Spatial Analysis of Plague in California - Recent Advances and Issues in Environmental Science - page 119

Figure 1. A conceptual model of the mechanisms by which climate influences plague transmission
and maintenance. Precipitation and temperature have been linked to plague outbreaks in prairie dogs,
and to human cases in the US. A proposed model for this relationship suggests that precipitation and
temperature may influence rodent abundance (by influencing rodent survival and food abundance),
and that increased rodent populations may affect flea abundance and/or plague transmission rates.
In addition to having a positive effect on rodent population dynamics, certain soil moisture, humidity
and temperature variables may influence flea ecology and the transmission of the plague pathogen.
MATERIALS AND METHODS
Data
The point inputs to the models developed in this study were derived from plague
surveillance data collected by the CDPH and other agencies [32]. Records of ap-
proximately 37,000 animals (33 different genera) collected throughout the state of
California during 1984-2004 were entered into an access database by public health
researchers.
Rodent Point Data
Rodent samples were obtained most often by active surveillance, which was conduct-
ed in areas with a known history of plague-positive rodents or human cases [32].
Rodent sera were tested by passive hemagglutination to F1 antigen of Y. pestis ; speci-
mens with antibody titer ≥ 1:32 were considered positive [33-35].
Rodent samples were geocoded based on an address or campsite name, which
allowed for location of rodent case point at a<1-km 2 spatial resolution. All rodent re-
cords were geo-located using National Geographic TOPO software (National Geo-
graphic Society 2001). Locations that could not be reliably located to a campground
or address were excluded from this analysis. All geocoded points were projected to
Teale Albers, NAD 1983 projection. The geocoded locations of the rodent case points
are located along the north-south transect of the Sierra Nevada range, and along the
southern coast and inland areas of Southern California. No positive rodents were