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What is malaria?

Malaria is an infectious disease caused by Plasmodium parasites. These parasites are primarily spread by the bite of infected female Anopheles mosquitos. There are four main types of Plasmodium (P) species that infect humans:

  • Plasmodium vivax and Plasmodium ovale, which cause a relapsing form of the disease, and
  • Plasmodium malariae and Plasmodium falciparum, which do not cause relapses.
  • Recently, it was recognized that a fifth species that normally infects macaques, Plasmodium knowlesi, can be naturally transmitted to humans and cases have been seen in parts of Southeast Asia.

Rarely, infection can be passed from a woman to her baby during pregnancy or labor and delivery (congenital infection) or transmitted through blood transfusion, organ transplant, or sharing of contaminated needles or syringes.

When a human is bitten by an infected mosquito, the parasites enter the bloodstream and travel to the liver. After a person is infected, there is usually an incubation period of 7-30 days, after which the parasites enter the person’s red blood cells (RBCs). They then multiply inside these cells, which rupture within 48 to 72 hours, causing many of the symptoms of malaria to develop. P. vivax and P. ovale cause relapsing disease as the parasite can stay dormant in the liver before re-entering the bloodstream and causing symptoms months, and even years, after the initial infection. While any malaria infection left untreated can cause severe illness and death, infection by P. falciparum is most likely to cause life-threatening disease, as can the newly recognized P. knowlesi.

Most malaria infections and most malaria deaths occur in Africa. Malaria also exists in regions in Central and South America, parts of the Caribbean, Asia (including South Asia, Southeast Asia, and the Middle East), Eastern Europe and the South Pacific. Globally, the World Health Organization (WHO) estimates that 3.3 billion people are at risk. In 2013, there were about 198 million cases of malaria resulting in 584,000 deaths, mostly among African children.

Cases of malaria in the United States are rare; they mostly occur among those who have travelled to parts of the world where malaria infections are common (endemic).


About Malaria

Risk to U.S. Travelers

It is important that people who intend to travel to areas where malaria is endemic talk to their healthcare provider about malaria protection before they travel. Their risk of exposure to malaria can be evaluated and proper precautions taken before, during, and after their trip. Even people who have had malaria previously can become reinfected. Similarly, people who originally come from endemic areas are at risk when they return for a visit.

For travelers, the risk of contracting malaria depends on the time of year, the country or countries visited, including specific areas visited within a country, the length of visit, and the type of activities. For example, a traveler to West Africa who will be backpacking and sleeping in tents for several weeks is at greater risk than a traveler who will spend a day or two in a country with relatively low malaria frequency and stay in a hotel with closed windows and air conditioning. According to the Centers for Disease Control and Prevention (CDC), the parts of the world with highest estimated relative risk of infection to U.S. travelers are West Africa and the tropical islands of the Pacific.

For details, see the CDC web page on Malaria Risk Information and Prophylaxis, by Country.

Signs and Symptoms

Signs and symptoms of malaria can be general and non-specific, so it is important for a health practitioner to take a thorough medical history and for people to tell their healthcare provider if they have traveled to endemic countries, even if they followed preventive measures carefully.

Often, malaria presents as a flu-like illness with fevers, chills, sweats, headaches, aches and malaise. Some people develop gastrointestinal symptoms of nausea, vomiting and diarrhea. Anemia and jaundice can occur. These symptoms can develop as early as 7 days after initial infection or as late as several months after returning to the U.S., but the most typical time is about 14 days after exposure.

Signs and symptoms develop after the parasites emerge from the liver stage and enter red blood cells to multiply, coinciding with the bursting of those red blood cells. The symptoms are cyclic, worsening and then improving every 2-3 days as more red blood cells are infected and burst. An infected person may have an enlarged liver or spleen, but there may be few other signs that can be detected by a physical examination.

Severe malaria, primarily caused by P. falciparum, is life-threatening. It can affect the brain, kidneys and lungs, causing symptoms such as seizures, mental confusion, acute respiratory distress and coma; it can result in multi-organ failure and death. When it affects the brain, it is called cerebral malaria.


Testing is performed to help diagnose malaria, to monitor for relapses, and to determine drug susceptibility of the parasite causing the infection.

