Malaria
Malaria is one of the oldest known infections. It is also the world's most deadly tropical parasitic disease. It kills more people than any other communicable disease except tuberculosis. The disease was first described in ancient Sanskrit and Chinese documents. Hippocrates also described the
disease in his writings. It is believed that the army of Alexander the Great was wiped out by the disease during its march across India.
Malaria is thought to have been introduced into the United States by European colonists and African slaves in the sixteenth and seventeenth centuries. It is now endemic in ninety-two countries worldwide. With approximately 41 percent of the world's population at risk, the disease poses a serious health threat globally. As many as two million people die annually; half of the deaths occur in children under five years of age. According to the World Health Organization, this amounts to one child dying every thirty seconds.
Malaria is characterized by both acute and relapsing infection in humans. Hallmark symptoms include periodic episodes of chills and fever, spleen enlargement, and anemia. The disease is caused by microscopic onecelled organisms called sporozoa, which belong to the genus Plasmodium. These parasites are transmitted to humans by several species of anopheles mosquitoes. Malaria is also found in apes, monkeys, birds, bats, reptiles, and rodents. While humans can be infected only by Anopheles mosquitoes, birds and other animals are known to have become ill after being bitten by mosquitoes from the genus Culex.
Four species of Plasmodium are known to cause human malaria: P. falciparum, P. vivax, P. malariae, and P. ovale. Diagnosis can be determined by a blood smear. The most common type, P. falciparum, requires relatively high environmental temperatures for development and is usually found in tropical areas such as western Africa. P. vivax malaria accounts for 43 percent of all cases and is widespread globally. It is known to occur even in cold-winter areas of Korea, Manchuria, and south Russia. The less common P. malariae malaria occurs in about 7 percent of all cases and is confined to the Mediterranean, while P. ovale malaria is rather rare and isolated within a small area of eastern Africa and the islands of the western Pacific.
Malarial attacks typically last four to ten hours during which a person experiences successive stages of chills, high fever, severe headache, and then profuse sweating. Between attacks, body temperature may be normal. The intervals between attacks are usually either forty-eight hours or seventy-two hours. The first attack typically occurs seven to nine days after a person is bitten by a disease-carrying mosquito.
Mosquito Transmission
Though malaria can occur in temperate regions, it is most common in the tropics and subtropics, where climatic conditions favor mosquito development. Mosquitoes lay their eggs in water where larvae mature and hatch into flying adults. Newly hatched female mosquitoes are especially bloodthirsty and require a blood meal to produce fertile eggs. When these mosquitoes bite a human who is already infected, they ingest the malarial parasite and the disease transmission cycle begins.
The life cycle of the Plasmodium starts in the stomach of the female mosquito. The organism's double life cycle has two phases—a sexual reproductive cycle and an asexual reproductive cycle. While the parasite is in its asexual, free-swimming stage, it is known as a sporozoite. When an infected mosquito bites, the sporozoite is injected along with saliva into the human bloodstream.
Once inside the bloodstream, the sporozoite enters a red blood cell. Inside the red blood cell, it changes shape and divides into smaller forms called merozoites. The red blood cell containing these merozoites ruptures, releasing them into the blood. The merozoites infect other red blood cells, and the life cycle is repeated. The rupturing of red blood cells causes the symptoms of fever and chills.
A mosquito biting an infected host at this stage can ingest merozoites. If this happens, the merozoites enter the mosquito's stomach and become male and female gametocytes. This kicks off a sexual reproductive life cycle where the separate male and female gametocytes unite together to form a single-celled zygote. This zygote grows to become an oocyst or large egglike sac, which eventually divides, releasing a multitude of asexual, freeswimming sporozoites.
These sporozoites move to the mosquito's head and salivary glands from which they can be injected into a human during the mosquito's next bite. This asexual cycle is repeated. During the asexual life cycle, the parasites grow and divide synchronously. The resulting merozoites produce the regularly occurring fever and chill attacks that are typical of malaria.
