Key facts
• Malaria is a life-threatening disease caused by parasites that are transmitted to people through the bites of infected mosquitoes.
• In 2012, malaria caused an estimated 627 000 deaths mostly among African children.
• Malaria is preventable and curable.
• Increased malaria prevention and control measures are dramatically reducing the malaria burden in many places.
• Non-immune travellers from malaria-free areas are very vulnerable to severe disease when they get infected especially if there are any delays in treatment.
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According to the latest estimates, there were approximately 207 million cases of malaria in 2012 and an estimated 627 000 deaths. Malaria mortality rates have fallen by more than 25% globally since 2000, and by 33% in the WHO African Region. Most deaths occur among children living in Africa where a child dies every minute from malaria. Malaria is caused by Plasmodium parasites. The parasites are spread to people through the bites of infective Anopheles mosquitoes, called “malaria vectors”, which bite mainly between dusk and dawn.
There are four parasite species that cause malaria in humans:
• Plasmodium falciparum
• Plasmodium vivax
• Plasmodium malariae
• Plasmodium ovale.
Plasmodium falciparum and Plasmodium vivax are the most common. Plasmodium falciparum is the most deadly.
In recent years, some human cases of malaria have also occurred with Plasmodium knowlesi – a species that causes malaria among monkeys and occurs in certain forested areas of South-East Asia.
Transmission
Malaria is transmitted exclusively through the bites of Anopheles mosquitoes. The intensity of transmission depends on factors related to the parasite, the vector, the human host, and the environment.
Globally, approximately 70 Anopheles species are capable of transmitting malaria of which approximately 20 are important malaria vectors. The most important malaria vector mosquito species preferentially bite humans although some species bite animals as well. In South Africa, Anopheles funestus and Anopheles arabiensis are the primary vectors of malaria.
All of the important vector species bite at night. Anopheles mosquitoes breed in water and each species has its own breeding preference; for example, some species, including Anopheles arabiensis, prefer shallow collections of fresh water, such as puddles and hoof prints. Other species prefer more permanent bodies of water, such as Anopheles funestus. Transmission is more intense in places where the vector mosquito lifespan is longer (so that the parasite has time to complete its development inside the mosquito) and where the females of local vector species bite several times in one night.
Transmission also 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 favour 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. For this reason, most malaria deaths in Africa occur in young children, whereas in areas with less transmission and low immunity, all age groups are at risk.
Symptoms
Malaria is an acute febrile illness. In a non-immune individual, symptoms appear seven days or more (usually 10–15 days) after the infective mosquito bite. The first symptoms – fever, headache, chills and vomiting – 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 involvement is also frequent. In malaria endemic areas, persons may develop partial immunity, allowing asymptomatic infections to occur.
For both P. vivax and P. ovale, clinical relapses may occur weeks to months after the first infection, even if the patient has left the malarious area. These new episodes arise from dormant liver forms known as hypnozoites (absent in P. falciparum and P. malariae); special treatment – targeted at these liver stages – is required for a complete cure.
Who is at risk?
Approximately half of the world’s population is at risk of malaria. Most malaria cases and deaths occur in sub-Saharan Africa. Approximately 5 million people (10% of total population) are at risk in South Africa.
South Africa’s malaria risk regions include the low-
Figure 1: Malaria risk map for South Africa (released by National Department of Health).
Specific population risk groups include:
• young children in stable transmission areas who have not yet developed protective immunity against the most severe forms of the disease;
• non-immune pregnant women as malaria causes high rates of miscarriage and can lead to maternal death;
• semi-immune pregnant women in areas of high transmission. Malaria can result in miscarriage and low birth weight, especially during first and second pregnancies;
• semi-immune HIV-infected pregnant women in stable transmission areas, during all pregnancies. Women with malaria infection of the placenta also have a higher risk of passing HIV infection to their newborns;
• people with HIV/AIDS;
• international travellers from non-endemic areas because they lack immunity;
• immigrants from endemic areas and their children living in non-endemic areas and returning to their home countries to visit friends and relatives are similarly at risk because of waning or absent immunity.
