Changes in Malaria Epidemiology in Africa and New Challenges for Elimination

Abstract

Although the burden of Plasmodium falciparum malaria is gradually declining in many parts of Africa, it is characterized by spatial and temporal variability that presents new and evolving challenges for malaria control programs. Reductions in the malaria burden need to be sustained in the face of changing epidemiology whilst simultaneously tackling significant pockets of sustained or increasing transmission. Large-scale, robust surveillance mechanisms that measure rather than estimate the actual burden of malaria over time from large areas of the continent where such data are lacking need to be prioritized. We review these fascinating developments, caution against complacency, and make the case that improving the extent and quality of malaria surveillance is vital for Africa as she marches on towards elimination.

Keywords: Africa; Plasmodium falciparum; elimination; epidemiology; malaria control.

Figure 1. Declining Burden of Malaria in Africa.

(A) The modeled Plasmodium falciparum parasite prevalence in children aged between 2 and 10 (PfPR_2–10) years shows a clear decline in Africa. In this figure, Africa includes all African countries and is different from the WHO designated AFRO region. Data from Malaria Atlas Project (MAP) (www.map.ox.ac.uk). (B) The trend is less clear with other malaria metrics and is obscured by huge uncertainty. The estimated total number of Plasmodium falciparum cases (in millions) in Africa in each year between 2000 and 2015. Point estimates and 95% confidence or credible intervals are presented from: (i) Malaria Atlas Project (MAP) (www.map.ox.ac.uk); (ii) Cibulskis et al. [25]; (iii) other sources (for 2000: http://archives.who.int/prioritymeds/report/append/610snow_wp11.pdf; for 2002: Snow et al. [91]; for 2004: http://archives.who.int/prioritymeds/report/append/610snow_wp11.pdf; for 2004: http://www.who. int/malaria/publications/atoz/incidence_estimations2.pdf; for 2007: Hay et al. [92]); (iv) World Malaria Reports (each data point is from the report in the following year). All estimates are from the WHO AFRO Region. World Malaria Reports do not specifically exclude other malaria species, but these cases are very rare in sub-Saharan Africa and therefore not likely to affect the comparison between sources. Single point estimates from other sources were offset slightly on the x-axis for clarity.

Figure 2. Uncertainty in the Estimates of Total Malaria Case Burden in Africa.

(A) The difference between the upper and lower 95% credible intervals for total estimated malaria cases per country in 2015. Malaria Atlas Project (MAP) (www.map.ox.ac.uk). Briefly, estimates are derived from inputting prevalence values extrapolated from survey data into functions describing the modeled relationship between prevalence and incidence. Case counts are generated by multiplying incidence and population data. (B) Trends in malaria burden in WHO African region from 2006 to 2015. Studies on malaria burden in Africa with data for more than two consecutive years were obtained from Pubmed (https://www.ncbi.nlm.nih.gov/pubmed). Studies with data from before 2006 and those designed specifically to test an intervention were excluded. A total of 51 longitudinal studies from 22 countries were included (see Table S1 in the supplemental information online for a complete list of studies). The trend in malaria cases, admissions, and/or parasite prevalence in each country was summarized year-by-year and indicated as: decline (green), increase (red), or as differing between studies (yellow). The island of Zanzibar is part of Tanzania but was shown separately as it has experienced a sustained decline in transmission.

Figure 3. Temporal Trends in Malaria Cases before and after Introduction of Rapid Diagnostic Tests.

Total annual malaria cases (confirmed and unconfirmed, red lines) reported from routine government surveillance in five countries are plotted before and after introduction of rapid diagnostic tests (RDTs). The proportions of total cases that were confirmed with microscopy or RDT are plotted for the same time period. The year RDTs were rolled out in each country is indicated by the yellow arrows. The blue lines show the proportion of confirmed cases. Select countries that showed a decline in total cases and had at least 5 years of diagnosis data were included to illustrate the temporal trends in cases and testing rates. Of interest, The Gambia reported more confirmed cases than total cases in 2012, which illustrates the question of data quality when quantifying the burden of malaria through routine reporting. All data are from World Malaria Report 2014 [21].