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Ticks, tortoises and tick-borne relapsing fever in Mojave desert tortoise habitat

Bechtel, Molly June (2022) Ticks, tortoises and tick-borne relapsing fever in Mojave desert tortoise habitat. Doctoral thesis, Northern Arizona University.

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Abstract

Argasid ticks, Ornithodoros parkeri and O. turicata occur throughout the Mojave Desert and are frequently observed on Mojave desert tortoises (Gopherus agassizii; hereafter referred to as desert tortoise). These ticks harbor and transmit tick-borne relapsing fever (TBRF) group Borrelia, resulting in TBRF in people. Tick-borne relapsing fever group Borrelia is endemic in the western U.S.; however, it is typically associated with the bite of an infected O. hermsi tick found in habitats at high elevations (above 1500 feet). This dissertation examines the relationship between O. parkeri and O. turicata, the TBRF group Borrelia species they carry, and their common host, the desert tortoise in Mojave desert tortoise habitat.Chapter two of this dissertation described the first documented cases of TBRF in people associated with Ornithodoros ticks in the Mojave Desert. This case report described TBRF contracted by two tortoise biologists exposed to Ornithodoros ticks when excavating desert tortoise burrows in Clark County, Nevada. A blood sample from one of the cases was able to be sequenced to Borrelia turicatae, a TBRF group Borrelia carried by the tick O. turicata. This chapter emphasized the risk of TBRF for individuals, such as biologists working in Mojave desert tortoise habitat, desert tortoise pet owners, or anyone that interacts with desert tortoises and their burrows where Ornithodoros ticks are commonly encountered. Despite the risk of contracting TBRF group Borrelia in Mojave desert tortoise habitat, limited research exists examining the relationships between O. parkeri and O. turicata and the desert tortoise. Chapter 3 of this dissertation examined this relationship using data collected from desert tortoise health assessments. As a threatened species, desert tortoise populations are monitored per U.S. Fish and Wildlife Services regulations and the health of individual tortoises is routinely assessed. Presence and abundance of ticks as well as clinical signs of disease and other morphometrics are collected for these health assessments. This chapter analyzed tick presence on desert tortoise health assessments as a function of season, location, sex, foraging behavior and evidence of clinical signs of disease. Ticks were more likely to be present on adult tortoises than juveniles and on captive tortoises versus wild tortoises. Ticks were also more likely to be observed on tortoises with observed clinical signs of disease and on tortoises without evidence of foraging. This analysis provided insight into the biology of ticks in relation to desert tortoises that may be useful for management of threatened tortoise populations where large tick infestations are detected and emphasized the risk of encountering ticks in tortoise burrows. Notably, no information regarding pathogen prevalence or tick species was collected for these health assessments; chapter four discussed pathogen prevalence and tick species of ticks collected throughout Mojave desert tortoise habitat. Collected ticks were identified to species using morphological characteristics and DNA sequencing. All ticks were also tested for pathogens. A total of 1,165 ticks were collected and identified as O. parkeri and O. turicata. One Otobius megnini was found in a desert tortoise burrow. Twenty-two ticks of the 1,165 tested positive for TBRF group Borrelia, five had enough bacterial DNA to be sequenced to B. turicatae. This study was the largest tick surveillance study conducted in the Mojave Desert and shed light on distributions of tick species and pathogens in Mojave desert tortoise habitat. Research from chapter four determined a low prevalence of TBRF group Borrelia in collected ticks (1.88%, CI = 1.07-3.27); chapter five examined potential borreliacidal (Borrelia killing) components of desert tortoise plasma that may contribute to this low pathogen prevalence in ticks. Limited research exists on relationships between TBRF group Borrelia and desert tortoises. However, studies of relationships between ticks, other Borrelia spp., and reptile hosts such as lizards, suggest that reptiles are incompetent reservoirs of Borrelia spp. Reptile hosts may reduce the prevalence of Borrelia infections in ticks where reptile hosts are common. For example, in the western US, where there is a high abundance of lizards with documented borreliacidal components in their blood, there is a low prevalence of Borrelia infected nymphal ticks. As noted, the prevalence of TBRF group Borrelia is low in ticks collected in desert tortoise habitat. A simple bactericidal killing assay was used in chapter 5 to determine if desert tortoises have a borreliacidal component in their blood. Results from chapter five showed that TBRF group Borrelia spirochetes do not survive in desert tortoise plasma, suggesting that desert tortoises may have a borreliacidal factor in their blood. The immunological mechanism is yet to be determined, but this borreliacidal factor would help maintain low pathogen prevalence in ticks collected in Mojave desert tortoise habitat. This dissertation helped close gaps of knowledge in regard to ticks, tick-borne zoonotic disease and desert tortoises in Mojave desert tortoise habitat. This research included the first documented human cases of TBRF in relation to Ornithodoros ticks in desert tortoise habitat, relationships between Ornithdoros ticks and desert tortoises, tick species and pathogen prevalence in a large sample size of ticks collected throughout the Mojave Desert and a potential borreliacidal component in desert tortoises. Although there is much to learn about the relationships between ticks, the pathogens they carry and desert tortoises, work from this dissertation has provided valuable insights into an understudied disease dynamic with likely relevance to disease ecology of wildlife and human infections.

Item Type: Thesis (Doctoral)
Publisher’s Statement: © Copyright is held by the author. Digital access to this material is made possible by the Cline Library, Northern Arizona University. Further transmission, reproduction or presentation of protected items is prohibited except with permission of the author.
Keywords: Desert tortoises; Argasid ticks; Mojave Desert; Tick-borne relapsing fever; Ornithodoros parkeri; Ornithodoros parkeri; Ornithodoros turicata
Subjects: Q Science > QL Zoology
NAU Depositing Author Academic Status: Student
Department/Unit: Graduate College > Theses and Dissertations
College of the Environment, Forestry, and Natural Sciences > Biological Sciences
Date Deposited: 07 Jun 2023 17:31
Last Modified: 07 Jun 2023 17:31
URI: https://openknowledge.nau.edu/id/eprint/5984

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