What is malaria?
Globally malaria is estimated to kill more than half a million people per year. Millions more suffer from the disease. According to the latest estimates from the World Health Organization 198 million cases occurred in 2013.
The disease develops when a person is infected with the microscopic parasite, Plasmodium.
The infection is passed from person to person by the bite of a female Anopheles mosquito: the ‘vector’ of the disease.
The mosquito picks up the infection when she bites someone carrying the malaria parasite in their blood. The parasite then develops inside the mosquito’s body for some days finally infecting her salivary glands. She can then pass the infection on each time she bites another person.
There are many types (species) of malaria parasite.
For example there are malaria parasites that infect different animals ranging from monkeys to crocodiles and even penguins. Five of the malaria species are known to cause disease in humans: Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale, Plasmodium malariae and Plasmodium knowlesi.
Types of malaria and their distribution
Falciparum malaria is found throughout the tropical and sub-tropical world and is the most lethal form of malaria. The vast majority of deaths from malaria are due to this type of the disease. Vivax malaria is common in Asia and South and Central America and may be the most common form of malaria worldwide. Vivax parasites can develop at lower temperatures than falciparum parasites so its range is wider than the tropical and sub-tropical range of falciparum malaria.
This type of malaria usually results in an unpleasant illness lasting several days. While it can lead to severe illness and sometimes death, typically it is not life-threatening. The economic toll of this form of malaria – to households and employers – is high. Vivax malaria can remain in a patient’s body after treatment if the treatment given fails to clear the parasite forms which lie dormant in the liver. If this happens then people who are infected can fall ill again and again. Treatment that can provide ‘radical’ cure by removing the dormant liver stages is available but the medicine can have serious side effects in a small proportion of people. It must therefore be prescribed with caution and ideally after testing whether the patient is likely to suffer an adverse reaction.
Falciparum and vivax malaria are responsible for the vast majority of malaria cases, but there are also three other forms of the disease that can occur in humans.
Ovale malaria has a similar disease profile to vivax malaria. It is found mostly in West Africa and in some islands of the Western Pacific but is also present in South and East Asia.
Malariae malaria is found worldwide though is not common. It is rarely fatal.
Plasmodium knowlesi, present in South East Asia, is a species of malaria previously thought to only infect monkeys but is now known to also infect humans. Knowlesi malaria has a disease profile similar to the deadly falciparum malaria and can cause fatalities.
The mosquito vector
There are many types of mosquito however malaria carrying mosquitoes are all part of the Anopheles group. There are almost 500 Anopheles species but only 40 or so species commonly carry malaria.
Non-anopheles mosquitoes also have impacts on human health; for example some Aedes mosquitoes spread dengue and yellow fever and some Culex mosquitoes spread Japanese Encephalitis and filariasis (elephantiasis). Often the distribution of these diseases overlaps with malaria; integrated vector control programmes can therefore sometimes be designed to reduce transmission of a number of mosquito borne diseases at once.
Map of overlap of mosquito borne diseases
Table of other mosquito borne diseases in the Asia Pacific
Anopheline mosquitoes can be distinguished from other types fairly easily by characteristic morphological features in most life stages. Trained entomologists can also identify individual species of Anopheles.
Anopheles mosquitoes lay their eggs in water. The eggs hatch after 2-3 days and the larvae eat and grow in the water for around a week, before moving into the pupal stage. After a couple of days as a pupa the mosquito adult emerges, waits for its wings to dry and then flies away. The time for this progression through the various life stages (metamorphosis) is very dependent on temperature; at lower temperatures development slows down.
The optimal temperature for mosquito metamorphosis is between 20 and 30°C; this range is also ideal for the malaria parasite to develop quickly within the mosquito.
The lifespan of an adult mosquito varies but on average is around 10-14 days. Some individuals will live longer than this. The life span is very important in the transmission of the disease. The malaria parasite develops inside the mosquito for 1-2 weeks before the mosquito is able to pass it on to anyone else. This means that only a small percentage of mosquitoes (the oldest ones) may actually pass on the disease.
