Plasmodium Falciparum: Life Cycle & Morphology

Instructor: Bridgett Payseur

Bridgett has a PhD in microbiology and immunology and teaches college biology.

Plasmodium falciparum is a parasite that causes malaria in humans. This lesson will look at the various stages of its complicated life cycle, which involves both mosquito and human hosts.

Human Malaria

Malaria is a disease in which red blood cells are infected by parasites. Malaria is a major public health concern, with up to 200 million cases occurring annually, resulting in over 1 million deaths. Most of these deaths happen in young children, making malaria an especially cruel infection.

Map showing risk of Malaria in red
Map of Malaria Risk

Plasmodium falciparum is one of the four or five parasites (this number will depend on which scientist you ask) that causes malaria in humans, and is responsible for most malaria deaths. What makes P. falciparum more deadly than other human malaria parasites is that it causes the infected red blood cells to get caught within capillaries, blocking blood flow to vital organs. If this happens in the brain, the result is cerebral malaria, which is very dangerous.

Malaria parasites have a complex life cycle, involving different hosts and different systems within those hosts. The life cycle of P. falciparum begins in the mosquito, and then goes to the liver and finally blood of a human. During the blood stage of infection, it is possible to observe P. falciparum morphology, or what it looks like, under a microscope.

Malaria Life Cycle
Malaria Life Cycle

Mosquito Stage

As a warning, a lot of the stages of malaria have complex, scientific names. On the plus side, it will help you feel smart when you can throw out large, impressive words at your next dinner party. This lesson will try to break down these words so you can get through without needing an encyclopedia.

We will start our talk about the malaria life cycle by looking at the mosquito stage. Gametocytes (gam-EET-oh-site) are the sexual reproductive form of the parasite. They are kind of like human gametes; they can be either male, like a sperm cell, or female, like an egg. The gametocytes are taken in by a female mosquito when she takes a blood meal from an infected human host.

The gametocytes will combine in the mosquito's gut, similar to how sperm and an egg combine in humans. However, rather than forming a new human, the gametocytes join to form a sporozoite (spore-oh-zo-ite), the form of the parasite that can infect human liver cells.

Many of the stages of malaria have the ending -zoite, which means small cell. For these stages, you know that the cell is small. Both the gametocyte and sporozoite form of the parasite are uninucleate in the mosquito, meaning they only have one nucleus.

The mosquito is acting as an unintentional transport system for the P. falciparum parasite to move from one human to the next.

Liver Stage

When a female mosquito takes a blood meal from a human, sporozoites are injected into small capillaries of the person. The sporozoites immediately go to the liver cells to begin replicating. Rather than simply copying themselves, the parasites form schizonts (SKIZ-onts), huge, multinucleated structures. It's easy to remember that schizonts are big cells, because they don't have the -zoite ending that most of the other forms have.

The schizont is filled with thousands of single-nucleus merozoites (me-ro-zo-ite). Once the merozoites are fully formed, the schizont will burst, like a scene from a 1950s horror movie, to release its little minions out into the rest of the body. It is the merozoites that will go on to infect red blood cells.

Blood Stage

As the schizont grows in liver cells, it eventually breaks open, releasing all of the merozoites. The merozoites can then move to the blood and begin infecting red blood cells. Once the parasite has invaded a red blood cell, it is called a trophozoite (trof-oh-zoh-ite). It helps to remember this name by thinking of the malaria parasite claiming the red blood cell as its 'trophy'.

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