Date of Award

Spring 2023

Document Type

Open Access Dissertation


Epidemiology and Biostatistics

First Advisor

Melissa Nolan


Tick-borne diseases are emerging worldwide due to environmental shifts caused by land fragmentation and climate change. Among the most important ones spotted fever group rickettsioses (SFGRs) are of particular interest due to their wide species variety and high pathogenicity. Given the global spotted fever group rickettsia (SFGR) distribution, evaluation and comparison of different populations is critical to better understand these pathogens' epidemiology and unravel the risk factors affecting disease occurrence. Consequently, this dissertation investigates SFGR highlighting the One Health framework, assessing animals, humans, and the environment. The first part includes an evaluation at the household level using human and canine biobanked samples from a rural community in Miraflores, Boyacá, Colombia. This chapter unravels first-time SFGR evidence in the department of Boyacá. Second, a statewide evaluation of human biobanked samples from historically underrepresented populations across South Carolina, USA. The study provides novel SFGR insights into a low-income, low-education, mostly Black race sample. Lastly, an eco-epidemiological study evaluates SFGR tick vector populations across South Carolina state parks and animal shelters providing important ecological factors linked to SFGR distribution.

The studies were approved by the ethics committee involved from the University of Antioquia, Medellín, Colombia, and the University of South Carolina, Columbia, South Carolina.

For the serological studies involving humans and canines, informed consent was obtained to extract blood samples that were analyzed using an indirect fluorescent antibody (IFA) test Rickettsia rickettsii-specific. For the eco-epidemiological study, ticks were collected across South Carolina state parks, and animal shelters, processed in pools for DNA extraction, and tested for Rickettsia amblyommatis and Rickettsia parkeri infection through RT-PCR.

Statistical analyses were performed separately for each aim, descriptive statistics using Fisher’s exact, ordinal regression, and t-tests; and a deeper analysis using multivariable logistic regression, and Bayesian logistic regression using R studio. Geospatial analysis was complementary used, performing a Getis Ord Gi* in ArcGIS Pro.

The results of this study yielded a seroprevalence of 20.4% among humans and 26.5% among canines in Miraflores. Human risk factors were animal seropositivity, and household-level factors included crop field presence and the number of animal species owned. In South Carolina, we described a 3.4% seroprevalence, with being male and spending time working outdoors as risk factors associated with seropositivity. Moreover, we found hot spots in five counties suggesting regions of potential previous outbreaks. Lastly, among ticks, the prevalence was 24.8% and 39.5% for R. amblyommatis and 19.0% and 22.4% for R. parkeri for parks and shelters respectively. R. amblyommatis and R. parkeri were negatively correlated, and land use classification variables were found to distinctively impact R. amblyommatis and R. parkeri presence. Landscapes associated with greater presence of R. amblyommatis were deciduous, evergreen, and mixed forests, while hay, pasture fields, and emergence herbaceous wetlands were associated with lower presence. On the other hand, deciduous forest, mixed forest, herbaceous vegetation, cultivated crops, woody wetlands, and emergent herbaceous wetlands were associated with a greater presence of R. parkeri whereas hay and pasture fields were associated with a lower presence.

The overall implications of these findings are suggestive of 1) domestic animals are key risk factors in the seroprevalence of SFGR among rural communities, 2) spending time working outdoors and being a male are common characteristics of SFGR seropositive individuals, 3) geographic distribution of cases occurs in clusters, 4) different Rickettsia sp. could be implicated in the development of SFGR antibodies, 5) different Rickettsia species are likely to interact with each other, and some may have inhibitory effects, and 6) tick distribution patterns and Rickettsia sp. positivity are determined by environmental factors. The results from this dissertation highlight the importance of evaluating SFGRs at multiple levels to understand the epidemiology of this disease through a One Health framework. Future investigations are warranted applying a One Health design, incorporating the different levels within humans, animal hosts, and the environment.


© 2023, Lidia Gual Gonzalez

Available for download on Saturday, August 31, 2024

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