Diagnosis and treatment
Early diagnosis and treatment of malaria reduces disease and prevents deaths. It also contributes to reducing malaria transmission.
The best available treatment, particularly for P. falciparum malaria, is artemisinin-based combination therapy (ACT).
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 15 minutes or less. Treatment solely on the basis of symptoms should only be considered when a parasitological diagnosis is not possible. More detailed recommendations are available in the Guidelines for the treatment of malaria (second edition).
Prevention
Vector control using insecticides is the main method of reducing malaria transmission at the community level. It is the only intervention that can reduce malaria transmission from very high levels to close to zero. Insecticides are used to spray the insides of houses and are also used to treat fabrics, especially bed-nets.
For individuals, personal protection against mosquito bites represents the first line of defence for malaria prevention. This can involve the use of insect repellents, the burning of insecticide coils, the wearing of long-sleeved clothing and socks in the evenings, sleeping under a bed-net and screening windows and doors.
The principal form of vector control in South Africa is indoor spraying with residual insecticides (IRS). This is a powerful method of rapidly reducing malaria transmission. Its full potential is realized when at least 80% of houses in targeted areas are sprayed. Indoor spraying is effective for 3–6 months, depending on the insecticide used and the type of surface on which it is sprayed. DDT can be effective for 9–12 months in some cases. Longer-lasting forms of existing IRS insecticides, as well as new classes of insecticides for use in IRS programmes, are under development. All three of South Africa’s malaria affected provinces – Mpumalanga, KwaZulu-Natal and Limpopo – conduct annual IRS operations.
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. In addition, WHO recommends intermittent preventive treatment with sulfadoxine-pyrimethamine for pregnant women living in high transmission areas, 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 is recommended delivered alongside routine vaccinations. In 2012, WHO recommended Seasonal Malaria Chemoprevention as an additional malaria prevention strategy for areas of the Sahel sub-Region of Africa. The strategy involves the administration of monthly courses of amodiaquine plus sulfadoxine-pyrimethamine to all children under 5 years of age during the high transmission season.
Surveillance
Tracking progress is a major challenge in malaria control. Malaria surveillance systems detect only around 10% of the estimated global number of cases. Stronger malaria surveillance systems are urgently needed to enable a timely and effective malaria response in endemic regions, to prevent outbreaks and resurgences, to track progress, and to hold governments and the global malaria community accountable. In April 2012, the WHO Director-General launched new global surveillance manuals for malaria control and elimination, and urged endemic countries to strengthen their surveillance systems for malaria. This was part of in a larger call to scale up diagnostic testing, treatment and surveillance for malaria, known as WHO’s T3: Test. Treat. Track initiative.
Elimination
Malaria elimination is defined as interrupting local mosquito-borne malaria transmission in a defined geographical area, i.e. zero incidence of locally contracted cases. Malaria eradication is defined as the permanent reduction to zero of the worldwide incidence of malaria infection caused by a specific agent; i.e. applies to a particular malaria parasite species. South Africa is scheduled to achieve malaria elimination by 2018.
Large-scale use of WHO-recommended strategies, currently available tools, strong national commitments, and coordinated efforts with partners, will enable more countries – particularly those where malaria transmission is low and unstable – to progress towards malaria elimination. In recent years, 4 countries have been certified by the WHO Director-General as having eliminated malaria: United Arab Emirates (2007), Morocco (2010), Turkmenistan (2010), and Armenia (2011).
Vaccines against malaria
There are currently no licensed vaccines against malaria or any other human parasite. One research vaccine against P. falciparum, known as RTS,S/AS01, is most advanced. This vaccine is currently being evaluated in a large clinical trial in 7 countries in Africa. A WHO recommendation for use will depend on the final results from the large clinical trial. These final results are expected in late 2014, and a recommendation as to whether or not this vaccine should be added to existing malaria control tools is expected in 2015.
Reference
World Health Organisation (WHO) World Malaria Report, 2013 http://www.who.int/malaria/publications/world_malaria_report_2013/report/en/
lying areas of Mpumalanga, KwaZulu-Natal and Limpopo, particularly the border regions with Mozambique, Swaziland, Botswana and Zimbabwe (figure 1).
