Bridgett has a PhD in microbiology and immunology and teaches college biology.
Malaria is a major public health concern in Sub-Saharan Africa, Central and South America, and parts of Southeast Asia. In 2013, there were nearly 200 million cases of malaria, resulting in a half million deaths.
Malaria is caused by protists, which are single-celled eukaryotes. There are four or five Plasmodium species that can cause malaria in humans. Each species can cause distinct symptoms and complications, and a well-trained technician can even differentiate between the four species with a microscope. However, all of the malaria parasites have a similar life cycle.
The life cycle begins in a female mosquito, who will bite a human to take a blood meal and pass on the malaria parasite to the human host. Once inside the human, the malaria parasite first goes to the liver. Here, it will replicate, building its forces to begin its attack on the red blood cells.
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Once the malaria parasites have replicated in the liver, they go on to infect the red blood cells. Inside the red blood cells, parasites will replicate again and break out of the cell to continue the infection process. The destruction of red blood cells is the stage of infection that results in malaria symptoms. Because the malaria parasites tend to replicate at the same time, the patient often shows the malaria symptoms in bursts. Some of the malaria parasites in the blood will develop into a form that can be taken up by a mosquito, to be passed on to a new host.
Now that we've got a handle on the life cycle of the malaria parasites, let's get a look at some of their individual characteristics.
Plasmodium falciparum is the most common and most lethal of the human malaria parasites. P. falciparum can cause infected red blood cells to stick together and clog up capillaries. This can prevent blood flow to major organs, which is why P. falciparum is so fatal. If the infected red blood cells block blood vessels in the brain, it is called cerebral malaria, and is very dangerous.
While P. vivax and P. ovale aren't as lethal as P. falciparum, they come with their own complicating factors. P. vivax and P. ovale are both able to remain dormant in the liver; that is, they can hide in liver cells without causing symptoms and without alerting the body's defenses to their presence. Both of these parasites can hide in liver cells for several years. This can make a diagnosis of malaria difficult, particularly if a person hasn't been in a malaria-prone area for some time. It also means that after a patient has been treated and cleared of malaria infection, they can relapse at a later time.
We've mentioned that malaria symptoms tend to come in bursts, because the infection of red blood cells often happens simultaneously. In most cases of human malaria, these bursts will happen every two days. However, P. malariae has a slightly longer replication time, so the symptoms will come three days apart instead. P. malariae can also cause a chronic infection of the red blood cells. This can last for a person's entire lifetime. The constant cycle of red blood cells being infected and destroyed can cause major strain on internal organs, particularly the kidneys.
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Including P. knowlesi as human malaria parasite is a bit controversial. P. knowlesi normally infects macaques, a type of monkey that lives in Asia. While unfortunate laboratory accidents have shown that P. knowlesi could also infect and cause malaria in humans, natural infection of humans hadn't been shown until recently. These cases have been very limited to small groups of people. It is incredibly important for scientists to monitor human infection of P. knowlesi, because it can replicate in about half the time of the other malaria parasites. Because of this, P. knowlesi parasites can quickly cause severe disease.
The parasites that cause human malaria have unique characteristics. P. falciparum is responsible for the most malaria-related deaths worldwide. P. vivax and P. ovale can hide in the patient's liver for years, only to re-emerge and cause a surprise case of malaria. P. malariae can continue infecting blood cells for years, leading to kidney damage. P. knowlesi replicates quickly, but has a limited range of humans to infect. Even though all of these parasite operate differently inside the human, they all share a similar life cycle going from mosquito, to human liver, to human blood, and back to mosquito.
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