The reason I want to spend a few minutes on malaria is because it has such a large reach. About half of the global population is at risk for getting malaria, and it causes a lot of deaths, about 500,000 per year, and half of those approximately are children. It's transmitted by the Anopheles mosquito, so just to differentiate, we said the Aedes aegypti is responsible for transmitting Zika. We said it's also responsible for chicken gunya, yellow fever, and dengue fever. So the Anopheles mosquito is a different species of mosquito that transmits malaria.
Malaria is caused by a protozoan parasite. The genus is Plasmodium, and there are several different species. So Plasmodium vivax is very common, Plasmodium ovale. Your book actually lists, I think, several more.
The other one though that I do want to mention is Plasmodium falciparum because this is the most lethal species. So the vast majority of deaths from malaria are the result of Plasmodium falciparum. I want to spend a moment talking about the life cycle of Plasmodium because it's actually super interesting.
And don't stress out because you don't need to memorize it, but it does help you understand a little bit about why this particular disease is very debilitating. So let's start where the mosquito is biting the human. So you can see the mosquito is biting the human and it's going to inject a life cycle stage called a sporozoite into the body.
It gets into the circulatory system and then those sporozoites go directly to the liver and they reproduce asexually in the liver and they over time turn into something called a merozoite. And the merozoites, what they do then is they travel to a red blood cell. They infect a red blood cell. They will divide and divide and divide in that red blood cell until the red blood cell ruptures. Then these merozoites, the new merozoites, they will infect other red blood cells and the cycle will start over again.
So those blood cells will rupture and then the new merozoites will infect new red cells and this can occur over and over and over again depending on the species of malaria every two to three days or so. And along with the rupture is when you see a lot of the symptoms of malaria occurring. So they kind of occur in this cyclical way as a result of these cyclical ruptures of red blood cells that all happen at the same time.
Some of the merozoites will then go on to another life cycle stage called a gametocyte. And this is where the male and female gametes are formed. The Plasmonium is actually one of the rare protozoan species that reproduces sexually.
So these will go back into the bloodstream and circulate. Eventually, they get taken up by a new mosquito. So this new mosquito bites you. You have malaria. It collects some of these gametocytes.
They're going to travel into the mosquito now and go through a bunch of other life cycle stages in the mosquito. Ultimately, they fuse. They form a zygote. And new sporozoites are formed, which then travel back to the salivary glands of the mosquito.
Now, this mosquito then bites another person. and it's going to inject those sporozoites into them and of course then the cycle starts over again. An individual with malaria typically exhibits some very classic symptoms.
Those include cycles of extreme fever and chills. This can also have headache and muscle aches, some nausea and vomiting as a result of this. Usually that's followed by profuse sweating and fatigue and this occurs as a result of this synchronous rupture of red blood cells. So every time those red blood cells rupture, you go through this cycle again.
If it's a two-day cycle, we talk about tertian malaria. If it's a three-day cycle, we say quartan malaria. If you've ever read any novels, historical novels, you might have heard people refer to tertian fever. So often it's the case that those descriptions actually are of somebody with malaria.
All of these repeated cycles of lysis of red blood cells can have some pretty serious consequences. Remember that the spleen has the responsibility to filter out things that don't belong in the blood. Sometimes people refer to somebody with malaria as having sludge blood because of all the hemolysis and sort of the debris from all these red blood cells being lysed. The other way that people often describe the blood of somebody with malaria is that it looks like Kool-Aid. So instead of that bright red rich color that you're used to, with all those red blood cells lysed, it can actually end up being kind of a pale red, so that's obviously pretty terrible.
If you don't have enough red blood cells, then you're not going to be able to get oxygen to your tissues. That's going to cause some of that exhaustion that's characteristic of malaria. The immune response to malaria can result in organ failure, and ultimately death can happen, especially if you are infected with Plasmodium falciparum.
Another complication of Plasmodium falciparum is called encephalopathy. So this is often referred to as cerebral malaria, and it's a neurological complication with a high mortality rate of about 15 to 25%. There are some excellent drugs to treat malaria. Artemisinin is actually the first-line drug currently and it's usually used in combination with a few other drugs in order to combat the development of resistance.
In spite of this, resistance is developing anyway and this usually means that the drugs work but they take longer to clear all the parasites out of the body. There is a vaccine approved by the World Health Organization just recently in 2021. It's for children under two. It has a moderate efficacy of only about 40%.
And in order to get to 40%, you need to have four injections over 18 months. So it's not a one-and-you're-done kind of vaccine. So many people don't feel that this is nearly good enough, and work will continue on trying to develop a better vaccine for malaria. But generally, when people think about preventing malaria, they think about a multimodal method where people use insecticides, bed nets, and the vaccine to try to reduce the levels of malaria transmission. Next up is another protozoan parasite, Trypanosoma cruzi, that causes Chagas disease.