Year of Award


Document Type


Degree Type

Doctor of Philosophy (PhD)

Degree Name

Organismal Biology and Ecology

Department or School/College

Division of Biological Sciences

Committee Co-chair

Kerry R. Foresman, Tom G. Schwan

Commitee Members

Fred W. Allendorf, Jonathan M. Graham, Vanessa O. Ezenwa


Disease Ecology, Maxent, Borrelia hermsii, SIR


University of Montana


In North America the primary cause of tick-borne relapsing fever (TBRF) is the spirochete Borrelia hermsii that is vectored by the tick Ornithodoros hermsi. Ecological investigations were combined with mathematical modeling and genetics to gain a clearer understanding of the interactions and mechanisms responsible for disease maintenance, distribution, and genetic diversity. The objectives of this research were to: 1) identify mammals associated with B. hermsii and O. hermsi by determining active infection and antibody presence, 2) develop a deterministic model to ascertain ecological and epidemiological parameters essential for disease persistence, 3) resolve the phylogeographic structure of B. hermsii and O. hermsi to identify dispersal events, and 4) determine the environmental requirements of B. hermsii and O. hermsi.

We identified 11 species of small mammals with antibodies to relapsing fever spirochetes, while pine squirrels (Tamiasciurus hudsonicus) and deer mice (Peromyscus maniculatus) had active infections. Interactions for the enzootic maintenance of B. hermsii include vectors and host(s). These interactions were incorporated into a SIR compartmental model to calculate R0, the basic reproductive number of the disease system. The effect of antigenic variation of the spirochete was assessed by adding relapsing classes to the model, which resulted in an increase of R0. We confirmed the suitability of coniferous forests at higher elevations for the presence O. hermsi and identified constraints on this distribution. O. hermsi and B. hermsii were sensitive to temperature extremes throughout the year. Models for global climate change predicted a shift in range of suitable conditions to higher elevations in the year 2050. Little is known about dispersal of these ticks and spirochetes. Phylogeographic analysis of single nucleotide polymorphisms in 49 B. hermsii isolates from western North America suggested that B. hermsii was introduced to Wild Horse Island, Montana, on at least three occasions. Further, sequence data from the mt16S rDNA of O. hermsi suggested that these ticks can move between the mainland and islands on Flathead Lake. Taken together these data define the complex dynamics of the underlying ecological interactions of an otherwise poorly understood system, and provide evidence for the potential emergence of this pathogen in naïve areas.



© Copyright 2012 Tammi Lynne Johnson