Malaria - the disease
The risk of malaria
Map of global distribution of malaria transmission risk, 2003)
World Health Report 2005
Anyone travelling to or living in an area with endemic malaria is at risk of infection from a single mosquito bite. It is also important to remember that regardless of the presence or absence of symptoms, anybody carrying the parasite is at risk of a break-through attack requiring treatment. Thus, the entire indigenous population in an area where there is malarial plasmodia is at risk.
Factors affecting risk
Risk varies according to geographic location, time, the immune status and degree of exposure of the person.
Geographic location :
The wide variation in the malaria burden seen in different regions of the world depends upon several factors:
- Global variation in the parasite–vector–human transmission dynamics influence the risk of disease and death from malaria. P. falciparum causes most of the severe disease, and is most prevalent in parts of Africa, South-East Asia and the Western Pacific. The less dangerous malaria species, P. vivax , is found in most of Asia, and in parts of the Americas, Europe and North Africa. The many species of Anopheline mosquitoes that transmit human malaria differ both in their transmission potential, and in their global distribution.
- Climate has a major influence. Tropical areas of the world have the best combination of adequate rainfall, temperature and humidity, allowing for breeding and survival of mosquitos.
- Different levels of socioeconomic development are also important. General poverty, quality of housing and access to health care and health education, as well as the existence of active malaria control programmes, all strongly influence the geographic location of the disease. The poorest nations generally have the least resources for adequate control efforts. In many poor countries, exposure to malaria is enhanced by migrations enforced by poverty and/or conflict.
Time :
Risk varies both seasonally and diurnally. In areas with seasonal climatic variation, it is highest during the rainy season when mosquito populations increase. At this time, parasite inoculation rates can exceed 300 infective mosquito bites per person per year.8 In regions with a stable, warm, humid climate, the risk is throughout the year. Diurnal activity peaks of anopheline mosquitoes occur from dusk to dawn, although this is variable because these insects have highly adaptable behaviour patterns.
Exposure and immunity :
Immune persons - immunity reduces the risk of developing clinically significant disease compared with non-immune travellers and infants. Although the population in areas with endemic malaria mostly acquires immunity during childhood and adolescence, this protection is partial and wanes when exposure has ceased. When previously immune people move outside a malaria area for some years, they are at risk of severe malaria when they return. Moreover, immune people are usually chronically infected and are therefore at risk of breakthrough attacks.9 - Non-immune travellers are at a substantial risk of acquiring falciparum malaria. Each year as many as 30,000 travellers fall ill with the disease. The mortality of those infected with P. falciparum is estimated at about 1%.10
Although this risk seems small, a brief visit to a country where malaria is endemic may be sufficient to contract the disease. It is important to note that frequent travel to endemic areas does not convey useful immunity against malaria.
High-risk groups
Some groups of people run a particularly high risk of acquiring malaria:
- Non-immune natives of countries with endemic malaria are heavily exposed to malarial parasites. Although only a few infections in countries with endemic malaria are ever reported statistically, around 60% of the cases of clinical malaria, and over 80% of deaths, occur in Africa South of the Sahara.1
Among Africans, children below age five are the main sufferers from malaria. They account for 60-70% of all fatal falciparum malaria in Africa, or two thirds of all malaria deaths worldwide.
- Pregnant women (especially primigravidas) form a high-risk group. They are more susceptible to malaria than other women and develop higher parasitaemias and more severe malaria. 11,12 The placenta harbours high numbers of malaria parasites, exposing the foetus to placental insufficiency, which leads to low birth weight and increased infant mortality. Even in uncomplicated disease, fever may lead to abortion or premature delivery.
Pregnant women are also at particular risk of hypoglycaemia (malaria- or quinine-induced).12
Infants of immune mothers maintain immunity for some months after birth, but then lose this protection.
Risk containment
Protective measures, such as appropriate clothing, repellents, impregnated bed nets, aerosolised insecticides, screens and air-conditioning (where available) significantly reduce the risk of transmission. If transmission occurs despite protective measures, appropriate chemoprophylactic drugs will usually suppress and even eradicate the parasites.
The role of standby therapy
Using Riamet® /Coartem®
