Abstract:
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There is substantial variation in the relapse frequency of
Plasmodium vivax malaria, with fast-relapsing strains in
tropical areas, and slow-relapsing strains in temperate areas
with seasonal transmission. We hypothesize that much of the
phenotypic diversity in P. vivax relapses arises from selection
of relapse frequency to optimize transmission potential in a
given environment, in a process similar to the virulence
trade-off hypothesis. We develop mathematical models of P. vivax
transmission and calculate the basic reproduction number R0 to
investigate how transmission potential varies with relapse
frequency and seasonality. In tropical zones with year-round
transmission, transmission potential is optimized at
intermediate relapse frequencies of two to three months:
slower-relapsing strains increase the opportunity for onward
transmission to mosquitoes, but also increase the risk of being
outcompeted by faster-relapsing strains. Seasonality is an
important driver of relapse frequency for temperate strains,
with the time to first relapse predicted to be six to nine
months, coinciding with the duration between seasonal
transmission peaks. We predict that there is a threshold degree
of seasonality, below which fast-relapsing tropical strains are
selected for, and above which slow-relapsing temperate strains
dominate, providing an explanation for the observed global
distribution of relapse phenotypes. |