Plague Circulation and Population Genetics of the Reservoir Rattus rattus: The Influence of Topographic Relief on the Distribution of the Disease within the Madagascan Focus.

International audienceBACKGROUND: 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. METHODOLOGYPRINCIPAL 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. CONCLUSIONSSIGNIFICANCE: 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

Genetic isolation by distance patterns in <i>Rattus rattus</i> populations of the four areas.

<p>Relationships (the regression line is shown only when significant) between pairwise Euclidean geographic distance between subpopulations and genetic differentiation, estimated as <i>F</i>_ST/(1-<i>F</i>_ST). (A) Moramanga, plateau area; (B) Mandoto, plateau area; (C) Betafo, mountainous area; (D) Ambositra, mountainous area.</p

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

<p>Mandoto, Moramanga: plateau areas; Betafo, Ambositra: mountainous areas.</p

Relationship between plague seroprevalence data and genetic structure in rats.

<p>Each subpopulation pair was characterized by its absolute difference between seroprevalence levels, and its pairwise <i>F</i>_ST. (A) Mandoto, plateau area: black circles indicated the subpopulations pairs involving MAhiV; (B) Betafo, mountainous area: black triangles indicated the subpopulation pairs involving BAtnV.</p

Seroprevalence and genetic estimates per subpopulation.

<p>Seroprevalence (SP) was calculated on all sampled individuals per subpopulation, and genetic estimates were calculated on genotyped individuals (N) per subpopulation. Mean and standard errors (<i>r, H_S</i>) or 95% confidence intervals (<i>F_IS, F_ST</i>) are reported for each area.</p>*<p>indicated significant <i>F_IS</i> values after correction for multiple tests.</p

Study areas and their characteristics.

<p>D_E = Euclidean distance among villages (km).</p

Results of Structure analyses.

<p>(A) Estimates of ΔK for each possible value of K within each of the four areas; (B) Structure bar plot for the run with the highest estimated posterior probability for Betafo at K = 2: subpopulations are ordered from south (left) to north (right).</p