Malaria is an infectious disease caused by a parasite that spends part of its life cycle in humans and part in mosquitoes.
Malaria is a potentially fatal disease that has infected humans as far back as 50,000 BCE. There are records of mosquito netting being used in Egypt in 2700 BCE to protect against malaria. Chinese medical records from the same period describe the symptoms of the disease. The English name of the disease comes from an eighteenth-century Italian doctor who wrote a textbook about it and attributed it to bad air, or mal'aria in Italian.
Humans develop malaria when they are infected with a protozoan called Plasmodium through the bite of a mosquito. The parasites enter the bloodstream and are carried to the liver, where they infect liver cells and multiply. The mature parasites are released back into the bloodstream, where they infect red blood cells. The red cells burst in two to three days, releasing more parasites that, in turn, invade more red blood
cells. Most of the symptoms of malaria (fever, chills, generally sick or flu-like feeling) are related to the destruction of red blood cells. The reason why the fever associated with malaria comes in two- or three-day cycles in some people is that the bursting of infected red blood cells and the infection of new red blood cells occurs every two to three days.
There are four different species of Plasmodium that infect people. One of these, Plasmodium falciparum, is much deadlier than the other three. It can cause failure of the lungs, kidneys, and central nervous system within a few hours or days. The different species also take different lengths of time to produce the first symptoms of malaria in humans. Some remain in the liver for long periods of time, thus causing flare-ups of the disease months or even years later. These variations are one reason why doctors try to identify the particular Plasmodium species when they test a person's blood for malaria. It is possible for a person to be infected with more than one species of the parasite at the same time.
People cannot get malaria from sharing a household with an infected person since it takes more than a week before a mosquito is able to transmit the disease. It is possible for a pregnant woman to transmit the disease to her unborn baby, however. It is also possible to get malaria through a transfusion of infected blood, but this form of transmission is rare in developed countries. In the United States, people who have traveled in an area with endemic malaria cannot donate blood for a full year after returning to the United States. They are forbidden to donate blood for three years if they have been treated for malaria.
Malaria is one of the most common infectious diseases in tropical and subtropical parts of the world. These countries account for 41 percent of the world's population. It is estimated that there are between 400 and 900 million cases of malaria around the world each year and 1 million to 3 million deaths, most of them African children under age five. In some parts of Africa, it is estimated that one person dies of malaria every thirty seconds. The disease is endemic in parts of Africa, southern Asia, and portions of South America.
Malaria Research and the Nobel Prize
One indication of the importance—and the complexity—of malaria is the number of Nobel prizes in medicine that have been awarded to doctors who studied the disease. Charles Louis Alphonse Laveran (1845–1922) was a French doctor working in a military hospital in Algeria in 1880 when he discovered that malaria is caused by a protozoan (a type of one-celled organism). This was the first time that a protozoan had been shown to cause any disease in humans. Laveran was awarded the Nobel Prize in Physiology or Medicine in 1907.
In 1898, Sir Ronald Ross (1857–1932), a British doctor working in India, was able to show that mosquitoes are the vectors that carry malaria to humans. Ross even demonstrated that the parasites are present in the mosquito’s salivary glands.
He was awarded the Nobel Prize in Physiology or Medicine in 1902. Austrian doctor Julius Wagner-Jauregg (1857–1940) won a Nobel prize in 1927. He showed that malaria could actually be used to treat patients suffering from late-stage syphilis, a common cause of dementia in the early twentieth century. The reason this treatment was thought to work was that the fever caused by the malaria parasite was high enough to kill the bacteria that cause syphilis.
The fourth Nobel Prize in Physiology or Medicine related to malaria research was given in 1948 to Paul Hermann Müller (1899–1965), a Swiss chemist who discovered dichloro-diphenyltrichloroethane (DDT) in 1939. DDT was the first insecticide that was effective in controlling the mosquitoes that spread malaria.
Malaria has not been endemic in the United States since 1951, when a four-year program was started in 1947 to spray areas in the southeastern part of the country that were known to harbor the mosquitoes that transmit malaria. By 1952 the Centers for Disease Control and Prevention (CDC) reported that malaria had been eradicated from the United States. Cases of malaria since that time were either acquired by people traveling to countries where malaria is endemic or were small local outbreaks caused by local mosquitoes biting infected travelers and then transmitting the malaria parasites to others nearby. In 2002 the CDC reported 1,337 cases of malaria in the United States, including eight deaths. All but five cases were acquired abroad.
People who are at greatest risk for malaria are those who have never been exposed to the disease, pregnant women, and young children. Some researchers also think that genetic factors may increase some people's risk of getting the disease.
The cause of malaria is a protozoan of the genus Plasmodium. The most deadly of the four species that can cause malaria in humans is Plasmodium falciparum. The classical symptom of malaria is a repeated cycle of chills and shaking followed by fever and sweating lasting four to six hours. People infected by P. falciparum may feel a tingling in the skin as well. In some cases, the early symptoms of malaria may be mistaken for the flu.
Children infected by P. falciparum may develop increased fluid pressure on the brain and suffer permanent brain damage even if they survive. Adults infected by P. falciparum may develop kidney failure, dehydration, an enlarged liver and spleen, rupture of the spleen, severe headache, loss of blood supply to the brain, and coma. Some die. The destruction of red blood cells by the parasite may cause the urine to look brown or black, which is why the disease got the name “blackwater fever.”
Diagnosis of malaria is based on a combination of the patient's history and the results of blood tests. The patient's history is critical, particularly the dates of any trips abroad, because two of the four species of
the malaria parasite can remain dormant in the liver for months or even years.