Adult male mosquitoes feed on plant sugars. It is only the adult female mosquitoes that feed on blood, using proteins in the blood to develop their eggs. Female mosquitoes need to consume a blood meal every second to third night on average.
Malaria vectors exhibit a range of behaviours relevant to transmission and control of the disease. Behaviours can vary between species, and, in particular in the Asia-Pacific region, behaviour can also vary within the same species depending on location. Behaviours of interest include:
- Favoured ‘breeding sites’, i.e. where females choose to lay eggs;
- Preference for biting indoors or outdoors
- Preference for resting indoors or outdoors
- Preference for feeding on humans versus other animals
- Preference for time of biting – some prefer dusk and dawn, some late night, some bite throughout the night. Some species show different biting behaviours at different times e.g. bite outdoors at dusk and indoors later in the night.
Malaria illness, immunity and vulnerable groups
The incubation period of malaria in humans varies. It is commonly between 1 week and 1 month which means that after a person is bitten by an infected mosquito they do not fall sick immediately. This is an important consideration when diagnosing a malaria-like illness. Even if the person is in a non-malarious setting, recent travel must be taken into account.
When a person falls ill with malaria the initial signs and symptoms are non-specific and often similar to those of flu. Symptoms can include fatigue, fever, chills, headaches, gastrointestinal disturbances and aching muscles.
If the infection is not treated promptly it can progress swiftly to become more serious. Falciparum malaria multiplies quickly in the blood and can rapidly develop into severe malaria. Vivax malaria multiplies more slowly; it will progress to more intense symptoms and can occasionally lead to severe malaria, including coma and death but it is very uncommon for vivax malaria to become life-threatening. For a fuller discussion of the potential for vivax malaria to become severe, see Box 8.3 on page 64 of the 2013 WHO World Malaria Report.
Severe falciparum malaria can take a number of forms. If cerebral malaria develops then the patient may experience convulsions followed by coma and death. Alternatively the patient may develop severe anaemia, again with possible fatal consequences.
Whether a patient experiences malaria as a mild or severe disease depends on their immune status, how promptly they are treated and whether the treatment is successful. The latter is largely dependent on the quality of case management provided. Where drug resistant parasites are involved the choice of drug may be critical.
Partial immunity to malaria develops in people who are repeatedly exposed to the infection, generally from childhood.
As immunity develops, older children and adults may experience only mild illness when they are infected. However this immunity will only develop through repeated infection and is therefore only seen in populations living in areas where malaria transmission is intense. Where malaria transmission is moderate, partial immunity may develop later or, as in areas of low transmission, not at all.
Partial immunity can be rapidly lost when people move to areas of lower malaria transmission, they are then equally at risk of severe illness and death as people from non-malarious areas.
Some people are more at risk of severe disease than others.
- In areas of high transmission:
- Young children, who have not yet developed immunity;
- Immuno-compromised people, such as people infected with HIV;
- Pregnant women: Women normally with a high level of immunity to malaria may develop severe life-threatening anaemia during pregnancy without displaying the classic symptoms of malaria. The risk is highest in first pregnancies. The chances of miscarriage or giving birth to an underweight baby are high. Malaria in pregnancy can increase infant mortality very significantly in high transmission settings.
- Visitors from non-endemic areas who have no immunity.
- Returning former residents who are not aware that they have lost their immunity as a result of spending a long time in non-endemic areas.
- In areas of low / no transmission, immunity will not develop and all ages groups will be at risk of contracting the disease. Those most at risk of serious outcomes include:
- Immuno-compromised people, such as those infected with HIV;
- Pregnant women; they are more at risk of severe illness than non-pregnant women. In these cases, spontaneous abortion or miscarriage is a risk across all pregnancies;
There are also groups of people who are more at risk of contracting the disease as a result of their lifestyle. This is particularly the case in Asia where transmission is often focal and largely restricted to specific forest settings due to the behaviour of the mosquito vectors. Forest and forest fringe workers (many of whom may be migrants) and rubber tappers therefore have a much higher-risk of contracting malaria than most.
on the disease, its transmission cycle and malaria illness