Understanding the contribution of malaria parasites to continued residual malaria in a pre-elimination setting in South Africa : implications for elimination strategies

Abstract

Remarkable progress has been made in averting malaria cases and reducing malaria mortality globally. However, an incomplete understanding of the factors driving the continued transmission in pre-elimination settings challenges further successes in malaria control. For the evidence-based deployment of effective strategies targeting the parasite reservoir in humans, a better understanding of the contribution of asymptomatic parasite carriage to continued transmission is necessary. Primaquine (PQ) is a drug that targets and clears mature gametocytes, the transmissible forms of the malaria parasite. However, the implementation of PQ treatment strategies to reduce Plasmodium falciparum transmission is rather slow. The perceived safety issue surrounding PQ use in glucose-6-phosphate dehydrogenase (G6PD)-deficient individuals and local drug regulations are partly responsible for this slow uptake. When exposed to high doses of PQ, G6PD-deficient individuals risk haemolysis that may be life-threatening. In addition, the human cytochrome P-450 enzymes modulate PQ efficacy. This thesis evaluated locally relevant G6PD-deficiency genotypes and CYP2D6 variants in a pre-elimination setting in South Africa. The low prevalence of these variants supports PQ deployment as a transmission-blocking strategy towards malaria elimination in the country. To better understand which populations should be targeted, the thesis subsequently investigated the contribution of asymptomatic parasite carriage to continued transmission in South Africa. We focussed on genetically characterising the currently circulating parasites in the Vhembe District in Limpopo Province, an area that continues to experience malaria transmission. While the parasites are homogenous based on genotypes, clear distinctions are shown for “local” parasites in contrast to those “imported” from neighbouring countries. When we analysed drug resistance markers, we observed high levels of resistance to sulphadoxine-pyrimethamine and no evidence for artemisinin resistance in the parasite population. This supports the need for continued surveillance to maintain and prolong the efficacy of the existing drugs. Importantly, this thesis uncovers the presence of apparently chronic low-density P. falciparum parasite infections, only detectable by sensitive telomere-associated repetitive element 2 qPCR. Elimination strategies need to consider these low-density infections that may play a role in ongoing transmission. The transmission potential of (low density) infection depends on the production of mature male and female gametocytes. Mosquitoes need to ingest at least one gametocyte of both sexes to become infected with malaria. The thesis describes the development of a novel multiplex qPCR for detecting P. falciparum male and female gametocytes. The P. falciparum male gametocyte enriched transcript (pfmget, PF3D7_1469900) and female specific (ccp4, PF3D7_0903800) mRNA targets were used in qPCR assays on mosquito blood meal samples. This approach fuelled a pilot study to explore the possibility to use mosquito blood meals as a source of parasite material for xenodiagnoses of malaria, either by qPCR or by rapid diagnostic tests. Taken together, this thesis addresses several relevant gaps in our understanding of the role of asymptomatic parasite carriers to ongoing transmission in South African settings. The novel data presented in this thesis may contribute to the rational implementation of malaria interventions in South Africa with the ultimate aim of achieving countrywide malaria elimination and ultimately global malaria eradication.