Spatial Analysis of Plague in California - Recent Advances and Issues in Environmental Science - page 118

between epizootic cycles are not well understood, it is generally accepted that the
disease cycles between enzootic infections and occasional epizootic outbreaks among
susceptible hosts [2]. Humans are presumably at greatest risk of infection during epi-
zootics, when infectious rodent fl eas seek a new host. Plague transmission to humans
may also occur through contact with infected pets or other animals, through exposure
to infected tissue, or via respiratory exposure to infectious air-borne droplets [3, 4].
The incidence of human plague cases is relatively low in the US: for example, a
total of 107 cases occurred in the US from 1990 to 2005 [5], compared to over 240,000
cases of Lyme disease, another vector-borne disease, during roughly the same period
(1992-2006) [6]. Because of this low incidence, plague surveillance in the western US
is often conducted on a limited budget. However, in contrast to Lyme disease, the case-
fatality ratio of plague can be high. If antibiotic treatment is not initiated promptly,
plague is fatal in 40-70% of bubonic cases and nearly 100% of pneumonic cases [1].
The combination of low incidence with high mortality presents unique surveillance
and public health challenges, because early detection through surveillance may not
always be feasible and infrequent clinical cases may be misdiagnosed.
In addition, there is concern that certain factors [2, 7-10] could increase the occur-
rence of plague epizootics as well as the risk of exposure and infection to humans. In
particular, the direct and indirect effects of climate change on land use, population dis-
tribution, and ecologic character are projected to contribute to an increase in the emer-
gence and incidence of infectious diseases [11], including plague. Climate change may
drive plague activity through several pathways (Figure 1), including infl uences on
fl ea burden, rodent population dynamics, and plague transmission [12-19]. A spatially
explicit understanding of how plague risk may shift with changing climate patterns
can help not only to direct prevention and control efforts, but can also alert health care
providers toward quicker recognition of exposure potential and initiation of appropri-
ate treatment of patients [20], which is critical for improving the health outcome of the
individual infected as well as reducing secondary transmission to other people.
Recent studies describing the relationships between future climatic and environ-
mental factors and plague activity in the US have focused on human cases, as well as
animal cases in the Southwestern US and Colorado plateau [12, 17, 19, 21-24]; here,
we focus on the potential distribution of plague in California. The point inputs to
the models developed in this study were derived from plague serology data collected
by the California Department of Public Health (CDPH) and other agencies. Because
active surveillance had most often been conducted in areas with a known history of
plague-positive rodents or human cases, we used ecological niche modeling (ENM) to
identify the potential distribution of plague throughout California (including in previ-
ously unsampled areas). Niche modeling has most often been applied to predict the
potential for plant and animal species occurrences (for example, [25, 26]), and is in-
creasingly being used to identify and map the distribution of diseases, such as Chagas
disease [27], fi lovirus disease [28], Marburg hemorrhagic fever [29], avian infl uenza
[30], and plague [15, 31]. In this study we evaluated Maxent, a presence-only niche
modeling technique, to describe the potential distribution of plague foci in California
under recent and future climatic conditions.