Predicting The Transmission Risk Of Plague And The Spatial Distribution Of Plague Foci Based On Environmental Factors

Background: Plague, caused by the bacterium Yersinia pestis, is maintained in complex cycles among wild rodents and fleas. Human infection most frequently occurs when an epizootic develops among rodents near human population. In the last two thousand years, plague has become widespread, affecting a large number of countries on most continents during three pandemics.?The natural foci of plague are situated in a broad belt in the tropical and sub–tropical latitudes and the warmer parts of the temperate latitudes around the globe between the parallels 55 degrees North and 40 degrees South. On every continent, primary natural foci of plague are connected with particular types of landscape in which environmental conditions are favourable for a high and stable number of rodent reservoirs and flea vectors of Y. pestis. In China, 12 types plague foci were classified according to their primary reservoirs, principal flea vectors, landscapes and biotypes of Y. pestis.Qinghai-Tibetan Plateau of China is known to contain natural foci of plague where Marmota himalayana is the primary host and sporadic human cases occurred almost every year. In 2005, plague epizootics were found in great gerbil (Rhombomys opimus) pupulation of the desert regions in Junggar Basin of Xinjiang. However, the specific spatial distribution of Marmota himalayana plague foci and Rhombomys opimus have not been revealed. Meriones unguiculatus natural foci of plague in Etuokeqian county are located in Ordos Plateau of the Inner Mongolia. There are quiet obvious silent periods and active periods of plague epizootic in Meriones unguiculatus. This phenomenon may be caused by environmental dynamics which need further study.Objectives:1. To identify the full extent of potential Marmota himalayana plague foci, and to estimate human-inhabited areas and the number of people at risk of exposure to plague in the Qinghai-Tibetan Plateau. 2. To identify the climatic characteristics of habitat of Rhombomys opimus, and to predicted spatial distribution of Rhombomys opimus in China. 3. To evaluate the relationship between climate factors and plague epizootics in Meriones unguiculatus population in Etuokeqian county. 4. To establish geographic information system for plague prevention and control.Methods: 1. We used Marmota himalayana plague surveillance data of the Qinghai-Tibetan Plateau from 2007-2009 and seven remotely sensed environmental variables in an ecological niche modeling (ENM) framework to explore the potential distribution of the plague foci and their environmental characteristics. 2. Rhombomys opimus occurrence points were collected from Chinese research articles which published recent year (2000-2011) and geocoded. Bioclimate factors were selected as indicators for ecological niche modeling to predict spatial distribution of great gerbil, and to?clarify the climatic characteristics of habitat of Rhombomys opimus. 3. Using the point biserial correlation and logistic regression analysis of climatic factors which contribute to plague epizootic in Rhombomys opimus population of Etuokeqian county. 4. Using ArcGIS Engine development tools, to integrate the database of spatial and temporal distribution of plague foci, the database of plague surveillance and epizootic investigation, the database of multi-dimensional environment remote sensing data and fundamental geographic information on GIS platform.Results: 1. We predicted a broad potential distribution of Marmota himalayana plague foci across Qinghai-Tibetan Plateau with high accuracy. The areas of the foci were estimated about 575,000 km2. Most human-inhabited areas at risk of exposure to plague are distributed in the east and south of the Qinghai-Tibetan Plateau. Elevation, temperature of land surface and normalized difference vegetation index play a large part in determining the distribution of Marmota himalayana plague foci.The suitable ranges of environmental characteristics are (2240 ~ 5254m), (20 ~ 233,91), (1.85 ~ 23.80℃) and (-13.97 ~ 1.71℃). 2. Average annual temperature, annual precipitation and seasonal variations of temperature (standard deviation of seasonal mean temperature) were selected in the predictive model. For habitat of Rhombomys opimus, the optimal average annual temperature of is about 8℃, the optimal annual precipitation is about 179mm, and greater seasonal variations of temperature are more suitable for Rhombomys opimus. A broad potential distribution of Rhombomys opimus was predicted in the desert of West China. 3.In Etuokeqian county, the results of point biserial correlation and logistic regression analysis showed that summer precipitation was significant correlated with plague epizootics with lags of 2-3 years, average temperature of spring and autumn was significant correlated with plague epizootics in this year.Conclusion: ENM proved to be suitable and effective in exploring and mapping the potential Marmota himalayana plague foci in the Qinghai-Tibetan Plateau, and the distribution of Rhombomys opimus in China. This study provided a more detailed view of spatial distribution of Marmota himalayana plague foci and the potential risk areas for human exposure to plague. All the maps could help public health authorities decide where to perform plague surveillance and take preventive measures. This study also evaluated the relationship between climate variation and plague epizootics in Etuokeqian county. At the same time, the study also established a GIS-based software for plague prevention and control.