Malaria is a serious vector-borne infectious disease.
Malaria is caused by five species of a protozoan parasite belonging to genus Plasmodium. The parasite is transmitted by the bite of infected female Anopheles mosquito. Malaria is not a contagious disease. It can only be passed from one human to another by an exchange of infected blood either through shared needles or a transfusion of infected blood. Infection with one species of Plasmodium does not create immunity against the other species. Although the different species of Plasmodium are generally geographically separate, there is some overlap, and 5–7% of people who develop malaria are infected with more than one species of Plasmodium. Other animals (e.g., reptiles, birds, rodents) can contract malaria, but human malaria parasite species do not infect animals, and animal malaria parasites do not infect humans.
Once in the body, the parasite immediately enters the liver where it multiplies. After several weeks, it leaves the liver and enters the bloodstream where it infects red blood cells. It continues to develop, and after a few days, the infected red blood cells burst. This causes the classic symptoms of recurrent cycles of chills, fever, and sweating.
Malaria is endemic in many tropical and subtropical countries and is a major public health problem particularly in developing countries in Africa and Southeast Asia. Isolated, small outbreaks sometimes occur within the United States; however, 95% of cases are from travelers infected in an endemic area and returning to the United States. Some scholars worry that global warming will allow the Anopheles mosquito to expand its range so that malaria outbreaks may become more widespread.
In the 1800s and early 1900s, malaria was common in the United States, especially in subtropical Florida and New Orleans. The disease was eliminated in the 1950s, but in 2016, the CDC reported about 1,700 cases of malaria among persons in the United States. Almost all were acquired by travelers coming from outside the United States. This was a 30% increase in cases from 2008. In Europe, between 10,000 and 30,000 travelers acquire malaria by traveling abroad to tropical areas.
The picture is far more devastating in areas where malaria is endemic. In 2016, the World Health Organization (WHO) reported 216 million cases of malaria worldwide that resulted in 445,000 deaths. The WHO estimates that 3.2 billion people live in areas where they are at risk for contracting malaria. People living in the poorest countries are the most vulnerable, including most parts of Africa, India, Southeast Asia, Oceania, and Central and South America. The spread of malaria is becoming more serious as the parasites that cause the disease develop resistance to the drugs used to treat the condition.
The amount of time between the mosquito bite and the appearance of symptoms varies, depending on the strain of parasite involved. The incubation period is usually between 8 and 12 days for Plasmodium falciparum (falciparum malaria), but it can be as long as a month for the other types. Symptoms from some strains of P.vivax may not appear until 8–10 months after the mosquito bite occurred.
The primary symptom of all types of malaria is the malaria ague (chills and fever). In most cases, the fever has three stages, beginning with uncontrollable shivering for an hour or two, followed by a rapid spike in temperature (as high as 106°F or 41°C), which lasts three to six hours. Then, just as suddenly, the patient begins to sweat profusely, which quickly brings down the fever. Other symptoms may appear, including fatigue, severe headache, and nausea and vomiting. As the sweating subsides, the patient typically feels exhausted and falls asleep. In many cases, this cycle of chills, fever, and sweating occurs every other day, or every third day and may continue for between a week and a month. Those with the chronic form of malaria may have a relapse as long as 50 years after the initial infection.
Falciparum malaria is far more severe than other types of malaria because the parasite attacks all red blood cells, not just the young or old cells, as do other types. It causes the red blood cells to become sticky. A patient with this type of malaria can die within hours of the first symptoms. The fever is prolonged. So many red blood cells are destroyed that they block the blood vessels in vital organs. They damage the kidneys, and the spleen becomes enlarged. There may be brain damage, leading to coma and convulsions. The kidneys and liver may fail. Malaria in pregnancy can lead to premature delivery, miscarriage, or stillbirth.
Individuals who become ill with chills and fever after being in an area where malaria exists must see a doctor and mention their recent travel to endemic areas. Individuals with the above symptoms who have been in a high-risk area should request a blood test for malaria. Malaria is often misdiagnosed by North American doctors who are not used to seeing the disease. Delaying treatment of falciparum malaria can be fatal.
Malaria is diagnosed by examining blood under a microscope. The parasite can be seen in the blood smears on a slide. These blood smears may need to be repeated over a 72-hour period to make a definitive diagnosis. Antibody tests are not usually helpful because many people develop antibodies from past infections, and the tests may not be readily available.
Falciparum malaria is a medical emergency that must be treated in the hospital. The World Health Organization (WHO) recommends artemisinin-based combination therapies (ACTs) as a first-line treatment for P. falciparum malaria. Eighty countries have adopted this strategy.