Thick and thin blood smears
Diagnosis of malaria involves performing blood smears. For a blood smear, a drop of blood is applied to and spread onto a glass slide. It is then treated with a special stain and examined under a microscope for the morphology of infected blood cells and the parasite. Typically, two thick smears and two thin smears are prepared. These tests are currently the “gold standard” for malaria detection and identification. They require examination by a trained and experienced laboratorian.

The number of malaria parasites present in the blood at a given time fluctuates. Therefore, if no parasites are seen on the initial set of smears and the health practitioner still suspects malaria, then additional blood samples will be obtained to be tested. The samples may be collected at 8 to 12 hour intervals over 2 to 3 days to increase the probability of detecting the parasites. It is advantageous if the sample collection coincides with the appearance of signs and symptoms as this is the time that the parasites will most likely be detected in the blood.

Thick smears are a more sensitive test for malaria infection. A greater volume of blood is examined under the microscope and the parasites are therefore more likely to be seen. Thin smears have fewer blood cells present and allow identification of the type of Plasmodium species causing the infection. The number of infected red blood cells can also be calculated to determine the degree to which a person is infected (parasite load). This information is essential for proper treatment.

Rapid diagnostic tests (antigen testing)
When microscopy is not readily available, rapid diagnostic tests may be used instead of blood smears. These tests detect malaria antigens (proteins) in a sample of a person’s blood (usually taken with a fingerstick) and indicate a positive result by a color change on the testing strip. They are sometimes called “dipstick” tests.

Different rapid diagnostic tests are available, and they have varying capabilities in what they detect. For example, some rapid tests may detect all four common species (P. falciparumP. vivaxP. ovaleP. malariae) but do not distinguish between them. Others are combination tests that can detect all four common species and will identify P. falciparum specifically if it is present. The type of rapid test used is dependent on the patient population and the goals of providing a rapid test result.

The U.S. Food and Drug Administration (FDA) has approved a rapid diagnostic test for malaria. It is approved for use by hospital and reference laboratories, but not for doctors’ offices or home testing. This rapid test may allow for faster diagnosis and treatment. However, it is recommended that positive results be followed with blood smears for confirmation and to determine the extent of infection.

Molecular tests (Polymerase chain reaction, PCR)
The polymerase chain reaction is a laboratory method that amplifies the parasite’s DNA and allows detection and identification of the Plasmodium species. This test can be used to confirm the diagnosis in laboratories where there is a lack of training and experience in the microscopic examination for malaria. It can also be used to determine the Plasmodium species if the results of a blood smear are unclear. Likewise, it is useful for cases in which the number of malaria parasites in the blood is low or when there are different types causing the infection (mixed) and examination using a microscope may be less accurate. The cost of these molecular testing techniques limits their use in many regions where malaria is endemic.

Antibody tests (serology)
Serology tests detect antibodies in the blood that are produced by the body in response to a malaria infection. They cannot diagnose an acute infection but help determine if a person was previously exposed. These tests are not routinely used in the U.S. since a diagnosis can be made sooner by detecting the parasite under the microscope or its DNA instead of waiting for an immune response to develop weeks later.

Susceptibility testing
Some malarial parasites have become resistant to the drugs commonly used to treat the infections. Some specialized laboratories can test the parasites from an infected person to determine their drug susceptibility. This can be done either by growing the parasites in the presence of increasing amounts of the drug and observing the effect of the drug on the parasite or by testing the DNA of the parasite to detect markers that indicate resistance. This latter method is still being evaluated.


Currently, there is no vaccine available for malaria, though research is underway. People visiting or moving to areas where malaria is common (endemic) should seek medical advice well in advance of travel in order to minimize risk of exposure. This includes immigrants who return to their country of origin to visit friends and relatives. The Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) have information on advised measures to take to prevent exposure and infection (see Related Content).

All travelers should use personal protection methods to avoid mosquito bites, particularly from dusk to dawn. One of the most important means of protection is the use of mosquito nets for sleeping, particularly if accommodations do not have air conditioning or screens. Light-colored, loose fitting, long-sleeved shirts and long pants should be worn, and mosquito repellent with adequate DEET should be used. Permethrin-impregnated clothing and nets can be used by those at high risk.

