Rodent Plague Surveillance and Species Diversity – Understanding Environmental Conditions Associated with an Epizootic Event, 2014 – 2018, Yosemite National Park, California Público

Cummings, Marissa (Spring 2019)

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Background: Plague has been responsible for three known pandemics, and entered the United States through the Port of San Francisco in 1900 via urban rats. Plague has since spread across the western United States into wild rodent populations. The disease is considered endemic in parts of California due the maintenance in wild rodents. Several rodent species are moderately resistant to the disease, serving as the primary reservoir of plague. However, several species are highly susceptible to infection and mortality, serving as amplifying hosts, causing plague epizootics, or outbreaks. The California Department of Public Health does routine surveillance sampling of rodents for plague across the state.

Objective: This study aims to understand certain environmental conditions and species diversity that may be associated with a known epizootic event that occurred from 2014 through 2018 at Yosemite National Park. During this time, there was heightened plague activity, causing spillover from animals to humans, with two humans infected.

Methods: Maps for spatial distribution of plague results were completed and compared to climate covariates. The Shannon-Weiner Diversity Index was used to calculate rodent species diversity at rodent collection locations. A generalized linear mixed model was used to determine association of epizootic activity between test results and environmental and non-environmental factors.

Results: The California Department of Public Health obtained 604 samples by trapping efforts or contributed mammal carcasses between 2014 and 2018, with 29 samples positive for plague, and a peak of positive samples in 2015. Monthly minimum and maximum temperature, yearly temperature average, yearly dew point temperature, monthly and yearly precipitation and elevation environmental covariates were statistically associated with the epizootic event that occurred. The Shannon-Weiner Diversity Index was statistically associated with the epizootic event.

Discussion: Several environmental conditions were associated with epizootic activity that occurred from 2014 – 2018, though many factors were correlated with elevation. The Shannon-Weiner Diversity Index was associated with disease activity, where low diversity was associated with the epizootic event. However, sampling varied in intensity, with extensive sampling in 2015 occurring due to the human spillover event. More consistent sampling needs to occur within Yosemite National Park.

Table of Contents

I. INTRODUCTION:          1

II. BACKGROUND:           4

Pathogen: Yersinia pestis:            4

Host-Vector Relationship:           4

Human Clinical Illness:  7

Environmental Stability and Persistence:              8

Environmental Suitability of Yersinia pestis:         9

Rodent Host Species Diversity:  9

Study Site:          11


V. RESULTS:       20

Rodent Host Species:    20

Positive, negative and total seroprevalence of samples: 21

Rodent Collection Locations:      22

Rodent Host Species and Elevation Interaction: 22

Rodent Host Species and Number of Fleas:          23

Shannon-Weiner Diversity Index:             23

Spatial Distribution of Plague Serology and Rodent Species:         23


Public Health Implications:          26

Limitations:        28

Future Directions:           29



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