Facts about malaria
Malaria is caused by Plasmodium parasites. Four Plasmodium species (Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale and Plasmodium malariae) give disease in humans, and humans are their only relevant reservoir.
Transmission requires an intermediate mosquito (anopheles) host, which is found worldwide. Following exposure (an infected mosquito bite) the incubation period varies between one and four weeks in most cases. Depending on the plasmodium species involved, much longer incubation periods are possible.
Once the Plasmodia multiply inside the red blood cells, fever and multi-organ disease may ensue, which can be life-threatening when P. falciparum is involved. Symptoms are much reduced if the patient is semi-immune by repeated previous infection. Several drugs are available for both treatment and prophylaxis.
During the 20th century, malaria was eradicated from many temperate areas, including the whole of the EU. As a result, the disease is now essentially limited to tropical countries. With global climate change, the potential for the reappearance of malaria in countries where it was previously eradicated exists but is relatively small. Anopheles mosquitoes are in fact still present in those areas, including in Europe.
Due to the large number of imported cases in Europe, malaria is mainly a travel medicine issue. Nonetheless, ‘airport malaria’ is sometimes reported in relation to the inadvertent transport of infected mosquitoes from endemic areas.
Name and nature of infecting organism
Malaria is an acute or subacute infectious disease caused by one of four protozoan species of the genus Plasmodium: P. falciparum, P. vivax, P. ovale, or P. malariae, mainly transmitted by Anopheles mosquitoes. Occasionally, transmission occurs by blood transfusion, organ transplantation, needle-sharing, or congenitally from mother to foetus.
Malaria is one of the major world public health problems, causing 350-500 million infections worldwide and approximately 1 million deaths annually. Malaria transmission occurs in large areas of Central and South America, Africa, Asia, Eastern Europe, and the South Pacific. In endemic areas such as tropical Africa, the incidence of malaria is higher in children younger than five years, due to low acquired immunity.
The clinical presentation of malaria depends very much on the pattern and intensity of malaria transmission in the area of residence, which determines the degree of protective immunity acquired and, in turn, the clinical disease profile.
The incubation period mainly varies between 7 and 15 days, but long incubation periods of several months have been observed, depending on patients and Plasmodium species. Malaria is characterised by fever and influenza-like symptoms, including chills, headache, myalgia, and malaise; these symptoms can occur at intervals. Malaria may be associated with anaemia and jaundice, and P. falciparum infections can cause seizures, mental confusion, kidney failure, coma, and death, particularly in young children.
There is no animal reservoir for malaria parasites; it is a purely human disease.
All four human Plasmodium species are transmitted by the bite of an infected female Anopheles mosquito. About 60–100 anopheline species are able to transmit malaria in the world.
Infected humans remain infectious to mosquitoes as long as they carry mature gametocyte forms of plasmodium. This period depends on the Plasmodium species, on the immune status of the patients, and on treatment provided; in endemic areas it generally lasts for several weeks.
In endemic areas, populations at higher risk of severe malaria are children under five years of age and pregnant women. In unstable or low endemic areas, as well as in travellers from non-malaria regions, all ages are at risk.
Prevention of malaria is currently based on two complementary methods: chemoprophylaxis and protection against mosquito bites. While several malaria vaccines are under development, none is available yet.
In Europe, malaria chemoprophylaxis is only for travellers to malaria endemic countries, which are classified in three (or four) groups, to determine which drug is recommended for chemoprophylaxis. The choice of drugs depends on the travel destination, the duration of potential exposure to vectors, parasite resistance pattern, level and seasonality of transmission, age and pregnancy. In endemic countries, chemoprophylaxis could also be recommended for autochthonous young children and pregnant women, depending on endemicity level and seasonality of transmission.
Personal protection measures against mosquito bites
Because of the nocturnal feeding habits of most of Anopheles mosquitoes, malaria transmission occurs primarily at night. Personal protective measures to reduce the risk of mosquito bites include the use of mosquito bed nets (preferably insecticide-treated nets), sleeping or resting in screened or air-conditioned rooms, the wearing of clothes that cover most of the body, and the use of mosquito repellent in accordance with the instructions indicated on the product label.
Vector control measures depend on vector species, mosquito biology, epidemiological context, cost and acceptability by populations. The main current measures are focused on reduction of the contact between mosquitoes and humans, the destruction of larvae by environmental management and the use of larvicides or mosquito larvae predators, and destruction of adult mosquitoes by indoor residual spraying and insecticide-treated bed nets.
The gold standard for laboratory confirmation of malaria remains the identification of malaria parasites in blood films. Alternate methods for laboratory diagnosis include 1) antigen detection using rapid diagnostic tests, a useful alternative in situations where reliable microscopic diagnosis is not available; 2) molecular diagnosis, which is more accurate than microscopy but also more expensive and (currently still) requiring a specialised laboratory; and 3) serology, using indirect immuno-fluorescence (IFA) or enzyme-linked immuno-sorbent assay (ELISA). Serology does not detect current infection but rather measures past malaria experience.
Management and treatment
Although malaria can be fatal, illness and death from malaria are largely preventable. Early diagnosis and prompt treatment are the basic elements of malaria control. Anti-malarial treatment policies vary between countries, and depend on the epidemiology of the disease, transmission, patterns of drug resistance and political and economic contexts. There is no universal treatment scheme. The drugs currently available for malaria treatment include chloroquine, amodiaquine, primaquine sulfadoxine-pyrimethamine, mefloquine, atovaquone-proguanil, quinine, doxycycline, and artemisin derivatives often used in combination therapy.
Key areas of uncertainty
Research into an effective vaccine is the subject of an ongoing international effort. Diagnostics to ensure rapid detection of infection, as well as the parasite’s resistance to the available drugs and the vector’s resistance to the available insecticides, are also the focus of continuing investigations.
- Baird JK. Effectiveness of antimalarial drugs. N Engl J Med 2005;352:1565-77.
- Matuschewski and Mueller.. Vaccines against malaria – an update. FEBS J 2007;274(18):4680-7.
- Shanks GD, Edstein MD. Modern malaria chemoprophylaxis. Drugs 2005;65:2091-110.
- WHO. Malaria Vector Control and Personal Protection. Report of a WHO Study Group, WHO Technical Report Series 2006; no 936.
- WHO. International Travel and Health. Geneva: WHO edition; 2007.