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.
Tropical diseases occur in a zone stretching roughly from 1,000 miles north of the equator to 1,000 miles south of the equator. Generally, most of their damage is done in poor, developing countries. Malaria has existed for thousands of years, though it was eradicated from industrialized nations through an 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 familiar pathogens 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 complex life cycles of these organisms and their parasitic nature; they invade, multiply, and thrive inside the body. If they’re very efficient, they don’t kill the hosts but sap their strength and take their vitality, but they keep them barely alive to ensure their own survival. When the host dies, then, often, so does the parasite.
This is a transcript from the video series The Human Body: How We Fail, How We Heal. Watch it now, on The Great Courses.
There is a big group of important diseases that live in the bloodstream. Specifically, it is the plasmodium species that causes 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 in the colon. These patients die because they can no longer contract their colons during the process of digestion.
In all, it’s a wide and varied group of organisms doing amazing damage and in huge numbers. To better understand the destructive power of these pathogens, let’s consider how the malaria parasite has impacted history.
The History of Malaria: 4000 B.C.E. to the Present
Around 4000 B.C.E., there was a convergence of domestic agriculture, wet growing fields, and people domesticating animals. The animals brought the mosquitoes to the people. This was probably where agriculture did its worst by introducing species that people weren’t interacting with before. About 2,000 years later, there was a disease with malaria-like symptoms that was described in Chinese medical texts. Herbal drugs were used against it, perhaps even 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 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. Nearly 1,700 years later, modern medicine has turned back to this plant.
Quite a bit later in 1600, the tree bark from the Cinchona tree was used to treat malaria, called quinine. Quina quina was the “bark of barks,” a true antimalarial drug. Horace Walpole, the English author, gave the term mal aria, “bad air.” People finally made the connection that victims of this disease tended 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. This dose of quinine probably had nothing to do with curing them or preventing malaria.
Mosquitoes, Parasites, and the Third World
Malaria has been around for centuries and 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, which means it’s unicellular, eukaryotic, and has a good nucleus. Compared with bacteria, it’s fairly large, 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 common way to get tropical diseases. They can cycle almost anywhere: in the intestines, the liver, the spleen, and in the blood circulation.
The Plasmodium species are intracellular parasites. Specifically, 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 five.
Twenty-five hundred children per day in the third world are dying of malaria, about 100 an hour, and about two per minute. The deaths are primarily in Sub-Saharan Africa in the malaria belt, the plus-and-minus 1,000 miles of the equator. It is the leading cause of death of children under five in this area of the world. The high rate of death should generate outrage because if these were white children living in America, South America, or Europe, other countries would put an end to it immediately, as we have done. Malaria existed in the United States but has been eradicated, with funding and effort. The issue is an outrage—both a social outrage and an ethical one, that it has not been wiped out fully.
There are about 1,500 cases per year of malaria in the United States, nicknamed “airport malaria” because there are no malarial vectors. Since the Anopheles mosquito has been wiped out, we’ve effectively broken the cycle. Most of these cases are from travelers. High volume, population-dense cities like New York City, may see more tropical diseases there than are seen in many tropical countries. Travel between other countries, including increased air traffic, can lead to people getting infected abroad. International travelers returning to New York, either by passing through to another destination, or those who live there permanently, spread the disease when they return home. During my time in New York in medical school, we saw a lot of tropical diseases.
Learn more about the risks of foreign travel
The Four Varieties of Malaria
The Plasmodium falciparum is the most deadly form and it’s by itself as a group. Patients who contract this disease can be dead within 48 hours.
There are four varieties of malaria. The Plasmodium falciparum is the most deadly form and it’s by itself as a group. Patients who contract this disease can be dead within 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 where it turns black or dark brown. These patients have urine that’s very, very dark—the black water—and 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 sick. They’re a major cause of illness and debilitation throughout the tropical third world.
Malaria and Evolution
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.
This has lead natural selection to select for a bad disease because malaria is worse. Because sickle-cell disease doesn’t tend to kill patients quite as much or as early in life, these patients go on to have children with the disease before they die of sickle-cell anemia. Meanwhile, they pass their resistance to malaria to their children.
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