Malaria: The King of Tropical Diseases

Malaria—a mosquito-borne infectious disease—is widespread in tropical and subtropical regions of Earth. One of a variety of tropical diseases, Malaria has had a long and insidious history of interaction with mankind.

image of a mosquito - the prime vector for malaria

Tropical diseases occur in a zone stretching roughly from 1,000 miles north of the equator to 1,000 miles south of the equator and generally do most of their damage in poor, developing countries. Malaria has existed for thousands of years, though it was eradicated from industrialized nations through aggressive attack on the Anopheles mosquito using the now-banned insecticide DDT.

The distinguishing features that tie most of them together are the very complex life cycles of these organisms and their parasitic nature.

Tropical diseases are very different from what we’re used to studying—bacteria, viruses, and diseases of the first world—because these diseases generally do the most damage in the third world and emerging countries. The distinguishing features that tie most of them together are the very complex life cycles of these organisms and their parasitic nature; they invade, they multiply, and they thrive inside the body. If they’re very efficient, they don’t kill the hosts but sap their strength, take their vitality, but they keep them barely alive to ensure their own survival. When they do fail and they’re not so efficient, the host dies and then, often, so does the parasite.

There is a big group of very important diseases that live in the bloodstream. The Plasmodium species cause malaria. The trypanosomes cause African sleeping sickness; the bite of the tsetse fly is pretty well known to most people. There is also a species of Trypanosoma that causes Chagas disease in South America. Another devastating disease that’s transmitted by the reduviid bug is called the “kissing bug” because it bites people on the face while they’re asleep and causes a disease that kills the nerve endings—talk about specialization—in the colon. These patients die because they can no longer contract their colons during the process of digestion. In all, it’s quite a wide and varied group of organisms doing amazing damage and in huge numbers.

The History of Malaria: 4000 B.C.E. to the Present

drawing of the bust of Hippocrates, who may have been the first person to describe malaria
Hippocrates described something that sounds like malaria in soldiers who were encamped in swampy areas.

Let’s look at the history of the malaria parasite. Around 4000 B.C.E., there was a convergence of domestic agriculture, wet growing fields, and people getting together with animals. The animals brought the mosquitoes to the people. This was probably where agriculture did its worst by introducing species that people weren’t contacting before. About 2,000 years later, there was a disease with malaria-like symptoms that was described in the Chinese medical texts. Herbal drugs were used against it, perhaps some of the drugs we’re looking at today. In 500 B.C.E., malaria was described in some of the Indian journals. Hippocrates also describes something that sounds like malaria in soldiers who were encamped in swampy areas. Caesar thought about draining the marshes where troops were in order to prevent malaria, even though they didn’t know what it was. Then, in 300 C.E., the Chinese dynasty of Yin used something called sweet wormwood, which was qinghaosu, to combat this disease. We’re starting to use this again, 1,700 years later.

portrait of Horace Walpole, who gave the disease Malaria a name
Horace Walpole, the English author, gave the term mal-aria, “bad air.”

Quite a bit later in 1600, the tree bark from the Cinchona tree was used to treat malaria, and this was quinine. Quina quina was the “bark of barks.” This was a true antimalarial drug. Horace Walpole, the English author, gave the term mal aria, “bad air.” People finally made the connection of the victims of this disease tending to be out and about at night, which is when the Anopheles mosquito—about which they knew nothing—goes out to feed. British soldiers began drinking tonic water with quinine in it in their gin and tonics after the sun went down over the yardarm. Actually, this dose of quinine probably had nothing to do with curing them or preventing malaria.

Mosquitoes, Parasites, and the Third World

microscopic image showing Release of malaria parasites from red blood cell
Release of malaria parasites from red blood cell

Malaria has been around for a long time. It is still wreaking havoc with populations, causing a great deal of mortality and misery, especially in the tropics and in the third world. It is a protozoan. This means it’s unicellular and eukaryotic, and has a good nucleus. It’s fairly big compared with bacteria, and it’s transmitted by insect vectors—malaria is specific to the Anopheles mosquito. The other protozoa can be transmitted by the oral-fecal route, another very common way to get these diseases. They can cycle almost anywhere: in the intestines, the liver, the spleen—lots of organs—and in the blood circulation.

