  Plasmodium vivax populations are more resistant to malaria control strategies
than Plasmodium falciparum, maintaining high genetic diversity and gene flow
even at low transmission. To quantify the impact of declining transmission on
P. vivax populations, we investigated population genetic structure over time
during intensified control efforts and over a wide range of transmission
intensities and spatial scales in the Southwest Pacific. Analysis of 887 P.
vivax microsatellite haplotypes (Papua New Guinea, PNG = 443, Solomon Islands =
420, Vanuatu =24) revealed substantial population structure among countries and
modestly declining diversity as transmission decreases over space and time. In
the Solomon Islands, which has had sustained control efforts for 20 years,
significant population structure was observed on different spatial scales down
to the sub-village level. Up to 37% of alleles were partitioned between
populations and significant multilocus linkage disequilibrium was observed
indicating substantial inbreeding. High levels of haplotype relatedness around
households and within a range of 300m are consistent with a focal and clustered
infections suggesting that restricted local transmission occurs within the
range of vector movement and that subsequent focal inbreeding may be a key
factor contributing to the observed population structure. We conclude that
unique transmission strategies, including relapse allows P. vivax populations
to withstand pressure from control efforts for longer than P. falciparum.
However sustained control efforts do eventually impact parasite population
structure and with further control pressure, populations may eventually
fragment into clustered foci that could be targeted for elimination.
