Spatial Analysis of Plague in California - Recent Advances and Issues in Environmental Science - page 117

Chapter 9
Spatial Analysis of Plague in California
Ashley C. Holt, Daniel J. Salkeld, Curtis L. Fritz, James R. Tucker,
and Peng Gong
INTRODUCTION
Plague, caused by the bacterium Yersinia pestis , is a public and wildlife health concern
in California and the western US. This study explores the spatial characteristics of
positive plague samples in California and tests Maxent, a machine-learning method
that can be used to develop niche-based models from presence-only data, for map-
ping the potential distribution of plague foci. Maxent models were constructed us-
ing geocoded seroprevalence data from surveillance of California ground squirrels
( Spermophilus beecheyi ) as case points and Worldclim bioclimatic data as predictor
variables, and compared and validated using area under the receiver operating curve
(AUC) statistics. Additionally, model results were compared to locations of positive
and negative coyote ( Canis latrans ) samples, in order to determine the correlation
between Maxent model predictions and areas of plague risk as determined via wild
carnivore surveillance.
Models of plague activity in California ground squirrels, based on recent climate
conditions, accurately identifi ed case locations (AUC of 0.913-0.948) and were sig-
nifi cantly correlated with coyote samples. The fi nal models were used to identify po-
tential plague risk areas based on an ensemble of six future climate scenarios. These
models suggest that by 2050, climate conditions may reduce plague risk in the south-
ern parts of California and increase risk along the northern coast and Sierras.
Because different modeling approaches can yield substantially different results,
care should be taken when interpreting future model predictions. Nonetheless, niche
modeling can be a useful tool for exploring and mapping the potential response of
plague activity to climate change. The fi nal models in this study were used to identify
potential plague risk areas based on an ensemble of six future climate scenarios, which
can help public managers decide where to allocate surveillance resources. In addition,
Maxent model results were signifi cantly correlated with coyote samples, indicating
that carnivore surveillance programs will continue to be important for tracking the
response of plague to future climate conditions.
Plague, caused by the bacterium Yersinia pestis , is a disease that has played an
important role in human history, most notably through the demographic impacts of
three major historical pandemics [1]. Plague was introduced to the US during the third
pandemic (ca. 1900), and spread from the Pacifi c coast to its current distribution in
the western states. Plague is maintained among wild rodents in distinct geographic
foci in the western US [2]. Although the mechanisms by which plague is maintained