K4A - About malaria

Key facts

  • Malaria is a life-threatening disease caused by parasites that are transmitted to people
    through the bites of infected mosquitoes.
  • In 2008, malaria caused nearly one million deaths, mostly among African children.
  • Malaria is preventable and curable.
  • 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.

In 2008, there were 247 million cases of malaria and nearly one million deaths – mostly among children living in Africa. In Africa a child dies every 45 seconds of Malaria, the disease accounts for 20% of all childhood deaths.

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 types of human malaria:

  1. Plasmodium falciparum
  2. Plasmodium vivax
  3. Plasmodium malariae
  4. 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. They breed in shallow collections of freshwater like puddles, ricefields, and hoofprints. Transmission is more intense in places where the mosquito is relatively long-lived (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 underlying reason why more than 85% of the world's malaria deaths are in Africa.

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.

Transmission also depends on climatic conditions that may affect the abundance 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.

Symptoms
Malaria is an acute febrile illness. 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 in endemic areas with severe disease frequently develop one or more of the following syndromic presentations: severe anaemia, respiratory distress in relation to metabolic acidosis, or cerebral malaria. In adults,
multi-organ involvement is also frequent.

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 2008, malaria was present in 108 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. Young children contribute the bulk of malaria deaths worldwide.
  • Non-immune pregnant women are at risk 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 the 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 are at increased risk of malaria 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 are at increased risk of malaria disease when infected.
  • International travellers from non-endemic areas are at high risk of malaria and its consequences 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).

World Health Organisation recommends that malaria be confirmed by parasite-based diagnosis 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.

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 early following the rapid clearance of malaria symptoms. This results in partial treatment and 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. Monotherapies are therefore the primary force 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 sulfacoxine-pyrimethamine (SP), the public health consequences could be dire, as no alternative antimalarial medicines will be available in the near future.

World Health Organisation recommends the routine monitoring of antimalarial drug resistance, and supports countries to strengthen their efforts in this important area of work.

Prevention
Vector control is the primary public health intervention for 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. In high transmission areas, it can reduce child mortality rates and the prevalence of severe anaemia. 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. These are:

  1. insecticide-treated mosquito nets (ITNs): Long lasting insecticide impregnated nets (LLINs) are the preferred form of insecticide treated nets for public health distribution programmes. WHO recommends universal vector control coverage, 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;
  2. 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.

Insecticide resistance

Mosquito control is being strengthened in many areas, but there are significant challenges, including:

  1. an increasing mosquito resistance to insecticides, including DDT and pyrethroids, particularly in Africa; and
  2. a lack of alternative, cost-effective and safe insecticides.

The development of new, alternative insecticides is an expensive and long-term endeavour. 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 is a decision that should always be informed by local and recent data on the susceptibility of the target vectors, and ensuring the availability of such data is a shared responsibility.

Economic 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
Historically, 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 has re-emerged.

Large-scale use of World Health Organisation-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.

  Contact Rosemary Kahiga for more details.