Increasingly inbred and fragmented populations ofPlasmodium vivaxwith declining transmission

The human malaria parasite Plasmodium vivax is resistant to malaria control strategies maintaining high genetic diversity even when transmission is low. To investigate whether declining P. vivax transmission leads to increasing P. vivax population structure that would facilitate elimination, we genotyped samples from a wide range of transmission intensities and spatial scales in the Southwest Pacific, including two time points at one site (Tetere, Solomon Islands) during intensified control. Analysis of 887 P. vivax microsatellite haplotypes from hyperendemic Papua New Guinea (PNG, n = 443), meso-hyperendemic Solomon Islands (n= 420), and hypoendemic Vanuatu (n=24) revealed increasing population structure and multilocus linkage disequilibrium and a modest decline in diversity as transmission decreases over space and time. In Solomon Islands, which has had sustained control efforts for 20 years, and Vanuatu, which has experienced sustained low transmission for many years, significant population structure was observed at different spatial scales. We conclude that control efforts will eventually impact P. vivax population structure and with sustained pressure, populations may eventually fragment into a limited number of clustered foci that could be targeted for elimination.