More than one million people die of malaria every year, mostly infants, young children and pregnant women, and most of them in Africa. Finding effective ways to control and eventually eradicate the disease is a high priority of the National Institute of Allergy and Infectious Diseases (NIAID).
Quick Facts
- More than 40 percent of the world’s population lives in areas where there is a risk of contracting malaria.
- A child dies of malaria every 30 seconds.
- More than one million people die of malaria every year, mostly infants, young children and pregnant women and most of them in Africa.
- Approximately 300-500 million cases of clinical malaria occur each year.
- Malaria accounts for at least $12 billion in economic losses each year in Africa, and a reduction in annual economic growth estimated at 1.3 percent.
Source: World Health Organization
Causes
Malaria is caused by a single-celled parasite from the genus Plasmodium. More than 100 different species of Plasmodium exist. They produce malaria in many types of animals and birds, as well as in humans.
Four species of Plasmodium commonly infect humans. Each one has a distinctive appearance under the microscope, and each one produces a somewhat different pattern of symptoms. Two or more species can live in the same area and infect a single person at the same time.
- Plasmodium falciparum is responsible for most malaria deaths, especially in Africa. The infection can develop suddenly and produce several life-threatening complications. With prompt, effective treatment, however, it is almost always curable.
- Plasmodium vivax, the most geographically widespread of the species, produces less severe symptoms. Relapses, however, can occur for up to 3 years, and chronic disease is debilitating. Once common in temperate climates, P. vivax is now found mostly in the tropics, especially throughout Asia.
- Plasmodium malariae infections not only produce typical malaria symptoms but also can persist in the blood for very long periods, possibly decades, without ever producing symptoms. A person with asymptomatic (no symptoms) P. malariae, however, can infect others, either through blood donation or mosquito bites. P. malariae has been wiped out from temperate climates, but it persists in Africa.
- Plasmodium ovale is rare, can cause relapses, and generally occurs in West Africa.
Transmission
The malaria parasite typically is transmitted to people by mosquitoes belonging to the genus Anopheles. In rare cases, a person may contract malaria through contaminated blood, or a fetus may become infected by its mother during pregnancy.
Because the malaria parasite is found in red blood cells, malaria can also be transmitted through blood transfusion, organ transplant, or the shared use of needles or syringes contaminated with blood. Malaria also may be transmitted from a mother to her fetus before or during delivery ("congenital" malaria).
Symptoms
Malaria typically produces a string of recurrent attacks, or paroxysms, each of which has three stages—chills, followed by fever, and then sweating. Along with chills, the person is likely to have headache, malaise, fatigue, muscular pains, occasional nausea, vomiting, and diarrhea. Within an hour or two, the body temperature rises, and the skin feels hot and dry. Then, as the body temperature falls, a drenching sweat begins. The person, feeling tired and weak, is likely to fall asleep.
The symptoms first appear some 10 to 16 days after the infectious mosquito bite and coincide with the bursting of infected red blood cells (RBCs). When many RBCs are infected and break at the same time, malaria attacks can recur at regular time periods—every 2 days for P. vivax malaria and P. ovale, and every 3 days for P. malariae.
With P. vivax malaria, the person may feel fine between attacks. Even without treatment, the paroxysms subside in a few weeks. A person with P. falciparum malaria, however, is likely to feel miserable even between attacks and, without treatment, may die. One reason P. falciparum malaria is so virulent is that the parasite can infect RBCs in all stages of development, leading to very high parasite levels in the blood. In contrast, P. vivax parasites infect only young RBCs, which means the number of parasites in the blood does not reach the same high levels as seen in P. falciparum infection.
Diagnosis
Healthcare providers should suspect malaria in anyone who has been in the tropics recently, who received a blood transfusion, and who develops a fever and other signs that resemble the flu. They examine blood smears taken from a finger prick under a microscope to confirm the diagnosis. A "thick" smear makes it possible to examine a large amount of blood. Then, the species of parasite can be identified by looking at a corresponding "thin" smear. Because mixed infections are possible, these techniques are important for deciding the best treatment. For example, a person can be infected with P. vivax as well as the more dangerous P. falciparum.
