Epidemiology of Latency and Relapse in Plasmodium vivax Malaria

Abstract

Malaria is a major contributor to health burdens throughout the regions where it is endemic. Historically, it was believed that there was limited morbidity and essentially no mortality associated with Plasmodium vivax; however, evidence from diverse settings now suggests that infections with P. vivax can be both severe and fatal. This awareness has highlighted a critical gap: the vast majority of research has been directed towards P. falciparum, leading to a decades-long neglect of epidemiological and clinical studies of P. vivax. There exists a large body of historical data on human experimental infections with P. vivax; these studies in controlled settings provided a wealth of wide-ranging statements based on expert opinion, which form the basis for much of what is currently known about P. vivax. In this thesis, portions of this evidence-base have been re-examined using modern epidemiological analyses with two aims: to critically examine this accumulated knowledge base, and to inform current research agendas towards global malaria elimination for all species of Plasmodium. Chapter 2 examines geographic variation in the epidemiology of P. vivax, especially the timing of incubation periods and of relapses, by origin of the parasites. Chapter 3 re-assesses the impact of sporozoite dosage upon incubation and pre-patent periods; Chapter 4 provides well-defined mathematical distributions for incubation and relapses periods in experimental infections, and explores their epidemiological impacts using simple transmission models. Chapter 5 examines the epidemiology of mixed-strain P. vivax infections and compares these results with studies in murine models and general ecological theory; and Chapter 6 clarifies the origin of the Madagascar strain of P. vivax.