The basic blood test involves looking at blood smears made by pricking the patient's finger at twelve- to twenty-four-hour intervals. One technique is called a thick smear, which is used to estimate the number of parasites in the blood. The other is called a thin smear and is used to determine the species. A rapid test called OptiMAL has been developed to distinguish P. falciparum from the other species. It gives results in less time than the smear method and can be used by doctors in areas without hospital laboratories.
Malaria is treated with various combinations of drugs, which may be given by mouth or intravenously. Patients infected by P. falciparum may be taken to a hospital for treatment because of the possibility of major organ damage. The oldest drugs used to prevent or treat malaria are quinine and chloroquine. Relatively inexpensive, they are not always effective because the malaria parasites have developed resistance to them in some countries. Newer drugs have been developed that are more effective for resistant strains. People who are traveling to parts of the world where malaria is endemic need to take these drugs while they are abroad.
The specific drug that is given to a patient and the length of treatment depend on the type of malaria, the source of the infection, the patient's age, and the severity of the symptoms when treatment began. People often feel weak and tired for a few weeks when taking these medications.
Malaria caused by P. falciparum is the most deadly; it can kill within days of the first symptoms. In some areas, the death rate from this type of malaria is as high as 20 percent. People infected by the other three species of Plasmodium usually recover, but some may have bouts of malaria for months until all the parasites have been cleared from their livers.
There is no vaccine effective against the malaria parasite, although scientists are working on several possibilities. For the time being, prevention is
the most effective way to lower one's risk of getting malaria. There are three basic strategies recommended by the CDC:
- Controlling the mosquito population by using insecticides and draining swampy areas. This approach is called vector control.
- Using mosquito netting treated with insecticide to cover beds and other sleeping areas. These nets have cut the number of deaths from malaria in parts of Africa by 20 percent.
- Taking medications to prevent the disease when visiting countries where malaria is endemic. The CDC has general advice for travelers and a list of countries where travelers are at risk for malaria at http://www.cdc.gov/malaria/travel/index.htm.
It is unlikely that the mosquitoes that carry malaria will ever be completely wiped out. At present, efforts to relieve the impact of malaria in Africa are aimed at preventing the disease. Methods include spraying insecticides, providing people with insecticide-treated mosquito netting, and supplying poor countries with antimalarial drugs. In 2005 the President's Malaria Initiative (PMI)—a cooperative program that involves the CDC, the National Institutes of Health, the World Health Organization, and the World Bank as well as the White House—began a five-year program to cut deaths from malaria by 50 percent in fifteen African countries.
The most promising area of malaria research as of 2008 was the development of vaccines against the disease. Scientists have identified the genetic sequence of the malaria parasite, which may help researchers target a particular stage in the life cycle of the parasite or prevent it from developing inside the mosquito. One possibility is developing a vaccine that would stop the parasite inside the human body before it gets to the point of infecting the red blood cells. Another group of researchers is experimenting with cultivating P. falciparum inside mosquitoes and treating the parasite with radiation in order to make a vaccine. This approach is described in the video listed below.
SEE ALSO Influenza; Syphilis; West Nile virus infection
WORDS TO KNOW
Dementia: Loss of memory and other mental functions related to thinking or problem-solving.
Endemic: A term applied to a disease that maintains itself in a particular area without reinforcement from outside sources of infection
Protozoan (plural, protozoa): A one-celled animal-like organism with a central nucleus enclosed by a member. Many protozoa are parasites that can cause disease in humans
Spleen: An organ located behind the stomach that cleans old blood cells out of the blood and holds a reserve of red blood cells
Vector: An insect or other animal that carries a disease from one host to another.
Abramovitz, Melissa. Malaria. Detroit, MI: Lucent Books, 2006.
Lynette, Rachel. Malaria. Farmington Hills, MI: KidHaven Press, 2006. Marcus, Bernard A. Malaria. Philadelphia: Chelsea House Publishers, 2004.
Appleton, Samantha. “Slide Show: Healing Africa.” New Yorker, October 24, 2005. Available online at http://www.newyorker.com/archive/2005/10/24/slideshow_051024?slide=1#showHeader (accessed April 24, 2008). This is a slide show of fifteen photographs of the 2005 malaria epidemic in Africa.
McNeil, Donald G., Jr. “Searching for a Cure for Malaria.” New York Times, December 11, 2007. Video available online at http://video.on.nytimes.com/index.jsp?fr_story=6e4647e2867283dce5b8cb2cf46003e7fdcd6267 (accessed April 25, 2008). This is a four-minute video about ongoing efforts to develop a vaccine against malaria.
Centers for Disease Control and Prevention (CDC). Malaria: Topic Home. Available online at http://www.cdc.gov/malaria/ (updated April 18, 2008; accessed April 24, 2008). Contains links to news items as well as basic information about malaria and its treatment.
Mayo Clinic. Malaria. Available online at http://www.mayoclinic.com/health/malaria/DS00475 (updated February 8, 2008; accessed April 25, 2008).
President's Malaria Initiative (PMI). Fast Facts. Available online at http://www.fightingmalaria.gov/resources/pmi_fastfacts.pdf (posted January 2008; accessed April 26, 2008).
Sumanas, Inc. Malaria: Cooperation among Parasite, Vector, and Host. Available online at http://www.sumanasinc.com/scienceinfocus/sif_malaria.html (accessed April 5, 2008). This is an animation of the life cycle of the malaria parasite with voiceover. It takes three minutes to play.
WGBH Educational Foundation. Rx for Survival: Malaria. Available at http://www.pbs.org/wgbh/rxforsurvival/series/diseases/malaria.html (posted 2005; accessed April 24, 2008). The right-hand side of the page includes links to three videos on malaria, each about five minutes in length.