Vector Control, Saving Lives

WHO Malaria fact sheet

According to the World Malaria Report 2011, there were 216 million cases of malaria and an estimated 655 000 deaths in 2010. 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 of malaria and the disease accounts for approximately 22% of all childhood deaths.

Key facts

  • Malaria is a life-threatening disease caused by parasites that are transmitted to people through the bites of infected mosquitoes.
  • In 2010, malaria caused an estimated 655 000 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.
  • Malaria can decrease gross domestic product by as much as 1.3% in countries with high disease rates.
  • Non-immune travelers from malaria-free areas are very vulnerable to the disease when they get infected.

Malaria is caused by Plasmodium parasites. The parasites are spread to people through the bites of infected 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 monkey malaria that 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.

About 20 different Anopheles species are locally important around the world. 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 prefer shallow collections of fresh water, such as puddles, rice fields, and hoof prints. 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. For example, the long lifespan and strong human-biting habit of the African vector species is the main reason why more than 85% of the world's malaria deaths are in Africa.

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. Immunity is developed over years of exposure, and while it never gives 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 (absent in P. falciparum and P.malariae), and special treatment – targeted at these liver stages – is mandatory 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. However, Asia, Latin America, and to a lesser extent the Middle East and parts of Europe are also affected. In 2010, malaria was present in 106 countries and territories.

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 (up to 60% in P. falciparum infection) and maternal death rates of 10–50%;
  • semi-immune pregnant women in areas of high transmission. Malaria can result in miscarriage and low birth weight, especially during first and second pregnancies. An estimated 200 000 infants die annually as a result of malaria infection during pregnancy;
  • 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 giving treatment. Results of parasitological confirmation can be available in a few minutes. 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.

Drug resistance
Growing resistance to antimalarial medicines has spread very rapidly, undermining malaria control efforts.
When treated with an artemisinin-based monotherapy, patients may discontinue treatment prematurely following the rapid disappearance of malaria symptoms. This results in incomplete treatment, and such patients still have persistent parasites in their blood. Without a second drug given as part of a combination (as is done with an ACT), these resistant parasites survive and can be passed on to a mosquito and then another person. Such monotherapies are therefore one of the primary forces behind the spread of artemisinin resistance.
If resistance to artemisinins develops and spreads to other large geographical areas, as has happened before with chloroquine and sulfadoxine-pyrimethamine (SP), the public health consequences could be dire, as no alternative antimalarial medicines will be available for at least five years.
WHO recommends the routine monitoring of antimalarial drug resistance, and supports countries to strengthen their efforts in this important area of work.
More comprehensive recommendations are available in the Global Plan for Artemisinin Resistance Containment (GPARC).

Prevention

Vector control is the main way to reduce malaria transmission at the community level. It is the only intervention that can reduce malaria transmission from very high levels to close to zero. For individuals personal protection against mosquito bites represents the first line of defence for malaria prevention.

Two forms of vector control are effective in a wide range of circumstances.

  • Insecticide-treated mosquito nets (ITNs)

Long-lasting insecticide impregnated nets (LLINs) are the preferred form of ITNs for public health distribution programmes. WHO recommends coverage for all at-risk persons; and in most places, the most cost effective way to achieve this is through provision of LLINs, so that everyone in high transmission areas sleeps under a LLIN every night.

  • Indoor spraying with residual insecticides

Indoor residual spraying (IRS) with insecticides is the most powerful way to rapidly reduce 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 IRS insecticides are under development.

Drugs 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. WHO recommends intermittent preventive treatment with sulfadoxine-pyrimethamine for pregnant women living in high transmission areas, during the second and third trimesters. Similarly, for infants living in high-transmission areas of Africa, 3 doses of intermittent preventive treatment with SP is recommended delivered alongside routine vaccinations.

Insecticide resistance
Much of the success to date in controlling malaria is due to vector control. Vector control is highly dependent on the use of pyrethroids, which are the only class of insecticides used on currently recommended ITNs or LLINs. Resistance to pyrethroids has emerged, especially Africa, although so far there have been only one or two cases of obvious control failure.
Currently there is a lack of alternative, cost-effective and safe insecticides. The development of new, alternative insecticides is a high priority, but is an expensive and long-term endeavour. Development of new insecticides for use on nets is a particular priority.
Detection of insecticide resistance should be an essential component of all national malaria control efforts to ensure that the most effective vector control methods are being used. The choice of insecticide for IRS should always be informed by recent, local data on the susceptibility target vectors.
Economic and health system impact
Malaria causes significant economic losses, and can decrease gross domestic product (GDP) by as much as 1.3% in countries with high levels of transmission. Over the long term, these aggregated annual losses have resulted in substantial differences in GDP between countries with and without malaria, particularly in Africa.
The health costs of malaria include both personal and public expenditures on prevention and treatment. In some heavy-burden countries, the disease accounts for:

  • up to 40% of public health expenditures;
  • 30% to 50% of inpatient hospital admissions;
  • up to 60% of outpatient health clinic visits.

Malaria disproportionately affects poor people who cannot afford treatment or have limited access to health care, trapping families and communities in a downward spiral of poverty.

Elimination
Many countries – especially in temperate and sub-tropical zones – have been successful in eliminating malaria. The global malaria eradication campaign, launched by WHO in 1955, was successful in eliminating the disease in some countries, but ultimately failed to achieve its overall goal, thus being abandoned less than two decades later in favour of the less ambitious goal of malaria control. In recent years, however, interest in malaria eradication as a long-term goal has re-emerged.
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. Results will be emerging from this trial in 3 stages, and each set of results will be reviewed by external WHO advisory committees. A WHO recommendation for use will depend on the final results from the large clinical trial. Final results are expected in 2014 . Other malaria vaccines are at earlier stages of research.

WHO response
The WHO Global Malaria Programme is responsible for charting the course for malaria control and elimination through:

  • forming evidence-based policy and strategy formulation;
  • keeping independent score of global progress;
  • developing approaches for capacity building, systems strengthening, and surveillance;
  • identifying threats to malaria control and elimination as well as new areas for action.

WHO is also a co-founder and host of the Roll Back Malaria partnership, which is the global framework to implement coordinated action against malaria. The partnership mobilizes for action and resources and forges consensus among partners. It is comprised of over 500 partners, including malaria endemic countries, development partners, the private sector, nongovernmental and community-based organizations, foundations, and research and academic institutions.

 

Extracts from the WHO Malaria Fact sheet N°94 
December 2011