Worldwide incidence of malaria in 2009: estimates, time trends, and a critique of methods

Abstract

Background: Measuring progress towards Millennium Development Goal 6, including estimates of, and time trends in, the number of malaria cases, has relied on risk maps constructed from surveys of parasite prevalence, and on routine case reports compiled by health ministries. Here we present a critique of both methods, illustrated with national incidence estimates for 2009.

Methods and findings: We compiled information on the number of cases reported by National Malaria Control Programs in 99 countries with ongoing malaria transmission. For 71 countries we estimated the total incidence of Plasmodium falciparum and P. vivax by adjusting the number of reported cases using data on reporting completeness, the proportion of suspects that are parasite-positive, the proportion of confirmed cases due to each Plasmodium species, and the extent to which patients use public sector health facilities. All four factors varied markedly among countries and regions. For 28 African countries with less reliable routine surveillance data, we estimated the number of cases from model-based methods that link measures of malaria transmission with case incidence. In 2009, 98% of cases were due to P. falciparum in Africa and 65% in other regions. There were an estimated 225 million malaria cases (5th-95th centiles, 146-316 million) worldwide, 176 (110-248) million in the African region, and 49 (36-68) million elsewhere. Our estimates are lower than other published figures, especially survey-based estimates for non-African countries.

Conclusions: Estimates of malaria incidence derived from routine surveillance data were typically lower than those derived from surveys of parasite prevalence. Carefully interpreted surveillance data can be used to monitor malaria trends in response to control efforts, and to highlight areas where malaria programs and health information systems need to be strengthened. As malaria incidence declines around the world, evaluation of control efforts will increasingly rely on robust systems of routine surveillance.

Figure 1. Distribution of confirmed malaria cases per 1,000 population, for the lowest administrative level possible in each country.

The number of countries providing data at different administrative levels (from national level 0 down to subnational level 5) were: level 0, 13; level 1, 71; level 2, 19; level 3, 2; level 4, 0; level 5, 1. The total of 106 countries affected by malaria, includes the 99 with ongoing transmission, and seven in the WHO “prevention of reintroduction” phase. Where national data were incomplete, the whole country is marked as such on the map.

Figure 2. Estimated number of malaria cases per 1,000 population in 99 endemic countries made by method 1 (56 non-African and nine African countries) and method 2 (34 African countries).

Figure 3. The percentage of reported malaria cases due to P. falciparum in 99 endemic countries.