Individuals who acquired falciparum malaria in the Dominican Republic, Haiti, Central America west of the Panama Canal, the Middle East, or Egypt can still be cured with chloroquine. Almost all strains of falciparum malaria in Africa, South Africa, India, and Southeast Asia are resistant to chloroquine. In Thailand and Cambodia, there are strains of falciparum malaria that have some resistance to almost all known drugs.
Patients with falciparum malaria need to be hospitalized and given antimalarial drugs in different combinations and doses depending on the resistance of the strain. Patients may need IV fluids, red blood cell transfusions, kidney dialysis, and assistance breathing.
The drug primaquine may prevent relapses after recovery from P. vivax or P. ovale. These relapses are caused by a form of the parasite that remains in the liver and can reactivate months or years later.
Another drug, halofantrine, is available abroad. While it is licensed in the United States, it is not marketed in the United States, and it is not recommended by the CDC. Individuals who get antimalarial drugs abroad or through the Internet should be aware that some of these products are counterfeit and are not effective.
Alternative treatments are those that replace Western pharmaceutical drugs. Complementary treatments are treatments used along with pharmaceutical drugs. The Chinese herb qinghaosu (the Western name is artemisinin) has been used in China and Southeast Asia to fight severe malaria, and it became available in Europe in 1994. Because this treatment often fails, it is usually combined with another antimalarial drug (mefloquine) to boost its effectiveness. It is not available in the United States and other parts of the developed world due to fears of its toxicity, in addition to licensing and other issues.
The Western herb wormwood (Artemisia annua) that is taken as a daily dose can be effective against malaria. Protecting the liver with herbs such as goldenseal (Hydrastis canadensis), Chinese goldenthread (Coptis chinensis), and milk thistle (Silybum marianum) can be used as preventive treatment. Taking precautions to prevent mosquitoes from biting is another possible way to avoid malaria.
The World Health Organization reports that $2.7 billion (USD) was available to fight malaria in 2017. This funding included monies for research for a malaria vaccine, provision of mosquito nets, mosquito abatement programs, treatment, and education. The Bill and Melinda Gates Foundation has been a leader among nongovernmental agencies in the fight against malaria. On April 18, 2018, the Foundation pledged another $1 billion through 2023 in the effort to eradicate malaria. Multiple vaccines against the disease were under development, but as of 2018, none had proven effective enough to be licensed.
If treated in the early stages, malaria can be cured. Those who live in areas where malaria is endemic, however, can contract the disease repeatedly, never fully recovering between bouts of acute infection. Prognosis is worse for young children, pregnant women, and those infected with P. falciparum.
Though the disease presents special problems for vaccine developers because the parasite goes through several separate stages, RTS, S (Mosquirix) is a malaria vaccine approved for use in 2015. The World Health Organization (WHO) continues to work to eliminate malaria by controlling mosquitoes.
Those who use the following preventive measures get fewer infections than those who do not:
In certain areas, seasonal treatment of sulfadoxine-pyrimethamine and amodiaquine prophyllaxis are given to children.
Individuals visiting endemic areas should take antimalarial drugs starting a day or two before they leave the United States. The drugs used are chloroquine or mefloquine, as well as atovaquone-proguanil, and also primaquine for treatment of P. vivax malaria. This treatment is continued for at least four weeks after leaving the endemic area. Even those who take antimalarial drugs and are careful to avoid mosquito bites can still contract malaria.
International travelers are at risk for becoming infected. Most Americans who have acquired falcipa-rum malaria visited sub-Saharan Africa; travelers in Asia and South America are less at risk. Travelers who stay in air-conditioned hotels on tourist itineraries in urban or resort areas are at lower risk than backpackers, missionaries, and Peace Corps volunteers. Rarely, people in cities where malaria does not usually exist may acquire the infection from a mosquito carried onto a jet. This is called airport or runway malaria.
See also Endemic ; Parasites ; World Health Organization .
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Bill & Melinda Gates Foundation, PO Box 23350, Settle, WA, 98102, (206) 709-3100, https://www.gatesfoundation.org .
U.S. Centers for Disease Control and Prevention, 1600 Clifton Rd., Atlanta, GA, 30329-4027, (404) 639-3534, (800) 232-4636; TTY: 888-232-6348, http://www.cdc.gov .
World Health Organization, Avenue Appia 20, 1211 Geneva 27, Switzerland, +2241 791 21 11, Fax: +2241 791 31 11, email@example.com, http://www.who.int .
Rebecca J. Frey, PhD
Revised by Tish Davidson, AM