Sometimes, anti-malarial medication is taken to prevent infection (prophylaxis); not everyone traveling to endemic areas will require it. Use of the medication carries risk of side-effects so drugs, such as chloroquine, can be used after consideration of risk. Factors that must be considered include specific areas of travel, season of travel, type and length of travel, and each person’s medical history. The type of medication recommended will depend on patterns of drug resistance and the individual’s history. Plasmodium falciparum is becoming increasingly resistant to the most commonly prescribed anti-malarial medications such as chloroquine, so those traveling to areas where this type of malaria is common may be prescribed other anti-malarials, including mefloquine, atovaquone/Proguanil, and doxycycline.

It is very important that the malarial prophylaxis be used as directed, including continuing the entire course for the prescribed amount of time, even after returning home from a trip, when exposure to malaria is no longer a risk.


Early diagnosis and treatment is critical as malaria can be life-threatening. Travelers who develop symptoms of malaria during or after travel should seek medical attention as soon as possible. Because of the delayed nature of the signs and symptoms of malaria, healthcare providers need to take a thorough travel history of their patients.

Treatment is guided by the type of Plasmodium causing the infection, the geographical area in which the infection was acquired (and local patterns of drug resistance), and how sick the person is. Pregnant and breastfeeding women and children require special consideration. People with uncomplicated malaria can be treated with oral medication; however, more severe disease requires that medication be administered directly into a vein (intravenous).

Chloroquine is often used to treat malaria; however, other medication may be needed in cases of chloroquine-resistant infections. The World Health Organization (WHO) recommends treatment, particularly for P. falciparum malaria, with artemisinin-based combination therapy (ACT).

Infections with P. vivax and P. ovale can relapse as the parasite can remain dormant in the liver, and they therefore require additional treatment.

View Sources

Sources Used in Current Review

Centers for Disease Control and Prevention (2012). Diagnosis and Treatment of Malaria in the Malaria-Endemic World. Available online at Accessed September 12, 2015.

Malaria Vaccine Initiative (2015). Accelerating Malaria Vaccine Development. Available online at Accessed September 14, 2015.

Maxmen, Amy (2013). Malaria: A race against resistance. Nature, Vol 503, 7475. Available online at Accessed September 15, 2015.

World Health Organization (2015). Malaria. Available online at Accessed September 15, 2015.

Sources Used in Previous Reviews

MedlinePlus Medical Encyclopedia. Malaria. Available online at Accessed April 2011.

National Institute of Allergy and Infectious Diseases. Understanding Malaria. Available online at Accessed April 2011.

World Health Organization. Malaria. Fact Sheet, April 2010. Available online at Accessed April 2011.

WHO. 10 facts on malaria. Available online at Accessed April 2011.

CDC. Malaria Surveillance — United States, 2006. MMWR. Available online at Accessed April 2011.

WHO. What is a malaria RDT? Available online at Accessed April 2011.

CDC. DPDx – Diagnostic Procedures: Blood Specimens, Specimen Collection. Available online at Accessed April 2011.

(February 28, 2011) CDC. Malaria Diagnosis and Treatment in the United States. Available online at Accessed April 2011.

(July 27, 2009) CDC. Yellow Book, Chapter 2, The Pre-travel Consultation, Malaria. Available online at Accessed April 2011.

(October 2007) UNICEF. Malaria diagnosis: A guide for selecting malaria rapid diagnostics test (RDT) kits-1st edition. PDF available for download at Accessed May 2011.

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Peter Van den Eede, Hong Nguyen Van, Chantal Van Overmeir, Indra Vythilingam, Thang Ngo Duc, Le Xuan Hung, Hung Nguyen Manh, Jozef Anné, Umberto D’Alessandro, Annette Erhart. Human Plasmodium knowlesi infections in young children in central Vietnam. Malaria Journal. 2009 8:249.

Stephanie P. Johnston, Norman J. Pieniazek, Maniphet V. Xayavong, Susan B. Slemenda, Patricia P. Wilkins, and Alexandre J. da Silva. PCR as a Confirmatory Technique for Laboratory Diagnosis of Malaria. J Clin Microbiol. 2006 (Mar) 44(3): 1087–1089.