Treatment
In most cases, healthcare providers can successfully treat people with malaria. To decide which medicine to use, they should try to identify the species of parasite responsible and the geographical location where the person was infected. International travel clinics, the Centers for Disease Control and Prevention, and the World Health Organization offer up-to-date information on the geography of malaria, including
- Which species are present in which areas
- Whether chloroquine-resistant parasites are present
- Which seasons of the year carry the greatest risk
Prevention
Before leaving home, anyone traveling to an area with malaria should consult a knowledgeable healthcare provider, an international travel clinic, a local health department, the Centers for Disease Control and Prevention (CDC), or the World Health Organization (WHO) to obtain advice on what medicines to take before, during, and after the trip. Health risks for malaria vary with the destination, conditions of travel, and types of activities the traveler will undertake. A traveler who spends even a single night in a malaria-endemic area risks getting infected.
The CDC and the WHO have information on how to limit contact with mosquitoes, as well as current guidelines on antimalarial drugs.
Life Cycle of the Malaria Parasite
- A female Anopheles mosquito carrying malaria-causing parasites feeds on a human and injects the parasites in the form of sporozoites into the bloodstream. The sporozoites travel to the liver and invade liver cells.
- Over 5-16 days*, the sporozoites grow, divide, and produce tens of thousands of haploid forms, called merozoites, per liver cell. Some malaria parasite species remain dormant for extended periods in the liver, causing relapses weeks or months later.
- The merozoites exit the liver cells and re-enter the bloodstream, beginning a cycle of invasion of red blood cells, asexual replication, and release of newly formed merozoites from the red blood cells repeatedly over 1-3 days*. This multiplication can result in thousands of parasite-infected cells in the host bloodstream, leading to illness and complications of malaria that can last for months if not treated.
- Some of the merozoite-infected blood cells leave the cycle of asexual multiplication. Instead of replicating, the merozoites in these cells develop into sexual forms of the parasite, called male and female gametocytes, that circulate in the bloodstream.
- When a mosquito bites an infected human, it ingests the gametocytes. In the mosquito gut, the infected human blood cells burst, releasing the gametocytes, which develop further into mature sex cells called gametes. Male and female gametes fuse to form diploid zygotes, which develop into actively moving ookinetes that burrow into the mosquito midgut wall and form oocysts.
- Growth and division of each oocyst produces thousands of active haploid forms called sporozoites. After 8-15 days*, the oocyst bursts, releasing sporozoites into the body cavity of the mosquito, from which they travel to and invade the mosquito salivary glands. The cycle of human infection re-starts when the mosquito takes a blood meal, injecting the sporozoites from its salivary glands into the human bloodstream.
* Time-frame depends on the malaria parasite species.
Glossary
Diploid: Cells containing a full set of chromosomes.
Gametes: Reproductive elements, male and female.
Gametocytes: Precursors of the sexual forms of the malaria parasite, which release either male or female gametes within the stomach of the mosquito.
Haploid: Cells containing a half set of chromosomes.
Merozoite: The form of the malaria parasite that invades red blood cells.
Oocyst: A stage of the malaria parasite within the mosquito which is produced when male and female gametes combine.
Ookinete: The actively moving zygote of the malarial organism that penetrates the mosquito stomach to form an oocyst under the outer gut lining.
Sporozoite: The infectious form of the malaria parasite, which is injected into people by mosquitoes.
Zygote: The diploid cell resulting from union of a male and a female gamete.
Source: National Institutes of Health; National Institute of Allergy and Infectious Diseases, NIAID
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This information is provided by the Cleveland Clinic and is not intended to replace the medical advice of your doctor or health care provider. Please consult your health care provider for advice about a specific medical condition. This document was last reviewed on: 7/7/2009…#14097
