Tropical Diseases

Malaria, a parasitic disease spread by mosquitoes, is the best-known tropical disease and infects the largest number of people internationally. It has been called the "most devastating disease in history" based on the number of people it has attacked or killed. Currently, malaria has been identified in about 100 countries, although 80% of the clinical cases are reported in Africa.

The history of malaria is different than the history of other tropical diseases, due to its immense reach and the concerted effort over time to defeat the disease. But in as much as it has been eliminated or controlled in developed countries and remains a major killer in those that are poor and less developed, its history is also similar to that of other tropical diseases.

The ancient Chinese wrote about malaria as early as 2700 B.C., describing the symptomatic fever and characteristic enlargement of the spleen. Ancient Greeks and Romans also suffered from malaria. Hippocrates (460-375 B.C.), offered an accurate account of malarial symptoms and theorized that the disease was caused by a miasma, or poisonous cloud, rising up from marshy land.

The first effective treatment for malaria was developed in the seventeenth century and utilized a traditional Peruvian treatment for fever, the cinchona tree bark. In 1630, Jesuit missionaries in Peru introduced the bark of the cinchona tree to Europeans. The active substance in the bark, quinine, was isolated in 1820. Though there were common, serious side effects to the drug, such as ringing of the ears and hearing loss, the substance was effective in treating malaria, which was common in Europe at that time.

Development of a treatment for malaria cleared the way for large scale exploration of tropical areas by Europeans. Development of a synthetic quinine, called chloroquine, in the 1940s, offered effective treatment with fewer common side effects. In the 1960s, malarial parasites became resistant to chloroquine, and the substance no longer worked in many areas.

The most recent effort to control malaria has been effective in many areas, including the southern United The life cycle of Schistosoma japonicum, one of three species of parasitic blood flukes that cause schistosomiasis. Illustration by Hans & Cassidy. Courtesy of Gale Group.
States. The pesticide DDT was used, successfully, in the Tennessee River Valley, Greece, Puerto Rico and other locations to kill the mosquitoes carrying malaria and eliminate the disease. A world campaign to eliminate malaria internationally used DDT from 1956-1969. However, mosquitoes became resistant to DDT and the effort failed.

There are four different types of malaria protozoa. One of the four, P. falciparum, is responsible for almost all deaths from the disease. Symptoms of the disease do not occur for one to two weeks. They often include chills, muscle aches, fatigue, and abdominal discomfort. They may include tremors and high fever, which comes and goes at regularly spaced intervals. Complications from severe malaria include renal failure, pulmonary edema, coma, and hypoglycemia.

The delayed appearance of malaria's symptoms reflects the measured course the disease takes in the body. The activity of the malaria parasite in the body is aggressive and thorough. The disease is spread by female anopheline mosquitos that inject malaria parasites into the bloodstream with their bite. The parasites, called sporozoites, travel to the liver, where they enter cells of the liver tissue. Once in the liver, the sporozoite changes to a spore, which replicates itself until there are thousands of spores in a cyst-like structure which has replaced the cell. Malaria manifests no symptoms while this process occurs.

Symptoms develop when the cyst bursts and the spores, called merozoites, are released into the blood stream. At this point, sweating and high fever can occur A skin lesion from cutaneous leishmaniasis. Biophoto Associates, National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission.

as the spores enter the host's red blood cells. The parasite consumes hemoglobin from within the red blood cells.

As the parasite grows from consuming the hemoglobin, its nucleus divides into from six to 32 parts, each of which becomes a spore of its own. The red cell bursts, and the spore moves to another cell.

After the merozoites have launched their process of reproducing in the red blood cells of the host, some of the merozoites are transformed to male or female game-tocytes. These sexual cells form the base for more parasites if the host is bitten again by the female anopheles mosquito. When this occurs, the gametocytes develop into a number of differentiated cells, called gametes, zygotes, ookinetes and oocysts, within the gut of the mosquito. Eventually the parasite reproduces and sporozoites that travel in the mosquito are formed. These sporozoites move up to the mosquito's salivary gland, where they are ready to infect another host.

Treatment of malaria is usually effective. Because many malaria parasites are resistant to chloroquine, physicians have returned to using quinine in many instances. Travelers and others at risk of malaria receive chloroquine or other drugs, including mefloquine, if journeying to areas where the drugs still work. However, there is no totally effective preventive measure against malaria.