Background: Landscape may affect the distribution of infectious diseases by influencing the population density and dispersal of hosts and vectors. Plague (Yersinia pestis infection) is a highly virulent, re-emerging disease, the ecology of which has been scarcely studied in Africa. Human seroprevalence data for the major plague focus of Madagascar suggest that plague spreads heterogeneously across the landscape as a function of the relief. Plague is primarily a disease of rodents. We therefore investigated the relationship between disease distribution and the population genetic structure of the black rat, Rattus rattus, the main reservoir of plague in Madagascar.

Methodology/principal findings: We conducted a comparative study of plague seroprevalence and genetic structure (15 microsatellite markers) in rat populations from four geographic areas differing in topology, each covering about 150-200 km(2) within the Madagascan plague focus. The seroprevalence levels in the rat populations mimicked those previously reported for humans. As expected, rat populations clearly displayed a more marked genetic structure with increasing relief. However, the relationship between seroprevalence data and genetic structure differs between areas, suggesting that plague distribution is not related everywhere to the effective dispersal of rats.

Conclusions/significance: Genetic diversity estimates suggested that plague epizootics had only a weak impact on rat population sizes. In the highlands of Madagascar, plague dissemination cannot be accounted for solely by the effective dispersal of the reservoir. Human social activities may also be involved in spreading the disease in rat and human populations.

The authors have declared that no competing interests exist.

Figure 1

Figure 1. Geographic location of the four...

Figure 1. Geographic location of the four areas studied and their sampled subpopulations in Madagascar.

Figure 2

Figure 2. Results of S tructure analyses.

(A)...

Figure 2. Results of S tructure analyses.

(A) Estimates of ΔK for each possible value of...

Figure 3

Figure 3. Genetic isolation by distance patterns...

Figure 3. Genetic isolation by distance patterns in Rattus rattus populations of the four areas.

Figure 4

Figure 4. Relationship between plague seroprevalence data...

Figure 4. Relationship between plague seroprevalence data and genetic structure in rats.

Each subpopulation pair...