Early Cases and Treatments
The first documented treatment of the disease occurred in 1630 when "Jesuit's bark," from a cinchona tree, was used to ease the fever of a Spanish magistrate in Peru. Amazingly, the magistrate recovered and eventually the substance quinine was isolated from the bark and processed commercially as a treatment. The Cinchona genus includes about forty species of plants, mostly trees, native to the Andes of South America. Certain species are also known to grow in India and Sri Lanka.
In the 1940s, the antimalarial drug chloroquine was introduced as an effective additional treatment. Chloroquine is a member of an important series of chemically related antimalarial agents, the quinoline derivatives. A global eradication program was initiated in the 1950s and 1960s by the World Health Organization (WHO), in Geneva, Switzerland, which led to a significant decrease in malaria cases in Asia and South America.
Drug Resistant Strains and Reemergence of Disease
Drug resistant strains of malaria began to emerge in the 1970s, making the disease harder to control. During the 1990s the prevalence of malaria escalated at an alarming rate, especially in Africa where control efforts have typically been piecemeal and uncoordinated. Additionally, the phenomenon of "airport malaria," or the importing of malaria by international travelers, is becoming commonplace. Persons who are not normally exposed to this mosquito in its natural habitat can acquire "airport malaria" through the bite of an infected mosquito that has traveled far from its home.
In one study, random searches of airplanes at Gatwick Airport in London found dozens of airplanes from tropical countries containing mosquitoes. After a mosquito leaves an aircraft, it may survive long enough to take a blood meal and transmit the disease, usually in the vicinity of the airport. Incidents of malaria transmitted this way are expected to become more common, since airport travel has increased by almost 7 percent a year since 1980
and is predicted to increase by 5 percent a year for the first twenty years of the twenty-first century.
Resurgence and increased risk of the disease appears to be linked to several factors. Changes in land use, such as mining, logging, and agricultural projects, particularly in the Amazon and Southeast Asia frontier area, are providing new mosquito breeding sites. Other reasons for the disease's spread include global climatic changes, disintegration of health services, armed conflicts, and mass movements of refugees into areas of high malaria transmission.
Reemergence of malaria through mobility occurred in Brazil, for example. Malaria had been practically eradicated from most areas of the Amazon region until massive population movements began to colonize new territories. New highways were built, linking the Amazon to the rest of the country and attracting laborers to work on road construction. In 1970, prior to new road construction in these new areas, there were approximately 50,000 cases of malaria reported; by 1990, reports had increased to more than 500,000, representing 10 percent of the world's reported cases outside Africa.
As a result of the explosion of international travel, imported cases of malaria are now showing up more in developed countries such as the United States Malaria is also reemerging in areas where it was previously under control or eradicated, such as in Korea. According to the WHO, global warming and other climatic events such as El Niño also play a role in increasing the disease. Malaria has now spread to highland areas of Africa, where El Niño effects such as increased rainfall have influenced mosquito breeding sites and hence the transmission of the disease. The emergence of multidrug-resistant strains of parasites is also exacerbating the situation.
Disease Prevention
Prevention of malaria encompasses a variety of measures. Some may protect against infection—these are directed against mosquitoes—whereas others focus on stopping the development of the disease in human beings. Although only a limited number of drugs are available, if these are used properly and targeted to those at greatest risk, malaria can be reduced.
Since the early 1990s, considerable progress has been made in the search for a malaria vaccine. More than a dozen candidate vaccines are currently in development; some of them are in clinical trials. An effective vaccine could be available within the first twenty years of the twenty-first century. In the meantime, there are a number of prescription drugs available on the market in developed countries that can help prevent malaria, especially in individuals traveling to high incidence areas. Some of the best-known preventatives include Mefloquine, Malarone, and Primaquine.
Medical researchers continue to discover new drug therapies. Most recently, Chinese scientists discovered a drug called artemether that is derived from the Chinese herb qinghaosu. The new drug appears to be as effective as quinine although much slower acting. It may even kill resistant strains of malaria.
Bibliography
Noble, Elmer R., Glenn A. Noble, Gerhard A. Schad, and Austin J. MacInnes. Parasitology. London: Lea & Febiger, 1989.
Turkington, Carol, and Bonnie Ashby. Encyclopedia of Infectious Disease. New York: Facts on File, Inc., 1998.
