The World Health Organization (WHO) has updated its fact sheet on malaria recently.
Background Information:
Malaria is caused by Plasmodium parasites. The parasites are spread to people through the bites of infected female Anopheles mosquitoes, called “malaria vectors.” There are 5 parasite species that cause malaria in humans, and 2 of these species – P. falciparum and P. vivax – pose the greatest threat.
- In 2017, P. falciparum accounted for 99.7% of estimated malaria cases in the WHO African Region, as well as in the majority of cases in the WHO regions of South-East Asia (62.8%), the Eastern Mediterranean (69%) and the Western Pacific (71.9%).
- P. vivax is the predominant parasite in the WHO Region of the Americas, representing 74.1% of malaria cases.
There are more than 400 different species of Anopheles mosquito; around 30 are malaria vectors of major importance. All of the important vector species bite between dusk and dawn.
Anopheles mosquitoes lay their eggs in water, which hatch into larvae, eventually emerging as adult mosquitoes. The female mosquitoes seek a blood meal to nurture their eggs. Each species of Anopheles mosquito has its own preferred aquatic habitat; for example, 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) and where it prefers to bite humans rather than other animals. The long lifespan and strong human-biting habit of the African vector species is the main reason why approximately 90% of the world’s malaria cases are in Africa.
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.
Key Messages:
In 2017, there were an estimated 219 million malaria cases and 435 000 malaria deaths in 87 countries.
In 2017, 5 countries accounted for nearly half of all malaria cases worldwide:
- Nigeria (25%),
- Democratic Republic of the Congo (11%),
- Mozambique (5%),
- India (4%) and
- Uganda (4%).
Children under 5 years of age are the most vulnerable group affected by malaria; in 2017, they accounted for 61% (266 000) of all malaria deaths worldwide.
The WHO African Region carries a disproportionately high share of the global malaria burden. In 2017, the region was home to 92% of malaria cases and 93% of malaria deaths.
In 2017, 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.
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,
- patients with HIV/AIDS,
- non-immune migrants,
- mobile populations and
- travellers.
Clinical Features
In a non-immune individual, symptoms usually appear 10–15 days after the infective mosquito bite. The first symptoms – fever, headache, and chills – may be mild and difficult to recognize as malaria. If not treated within 24 hours, P. falciparum malaria can progress to severe illness, often leading to death.
Children with severe malaria frequently develop one or more of the following symptoms:
- severe anaemia,
- respiratory distress in relation to metabolic acidosis, or cerebral malaria.
In adults, multi-organ failure is also frequent.
In malaria endemic areas, people may develop partial immunity, allowing asymptomatic infections to occur.
Diagnosis and Treatment
WHO recommends that all cases of suspected malaria be confirmed using parasite-based diagnostic testing (either microscopy or rapid diagnostic test) before administering treatment. Results of parasitological confirmation can be available in 30 minutes or less. Treatment, solely on the basis of symptoms should only be considered when a parasitological diagnosis is not possible.
Early diagnosis and treatment of malaria reduces disease, prevents deaths; and helps reduce malaria transmission. The best available treatment, particularly for P. falciparum malaria, is artemisinin-based combination therapy (ACT).
Antimalarial Drug Resistance
Resistance to antimalarial medicines is a recurring problem. Resistance of P. falciparum malaria parasites to previous generations of medicines, such as chloroquine and sulfadoxine-pyrimethamine (SP), became widespread in the 1950s and 1960s, undermining malaria control efforts and reversing gains in child survival.
Protecting the efficacy of antimalarial medicines is critical to malaria control and elimination. Regular monitoring of drug efficacy is needed to inform treatment policies in malaria-endemic countries, and to ensure early detection of, and response to, drug resistance.
Prevention and Control
Vector control is the main way to prevent and reduce malaria transmission. If coverage of vector control interventions within a specific area is high enough, then a measure of protection will be conferred across the community.
Two forms of vector control – insecticide-treated mosquito nets and indoor residual spraying – are effective in a wide range of circumstances.
Insecticide-treated mosquito nets (ITNs)
Sleeping under an insecticide-treated net (ITN) can reduce contact between mosquitoes and humans by providing both a physical barrier and an insecticidal effect. Population-wide protection can result from the killing of mosquitoes on a large scale where there is high access and usage of such nets within a community.
In 2017, about half of all people at risk of malaria in Africa were protected by an insecticide-treated net, compared to 29% in 2010. However, ITN coverage increased only marginally in the period 2015 to 2017.
Indoor Residual Spraying (IRS)
Indoor residual spraying (IRS) 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.
Globally, IRS protection declined from a peak of 5% in 2010 to 3% in 2017, with decreases seen across all WHO regions. The declines in IRS coverage are occurring as countries switch from pyrethroid insecticides to more expensive alternatives to mitigate mosquito resistance to pyrethroids.
Antimalarial Drugs
Antimalarial medicines can also be used to prevent malaria.
For travellers, malaria can be prevented through chemoprophylaxis, which suppresses the blood stage of malaria infections, thereby preventing malaria disease.
For pregnant women living in moderate-to-high transmission areas, WHO recommends intermittent preventive treatment with sulfadoxine-pyrimethamine, at each scheduled antenatal visit after the first trimester.
Similarly, for infants living in high-transmission areas of Africa, 3 doses of intermittent preventive treatment with sulfadoxine-pyrimethamine are recommended, delivered alongside routine vaccinations.
Malaria Vaccine
RTS,S/AS01 (RTS,S) is the first and, to date, the only vaccine to show partial protection against malaria in young children. It acts against P. falciparum, the most deadly malaria parasite globally and the most prevalent in Africa. Among children who received 4 doses in large-scale clinical trials, the vaccine prevented approximately 4 in 10 cases of malaria over a 4-year period.
In view of its public health potential, WHO’s top advisory bodies for malaria and immunization have jointly recommended phased introduction of the vaccine in selected areas of sub-Saharan Africa. The vaccine will be introduced in 3 pilot countries – Ghana, Kenya and Malawi – in 2019.
Elimination
Countries that have achieved at least 3 consecutive years of 0 local cases of malaria are eligible to apply for the WHO certification of malaria elimination. In recent years, 9 countries have been certified by the WHO Director-General as having eliminated malaria:
- United Arab Emirates (2007),
- Morocco (2010),
- Turkmenistan (2010),
- Armenia (2011),
- Maldives (2015),
- Sri Lanka (2016),
- Kyrgyzstan (2016),
- Paraguay (2018) and
- Uzbekistan (2018).
Useful Links:
Link to the updated fact sheet:
https://www.who.int/news-room/fact-sheets/detail/malaria
Link to World Malaria Report 2018:
https://www.who.int/malaria/publications/world-malaria-report-2018/en/
Link to Question and Answer on the Malaria Vaccine Implementation Programme (MVIP):
https://www.who.int/malaria/media/malaria-vaccine-implementation-qa/en/
Link to WHO’s brochure on the malaria vaccine:
https://www.who.int/malaria/publications/atoz/first-malaria-vaccine/en/
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