No evidence for a classic transmission-duration tradeoff in human malaria infections

Classic theory posits that pathogenic organisms are subject to a tradeoff between the rate and duration of transmission imposed by within-host ecology, an assumption that underpins much evolutionary theory. Here we test for a transmission-duration tradeoff using historical malaria infection data from an era prior to widespread use of antibiotics when humans were deliberately infected with malaria parasites as treatment for neurosyphilis (malariatherapy). Time series follow individual human infections until recovery or treatment with antimalarial drugs due to acute need (a proxy for virulence) and include data on the abundance of specialized transmission stages that govern parasite fitness. We fit a model to estimate initial parasite multiplication rates (PMRs) and find that initial PMRs exhibit strain-specific differences required for evolution by natural selection. However, faster PMRs extend time until recovery and enhance parasite fitness without a consistent cost of increased virulence, challenging the generality of a transmission-duration tradeoff.