II. MALARIA BURDEN
As of 2004, 107 countries and territories have reported areas at risk of malaria transmission (Map 1). Although this number is considerably less than in the 1950s, with 140 endemic countries or territories (15), 3.2 billion people are still at risk. Present estimates are that around 350–500 million clinical disease episodes occur annually (2). Around 60% of the cases of clinical malaria (Box 2 and Map 3) and over 80% of the deaths (1) occur in Africa south of the Sahara. Of the more than 1 million Africans who die from malaria each year (1), most are children under 5 years of age. In addition to acute disease episodes and deaths in Africa, malaria also contributes significantly to anaemia in children and pregnant women, adverse birth outcomes such as spontaneous abortion, stillbirth, premature delivery and low birth weight, and overall child mortality. The disease is estimated to be responsible for an estimated average annual reduction of 1.3% in economic growth for those countries with the highest burden (3).
The wide variation seen in the burden of malaria between different regions of the world is driven by several factors. First, there is great variation in parasite– vector–human transmission dynamics that favour or limit the transmission of malaria infection and the associated risk of disease and death. Of the four species of Plasmodium that infect humans—P. falciparum, P. vivax, P. malariae and P. ovale— P. falciparum causes most of the severe disease and deaths attributable to malaria and is most prevalent in Africa south of the Sahara and in certain areas of South- East Asia and the Western Pacific (Map 4). The second most common malaria species, P. vivax, is rarely fatal and commonly found in most of Asia, and in parts of the Americas, Europe and North Africa. There are over 40 species of anopheline mosquitoes that transmit human malaria (Map 2), which differ in their transmission potential. The most competent and efficient malaria vector, Anopheles gambiae, occurs exclusively in Africa and is also one of the most difficult to control. Climatic conditions determine the presence or absence of anopheline’s vectors. Tropical areas of the world have the best combination of adequate rainfall, temperature and humidity allowing for breeding and survival of anophelines.
The second major factor contributing to regional and local variability in malaria burden is differences in levels of socioeconomic development. Determinants include general poverty, quality of housing and access to health care and health education, as well as the existence of active malaria control programmes providing access to malaria prevention and treatment measures. The poorest nations generally have the least resources for adequate control efforts. In many poor countries, exposure to malaria of vulnerable populations is enhanced by migrations enforced by poverty and/or conflict.
As a result of differing intensities of malaria transmission, the population groups at risk of malaria also differ between world regions. The majority of deaths in tropical Africa occur in areas of stable transmission of falciparum malaria. In these areas, the two groups at highest risk are very young children, who have not yet acquired clinical immunity, and pregnant women, whose immunity to malaria is temporarily impaired. In areas of unstable or highly seasonal falciparum malaria transmission, which is common in most regions outside Africa, the lack of frequent exposure to malaria infection early in life delays the acquisition of clinical immunity, and thus older age groups remain at relatively high risk for malarial disease when exposed (16). In fact, in some of these areas, adult groups such as forest workers in South-East Asia or migrant workers in Latin America are those most likely to be exposed to malaria and thus at highest risk for severe disease and death.
During the 20th century, human efforts to control malaria, and general socioeconomic development, including access to health care, have markedly reduced the spread of malaria. These gains are most evident in areas where transmission previously occurred only at low intensity, in the Americas, Asia, Europe and Transcaucasia. During the Global Malaria Eradication Programme between 1957 and 1972, vector control— mainly through DDT spraying combined with improved access to treatment—reduced or eliminated malaria transmission in considerable parts of these regions. In contrast, most of Africa south of the Sahara and some foci elsewhere continued to suffer malaria transmission at high intensity. In some areas malaria has resurged after interruption of eradication efforts that were not sustainable (17).
More recently, there is evidence that, compared with the 1980s, the burden of malaria increased during the 1990s in several areas in terms of proportions of population at risk, the severity of infections and the number of deaths. Malaria re-emerged in several countries in Central Asia and Transcaucasia with an increased frequency of epidemics and with the re-establishment of stable endemic transmission. In rural Africa south of the Sahara, child mortality caused by malaria is estimated to have increased by up to twofold during the 1980s and the early 1990s, while mortality resulting from other causes decreased over the same period (18, 19). Factors contributing to the increase in malaria include: (i) resistance of parasites to commonly used antimalarial drugs; (ii) breakdown of control programmes; (iii) complex emergencies; (iv) collapse of local primary health services; and (v) resistance of mosquito vectors to insecticides. Within this same period, however, malaria was well-controlled in the five northernmost African countries, and elimination or a very low level of transmission was maintained in some of the islands off the coast of Africa. Throughout the past decades, malaria was generally much less intense in Central America and South America than in Africa and South-East Asia, where transmission is mostly limited to P. vivax—except for the Amazon basin—and a relatively low but fairly stable incidence was reported throughout the 1990s.
From the available data, it is not yet possible to determine with sufficient confidence whether the global burden of malaria has changed substantially, for better or worse, since 2000 when RBM implementation began in many countries. In some areas, fluctuations in malaria transmission from year to year potentially confound evaluations of broader trends. Therefore, conclusions typically require an analysis of epidemiological data over multiple years. For the high-burden continent of Africa, reliable data on under-5 mortality from birth history surveys and demographic surveillance will only become available after a time lag of several years (18, 20) (Annex 4).
Nevertheless, for some countries and areas throughout the world there is evidence that successful control has had an impact on malaria disease burden. These success stories are presented in the respective sections of each region.