Malaria is caused by Plasmodium parasites. The parasites are spread to people through the bites of infected female Anopheles mosquitoes. There are currently 5 parasite species that cause malaria in humans: Plasmodium falciparum, P. vivax, P. ovale, P.malariae, P. knowlesi. Among those species, P. falciparum and P. vivax pose the greatest threat. In recent years, nearly half of the world’s population was at risk of malaria. Most malaria cases and deaths occur in sub-Saharan Africa. However, the WHO regions of South-East Asia, Eastern Mediterranean, Western Pacific, and the Americas are also at risk. P. falciparum accounted for 99.7% of estimated malaria cases in the African region, as well as in the majority of cases in the countries of South-East Asia (62.8%), the Eastern Mediterranean (69%) and the Western Pacific (71.9%). Meanwhile, P. vivax is the predominant parasite in the region of the Americas, representing 74.1% of malaria cases. P. knowlesi is a primate malaria parasite commonly found in Southeast Asia, particularly in Malaysian Borneo and neighboring countries. It causes malaria in long-tailed macaques (Macaca fascicularis), but it may also infect humans. P. malariae and P. ovale are responsible for a relatively small proportion of malaria cases.
Malaria is mainly transmitted through the bites of female Anopheles mosquitoes. There are more than 400 different species of Anopheles mosquito, around which 30 are malaria vectors of major importance. All of the important vector species bite between dusk and dawn. The intensity of transmission depends on factors related to the parasite, the vector, the human host, and the environment. Anopheles mosquitoes lay their eggs in water, which hatch into larvae, eventually emerging as adult mosquitoes. Each species of Anopheles mosquito has its own preferred aquatic habitat: some prefer small, shallow collections of fresh water, such as puddles and hoof prints, which are abundant during the rainy season in tropical countries. Transmission is more intense in places where the mosquito lifespan is longer, so that the parasite has time to complete its development inside the mosquito. Transmission also intensifies where it prefers to bite humans rather than other animals. Transmission depends on climatic conditions that may affect the number and survival of mosquitoes, such as rainfall patterns, temperature and humidity. In many places, transmission is seasonal, with the peak during and just after the rainy season. Malaria epidemics can occur when climate and other conditions suddenly favor transmission in areas where people have little or no immunity to malaria. They can also occur when people with low immunity move into areas with intense malaria transmission, for instance to find work, or as refugees. Human immunity is another important factor, especially among adults in areas of moderate or intense transmission conditions. Partial immunity is developed over years of exposure, and while it never provides complete protection, it does reduce the risk that malaria infection will cause severe disease.
Malaria is an acute febrile illness. Some population groups are at considerably higher risk of contracting malaria, and developing severe disease, than others. These include infants, children under 5 years of age, pregnant women and patients with HIV/AIDS, as well as non-immune migrants, mobile populations and travelers. In a non-immune individual, symptoms usually appear 10-15 days after the infective mosquito bite. The first symptoms, including fever, headache, and chills, may be mild and difficult to recognize as malaria. If left unreated, P. falciparum malaria can progress to severe illness, often leading to death. Persons with severe malaria frequently develop one or more of the following symptoms: severe anaemia, respiratory distress in relation to metabolic acidosis, or cerebral malaria. Multi-organ failure is also frequent. Early diagnosis and treatment of malaria reduces disease and prevents deaths. The WHO recommends that all cases of suspected malaria be confirmed using parasite-based diagnostic testing (either microscopy or rapid diagnostic test) before administering treatment. Treatment, solely on the basis of symptoms should only be considered when a parasitological diagnosis is not possible. The best available treatment, particularly for P. falciparum malaria, is artemisinin-based combination therapy (ACT). However, treatment of choice may also depend on the resistance status of antimalarial drugs in the regions. Resistance to antimalarial drugs is a recurring problem, particularly against chloroquine, sulfadoxine-pyrimethamine (SP), and, in Greater Mekong region, artemisinins.
Vector control is the main way to prevent and reduce malaria transmission. Generally, two forms of vector control are effective in a wide range of circumstances: insecticide-treated mosquito nets (ITNs) and indoor residual spraying (IRS). Sleeping under an ITN can reduce contact between mosquitoes and humans by providing both a physical barrier and an insecticidal effect. Indoor residual spraying with insecticides involves spraying the inside of housing structures with an insecticide, typically once or twice per year. To confer significant community protection, IRS should be implemented at a high level of coverage. For travelers, malaria can be prevented through antimalarial medicines (chemoprophylaxis), which suppresses the blood stage of malaria infections, thereby preventing malaria disease. For infants and pregnant women living in moderate-to-high transmission areas, the WHO recommends intermittent preventive treatment with sulfadoxine-pyrimethamine. Seasonal malaria chemoprevention is recommended as an additional malaria prevention strategy for areas of the Sahel sub-region of Africa. Recently, the RTS,S/AS01 (RTS,S) is the first vaccine to show partial protection against P. falciparum and is currently undergoing trials in 3 pilot countries: Ghana, Kenya and Malawi.
Since early 2000s, progress in malaria control has resulted primarily from expanded access to vector control interventions. However, these gains are threatened by emerging resistance to insecticides among Anopheles mosquitoes. According to the latest World malaria report, 68 countries reported mosquito resistance to at least 1 of the 5 commonly-used insecticide classes in the period 2010-2017. Among these countries, 57 reported resistance to 2 or more insecticide classes. To prevent an erosion of the impact of core vector control tools, the need for all countries with ongoing malaria transmission to develop and apply effective insecticide resistance management strategies is critical.
Surveillance entails tracking of the disease and programmatic responses, and taking action based on the data received. Currently, many countries with a high burden of malaria have weak surveillance systems and are not in a position to assess disease distribution and trends, making it difficult to optimize responses and respond to outbreaks. Effective surveillance is required at all points on the path to malaria elimination. Malaria elimination is defined as the interruption of local transmission of a specified malaria parasite species in a defined geographical area as a result of deliberate activities. Continued measures are required to prevent re-establishment of transmission. Malaria eradication is defined as the permanent reduction to zero of the worldwide incidence of malaria infection caused by human malaria parasites as a result of deliberate activities. Interventions are no longer required once eradication has been achieved.
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World Health Organization. Guidelines for the treatment of malaria. 3rd ed. Geneva: Global Malaria Programme, World Health Organization; 2015.
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World Health Organization. Malaria [updated 27 March 2019]. Available from: https://www.who.int/en/news-room/fact-sheets/detail/malaria.
