked bite leads from box 4 to the box in which the mosquito sucks blood with the gametocytes. Then arrows lead to: in the gut the gametes fuse and form a zygote; the oocyst in the gut wall makes sporozoites; and the sporozoites reach the salivary glands. A long arrow marked bite leads from the salivary glands back to box 1 in the human.
In the human
An infected female Anopheles bites. Her saliva carries sporozoites, which enter the blood. This is the stage that infects a human.
The sporozoites reach the liver and enter the liver cells. They multiply there, and the liver cells burst and release merozoites.
The merozoites attack the red blood cells. They multiply inside, and the red cells burst and release new merozoites. These enter fresh red cells, and the cycle repeats.
When the red cells burst, a toxic substance called haemozoin is released. NCERT says that it causes the chill and high fever, which come back every three to four days.
Some of the parasites become the sexual forms, the gametocytes, inside red blood cells.
In the mosquito
A female Anopheles bites an infected person and sucks up blood with the gametocytes. This is the stage that infects the mosquito.
In the gut of the mosquito the gametes fuse, which is fertilisation. The zygote grows into an oocyst in the wall of the gut.
The oocyst divides and makes many sporozoites. They move to the salivary glands.
At the next bite the sporozoites enter a new person, and the cycle starts again.
The names merozoite and oocyst come from standard textbooks. NCERT uses the names sporozoite and gametocyte, and they are enough for NEET.
Why does the fever come in waves?
The red cells do not burst one by one. The parasites in one batch of red cells grow at the same pace, so many cells burst almost together. Then the patient has a chill, a high fever and sweating. Then the fever falls until the next batch bursts.
The time for one cycle depends on the species. In P. vivax it is about 48 hours, so the fever returns every second day. This is tertian fever. In P. malariae it is about 72 hours, so the fever returns every third day. This is quartan fever. The fever of P. falciparum is often less regular.
NCERT says that the fever recurs every three to four days. That counts the day of the first fever as well. The gap between fevers is 2 days in tertian fever, and 3 days in quartan fever.
Two graphs of body temperature in degrees Celsius against the day, for 14 days. In the upper graph, tertian fever, the temperature stays at 37 degrees until day 2 and then rises sharply to about 40 degrees on days 2, 4, 6, 8, 10, 12 and 14, a peak every 2 days (48 hours), and falls back towards 37 degrees between the peaks. In the lower graph, quartan fever, the temperature stays at 37 degrees until day 3 and then rises sharply to about 40 degrees on days 3, 6, 9 and 12, a peak every 3 days (72 hours), and falls back towards 37 degrees between the peaks.
You should find that the gap between peaks follows the species. A patient with a 72 hour cycle has fewer fevers in two weeks than a patient with a 48 hour cycle.
Treatment and prevention
Malaria is treated with antimalarial medicines, which a doctor must prescribe. It is prevented by avoiding the bites of the mosquito, and by removing the places where she breeds. The next lesson describes these steps.
Try it in Python
The program lists the days of fever.
Practise with new numbers
Common mistakes
Calling the parasite of malaria a bacterium or a virus.Plasmodium is a protozoan.
Saying that the male mosquito spreads malaria. Only the female sucks blood.
Mixing up the two infective stages. The sporozoites pass from the mosquito to the human. The gametocytes pass from the human to the mosquito.
Putting the fertilisation in the human. The gametes fuse in the gut of the mosquito.
Saying that the fever is caused by the entry of the parasite into the red cell. It comes when the cells burst and release haemozoin.
Remember these
Malaria: Plasmodium, a protozoan. NCERT names P. vivax, P. malariae and P. falciparum. P. falciparum causes malignant malaria.
Vector: the female Anopheles. Infective stage for the human: sporozoites. Infective stage for the mosquito: gametocytes.
Liver cells, then red blood cells. The cells burst and release haemozoin: chill and high fever.
Fertilisation in the gut of the mosquito. Sporozoites are stored in the salivary glands.
Tertian fever: every 48 hours. Quartan fever: every 72 hours.
Check yourself
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
Which stage of Plasmodium enters the human, and which stage enters the mosquito?
Where does the parasite multiply in the human?
What causes the chill and fever of malaria?
Where does fertilisation take place, and where are the sporozoites stored?
Why is the human called the intermediate host and the mosquito the definitive host?
References
World Health Organization (2024). Fact sheet: Malaria.
Cowman A. F. et al. (2016). Malaria: biology and disease. Cell, 167, 610 to 624.
NCERT (2023). Biology, Textbook for Class XII, chapter 7.
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