Pascal Handschumacher

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.

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 km2 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.

Do Pedo-Epidemiological Systems Exist?

Human health is a field for specialists but also a universal concern. The increase in information and analysis in the various kinds of media make it possible for every one to form an opinion, right or wrong, on the health hazards that we will incur. In parallel, progress on experimental and investigative techniques, and dramatic increases in the masses of knowledge in the biological and medical disciplines has lead to an increasing splitting of research, to the detriment of the understanding on the overall pathogenic systems. This double situation very often leads to a division of simple relations between risk factors and medical impact, largely subdivided by disciplinary approaches The reality is that the emergence and/or the space-time variability of health hazards can be seldom expressed in simple relations of cause and effect. In France, during the period 1970–1980, H. Picheral, health geography Professor at the Montpellier University forged a new concept—the pathogenic system. For him, disease is the result of multiple interactions and reveals the physical and social structure of space (Picheral 1983). The initial object of research in health geography is thus the spatial, social and temporal variability of the disease and its factors—something that it has in common with epidemiology. But the final objective is to understand how the spatial heterogeneity of disease risk came into being, and how pathogenic systems function. Picheral distinguished four principal pathogenic systems: