Malaria Transmission and the Impact of Control Efforts in Southern and Central Africa
Lead Institution: Johns Hopkins University
The overall goals of the Southern and Central Africa ICEMR are to study barriers to malaria control and elimination and inform national strategies. The Southern and Central Africa ICEMR addresses these challenges in low, moderate, and high transmission settings in the region.
- In the low transmission setting in southern Zambia, ICEMR researchers study how malaria transmission is sustained at low levels and what strategies are needed to achieve elimination.
- In the high transmission setting in northern Zambia, the team investigates why the standard package of malaria control interventions, including indoor residual spraying, insecticide treated bed nets and community case management, have not led to a substantial reduction in malaria burden.
- In the moderate transmission setting in eastern Zimbabwe on the border with Mozambique, the team assesses the contributions of cross-border malaria transmission to sustained seasonal malaria in Zimbabwe.
These research questions are addressed using a multidisciplinary approach that combines epidemiology, entomology, parasite genomics, serology, spatial sciences, and modeling in each of the three transmission settings.
Understanding low-level transmission is critical to achieving elimination in southern Zambia
ICEMR researchers documented a dramatic decrease in the burden of malaria in southern Zambia as reflected in a decrease in pediatric hospitalizations for severe malaria, a decrease in malaria incidence at health centers, and a decrease in parasite prevalence through community-based surveys. However, malaria transmission persists, likely through a combination of parasite importation and local transmission arising from pauci-symptomatic reservoirs and hot spots. The National Malaria Elimination Programme in Zambia uses a reactive test-and-treat strategy to identify and treat parasitemic individuals who do not present for care. Although Anopheles arabiensis is known to be the major vector, understudied secondary vectors, with different biting behaviors, may also contribute to ongoing transmission. Studies conducted by the ICEMR found that:
- Most infected individuals reside in the index case household in the context of reactive test-and-treat. Beyond the index case household there is a background parasite prevalence of 1-2% that extends beyond the test-and-treat radius. Findings suggest that focal drug administration in the index case household would be a simpler, less costly, and more efficient reactive strategy.
- Environmental features, particularly proximity to streams, could be used to improve the efficiency of reactive test-and-treat strategies compared to the commonly used circular radius around the index case household.
- Parasite genotyping demonstrated the focal nature of malaria transmission in this low transmission setting and suggested distinct parasite populations detected through active and passive surveillance.
- As transmission declines, understudied secondary vectors were shown to contribute to malaria transmission, including Anopheles squamosus, and these vectors may require alternative vector control strategies to achieve elimination.
As more countries move toward malaria elimination, these findings can help guide reactive strategies such as test-and-treat and focal drug administration to identify parasitemic individuals, and understudied vectors may require modifications to vector control strategies.
Understanding why standard control interventions have had limited impact in high transmission area of northern Zambia
Malaria transmission in Nchelenge District in northern Zambia is intense, with transmission occurring throughout the year and parasite prevalence greater than 50%. Standard malaria control interventions, including spatially-targeted indoor residual spraying, insecticide treated bed nets and community case management, have not led to a substantial reduction in malaria burden.
Researchers investigated reasons for the limited impact of these interventions to guide alternative control strategies and found:
- Minimal impact on parasite prevalence and vector abundance following the switch from pyrethroid to pirimiphos-methyl insecticides for indoor residual spraying after the ICEMR identified high levels of resistance to pyrethroids in the major malaria vectors.
- Complex vector bionomics of the two major vectors Anopheles gambiae and Anopheles funestus, with Anopheles funestus peaking in abundance at the end of the rainy season.
- High malaria risk and vector breeding sites throughout the district, beyond areas targeted for indoor residual spraying.
- High human mobility between sprayed and unsprayed areas using GPS tracking loggers that may diminish the impact of targeted indoor residual spraying.
Findings suggest that twice-yearly indoor residual spraying of the entire district is likely to be required to reduce transmission in this holoendemic setting. However, resource constraints limit the ability to conduct such widespread and frequent spraying. The ICEMR continues to work with the National Malaria Elimination Programme and key partners to monitor the impact of interventions and plan new strategies.
Investigating recurrent outbreaks and cross-border transmission in eastern Zimbabwe
Malaria transmission in eastern Zimbabwe is highly seasonal. ICEMR researchers documented a high level of resistance to pyrethroids and a substantial decrease in malaria burden after the introduction of an organophosphate insecticide for indoor residual spraying. However, the malaria burden did not further decrease.
The ICEMR team is exploring the role of cross-border malaria transmission from neighboring Mozambique as a potential barrier to malaria control and elimination and recently documented the return of chloroquine sensitivity.
The Southern and Central Africa ICEMR works across a diverse range of transmission settings in southern and central Africa, including study sites in southern and northern Zambia and eastern Zimbabwe, with smaller projects in western Mozambique and southeastern Democratic Republic of the Congo (DRC). This geographic scope captures an important north-south longitudinal gradient from DRC through Zambia, to Mozambique and Zimbabwe, and from high to low transmission settings.
View Associated sites for the Southern Africa ICEMR in a larger map.
Map description: Associated sites for the Southern Africa ICEMR: Zambia (Choma District, Nchelenge District); Zimbabwe (Mutasa District); Democratic Republic of the Congo (Haut-Katanga District)
Principal Investigator: William Moss, M.D., M.P.H.
- William Moss, Johns Hopkins Bloomberg School of Public Health
- Douglas Norris, Johns Hopkins Bloomberg School of Public Health
- Jennifer Stevenson, Johns Hopkins Bloomberg School of Public Health (2017-2020)
- Macha Research Trust, Macha, Zambia
- Tropical Diseases Research Centre, Ndola, Zambia
- Biomedical Research and Training Institute, Harare, Zimbabwe
- National Institutes of Health Research, Harare, Zimbabwe
- Africa University, Mutare, Zimbabwe
- University of the Witwatersrand, Johannesburg, South Africa
- Université Protestante au Congo, Kinshasa, Democratic Republic of Congo
- University of North Carolina-Chapel Hill, Chapel Hill, North Carolina
- Brown University, Providence, Rhode Island