Mapping the distribution of the main host for plague in a complex landscape in Kazakhstan: An object-based approach using SPOT-5 XS, Landsat 7 ETM+, SRTM and multiple Random Forests

Plague is a zoonotic infectious disease present in great gerbil populations in Kazakhstan. Infectious disease dynamics are influenced by the spatial distribution of the carriers (hosts) of the disease. The great gerbil, the main host in our study area, lives in burrows, which can be recognized on high resolution satellite imagery. In this study, using earth observation data at various spatial scales, we map the spatial distribution of burrows in a semi-desert landscape.

The study area consists of various landscape types. To evaluate whether identification of burrows by classification is possible in these landscape types, the study area was subdivided into eight landscape units, on the basis of Landsat 7 ETM+ derived Tasselled Cap Greenness and Brightness, and SRTM derived standard deviation in elevation.

In the field, 904 burrows were mapped. Using two segmented 2.5 m resolution SPOT-5 XS satellite scenes, reference object sets were created. Random Forests were built for both SPOT scenes and used to classify the images. Additionally, a stratified classification was carried out, by building separate Random Forests per landscape unit.

Burrows were successfully classified in all landscape units. In the ‘steppe on floodplain’ areas, classification worked best: producer's and user's accuracy in those areas reached 88% and 100%, respectively. In the ‘floodplain’ areas with a more heterogeneous vegetation cover, classification worked least well; there, accuracies were 86 and 58% respectively. Stratified classification improved the results in all landscape units where comparison was possible (four), increasing kappa coefficients by 13, 10, 9 and 1%, respectively.

In this study, an innovative stratification method using high- and medium resolution imagery was applied in order to map host distribution on a large spatial scale. The burrow maps we developed will help to detect changes in the distribution of great gerbil populations and, moreover, serve as a unique empirical data set which can be used as input for epidemiological plague models. This is an important step in understanding the dynamics of plague.

[The size of small mammals and the number of fleas parasitizing them]

Correlation analysis of many-year's data on the distribution and number of fleas parasitizing on 281,627 mammals of 21 species in the pest natural foci of the Aral Sea coastal area has demonstrated that the number of mammalian ectoparasites was determined by ecological features as well as by the size of animals: higher numbers of bloodsucking insects could parasitize on animals of a larger size. Values of correlation coefficients of Spearman ranks between body size and number of fleas are high and significant. This fact has remained unnoticed, probably, due to extreme variability in the fleas number indicators obtained in epizootiological studies. This makes their suitability to important studies requiring high precision of the initial data rather doubtful.

[Experience in the comparative evaluation of the population sensitivity of mammals to the plague microbe based on the results of an epizootiological survey]

An essentially new method for the evaluation of the population susceptibility of mammals to Pasteurella pestis without the experimental infection of the animals is proposed. This method consists in using the conjugate result of the mass bacteriological and serological survey of plague carriers (the ratio of the number of infected animals to that of the animals having had the disease). The susceptibility of 12 rodent and Martes species has been studied with the use of this criterion on the basis of the data obtained in the epizootological survey of the plague foci in the vicinity of the Aral Sea in 1950-1979. Among all studied animals, even those belonging to the species universally known as highly susceptible, a considerable prevalence of the animals having had the disease over the infected animals has been established.