Date of Award
Master of Science (MS) in Biology
The tick species Dermacentor andersoni and Dermacentor variabilis are known vectors of pathogens. One such pathogen is the bacteria Rickettsia rickettsii, which causes Rocky Mountain spotted fever. The detection of this bacterium in ticks at Turnbull National Wildlife Refuge (TNWR) indicates a potential risk to human health. In order to follow up on this discovery, we had two separate objectives. First we developed a high resolution predictive map for Dermacentor spp. distribution across the public use area of TNWR. To do so, 50m transects (27 in total) were established across the public use area. Ticks were collected weekly within the transects from March 26th – June 5th, 2018. The transects were characterized by measuring percent vegetative cover, small mammal abundance, and large mammal usage (20 transects). Additionally, land cover class and 2017 burn status were determined for each transect. Generalized linear modeling (GLM) was used to evaluate which factors were the strongest predictors of Dermacentor spp. abundance. The strongest predictors included 2017 burn status, percent forb cover, percent shrub cover, and land cover class. All factors with the exception of forb cover were mapped across the public use area of TNWR at a 10m resolution using LiDAR and data from TNWR. These predictors were used with the corresponding GLM to map Dermacentor spp. density across the public use area of TNWR at a 10m resolution. The highest predicted Dermacentor density occurred in open shrubland cover class with high shrub density, which was not burned in 2017. To confirm previous detections of Rickettsia rickettsii in ticks, 452 Dermacentor ticks collected in 2018 were tested for Rickettsia spp. by PCR amplification of the rOmpB gene fragment. Ticks that were putatively infected with R. rickettsii (positive for rOmpB) were further assessed by amplifying fragments of the gltA and rOmpA genes. Sequencing of the rOmpB gene fragment showed 21 ticks positive for Rickettsia spp. and 6 ticks positive for R. rickettsii. However, both gene fragments (gltA and rOmpA) were positive for Rickettsia peacockii, a non-pathogenic Rickettsia spp. that may also block the infection of R. rickettsii in ticks. While our results are inconclusive, they suggest that Rickettsia rickettsii is likely not present at TNWR.
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Donahue, Justin L., "High resolution modeling of tick density and detection of Rickettsia spp. in Dermacentor spp. ticks at Turnbull National Wildlife Refuge, WA" (2019). EWU Masters Thesis Collection. 558.