The Plasmodium species are intracellular parasites. Basically, they’re obligate intracellular parasites, but they have extracellular cycling time. They infect about 350 to 500 million people every year, and they kill about 1 to 3 million a year. Close to a million of those are children under the age of 5.

little girl drinking from a clean, malaria free water source
Twenty-five hundred children per day in the third world are dying of malaria, about 100 an hour, and about 2 per minute.

Twenty-five hundred children per day in the third world are dying of malaria, about 100 an hour, and about 2 per minute. The deaths are primarily in Sub-Saharan Africa in the malaria belt, that plus-and-minus 1,000 miles of the equator. It is the leading cause of death of children under 5 in this area of the world. It really should generate outrage because if these were white children living in America or South America or Europe, we would put an end to it immediately, as we have done. We had malaria here; we don’t have it anymore. It’s really only a matter of money and the determination to do it, and it’s really an outrage, a social outrage and an ethical outrage, that we’re not putting an end to this.

There are about 1,500 cases per year of malaria in the United States—we call it airport malaria because there are no malarial vectors. Since we’ve gotten rid of the Anopheles mosquito, we’ve effectively broken the cycle. Most of these cases are from travelers. I was fortunate to go to medical school in New York City, and we saw more tropical diseases there than they see in many tropical countries. There’s so much air traffic that people get infected abroad. They come to New York either in passage, or they live there, and they get the disease when they are back home. So we got to see a lot of tropical diseases.

The Four Varieties of Malaria

The Plasmodium falciparum is really the most deadly form, and it’s kind of by itself as a group. Patients who contract this disease can be dead in 48 hours. 

There are four varieties of malaria. The Plasmodium falciparum is really the most deadly form, and it’s kind of by itself as a group. The Plasmodium falciparum is really the most deadly form, and it’s kind of by itself as a group. Patients who contract this disease can be dead in 48 hours. It’s called “black water fever” because the parasite disrupts the red cells, releasing massive amounts of hemoglobin into the serum, which passes through the kidney, and it turns black or dark brown. These patients have urine that’s very, very dark—the black water. They die very, very quickly.

The other three groups—Plasmodium ovale, vivax, and malariae—are less virulent. They don’t kill quite as fast, but they still make patients very, very sick, and they’re a major cause of illness and debilitation throughout the tropical third world.

Malaria and Evolution

illustration of sickle cells in the bloodstream
People with Sickle Cell trait have a higher survivability of Malaria than average.

A couple of things have happened over the years. There have been a couple of mechanisms to help host resistance in malaria patients. One has been an interesting piece of evolution. Patients who have a disease called sickle-cell anemia—an inherited disease mostly in people who are of African origin—are immune to malaria. There are two forms of the disease—sickle-cell disease and sickle-cell trait. The patients who have the disease have two bad chromosomes; the people who have the trait have one bad chromosome. One patient is sicker than the other. Red cells become shaped like a sickle; they curl up in conditions of low oxygen saturation. These patients are miserable, but the malaria parasite can’t replicate in their cells. So there has been natural selection to select for a very bad disease because it’s not as bad as malaria. It doesn’t tend to kill patients quite as much or as early in life, so these patients go on to have children with the disease before they die of sickle-cell anemia. Meanwhile, they’re passing on resistance to malaria to their children.

From the lecture series The Human Body: How We Fail, How We Heal
Taught by Professor Anthony A. Goodman, M.D., F.A.C.S., Montana State University
You can Learn More about Malaria in the lecture series An Introduction to Infectious Diseases
Taught by Professor Barry Fox, M.D., University of Wisconsin

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