Uncertainty in mapping malaria epidemiology: implications for control

Abstract

Malaria is a location-specific, dynamic infectious disease transmitted by mosquitoes to humans and is influenced by environmental, vector, parasite, and host factors. The principal purposes of malarial epidemiology are 1) to describe the malarial distribution in space and time along with the physical, biologic, and social etiologic factors and 2) to guide control objectives for either modeling impact or measuring progress of control tactics. Mapping malaria and many of its causative factors has been achieved on many different levels from global distribution to biologic quantitative trait localization in humans, parasites, and mosquitoes. Despite these important achievements, a large degree of uncertainty still exists on the annual burden of malarial cases. Accurate, sensitive detection and treatment of asymptomatic reservoirs important to infectious transmission are additional components necessary for future control measures. Presently spurred by the leadership and funding of Bill and Melinda Gates, the malarial community is developing and implementing plans for elimination of malaria. The challenge for malariologists is to digitally integrate and map epidemiologic factors and intervention measures in space and time to target effective, sustainable control alongside research efforts.

Figure 1.

Population burden of malaria. The 16 top African countries and 8 non-African countries account for more than 80% of all yearly malaria cases. For African countries, the population at risk (filled square) is close to the total population (white bar), while for Southeast Asia nations the population at risk is more than the total population of many African nations. The reported numbers of malaria cases (filled circles) are shown with low and high estimates (triangles). Data are from the World Malaria Report 2008 from the World Health Organization (18). “D. R. Congo” is Democratic Republic of Congo.

Figure 2.

Relation of malaria intensity measures. In A, age-dependent parasite prevalence rates grouped into the 4 traditional endemic transmission regions with human measures of intensity show a peak in young children. Large reductions in mosquito transmission measures can still sustain high parasite prevalences. Parasite prevalence rates decrease little with 10-fold decreases in entomologic inoculation rates (EIRs) (B) and almost 100-fold decreases in the basic reproductive ratio (R₀) (C). In D, the rates of seropositivity to malaria antigens quickly saturate to near 100% at both prevalence near 25% with low EIR and R₀ less than 10. In E, the less used measure of intensity associating infant malaria rates is useful in hyperendemic and holoendemic settings with prevalence over 50%.