Diversity of Ticks and Rickettsiae in the Southwestern United States: Implications for Public Health

<b><i>Background:</i></b> Surveillance is important in addressing the significant public health concerns posed by tick-borne diseases. However, the southwestern U.S. presents particular challenges due to diverse tick fauna and varied ecologies. <b><i>Methods:</i></b> From 2021 to 2022, we conducted a partner-based tick surveillance program in Arizona and California to assess the presence of <i>Rickettsia</i> spp. pathogens and species composition of tick vectors. <b><i>Results:</i></b> A total of 913 ticks was collected, comprising in descending abundance <i>Rhipicephalus sanguineus</i>, <i>Dermacentor similis</i>, <i>Ixodes pacificus (I. pacificus)</i>, <i>Argas sp</i>., <i>Otobius megnini,</i> and <i>Haemaphysalis leporispalustris</i>. Arizona submitted predominantly brown dog ticks (90.05% of all ticks from Arizona), while California showed greater tick species richness with five species identified. No <i>Rickettsia rickettsii</i> was detected, but a variety of other <i>Rickettsia</i> spp. was found in ticks from both Arizona and California and included <i>Rickettsia rhipicephali (R. rhipicephali)</i>, Rickettsia <i>massiliae</i>, and <i>Rickettsia monacensis</i>-like rickettsial agents of <i>I. pacificus</i>, and two rickettsial organisms that were not identified to species: one <i>Rickettsia montanensis</i> or <i>Rickettsia raoultii</i>-like, and the other most similar to <i>Candidatus Rickettsia tarasevichiae (R.</i> tarasevichiae). <b><i>Conclusion:</i></b> This research contributes to our understanding of tickborne diseases in the southwestern U.S., and emphasizes the need for targeted surveillance and intervention initiatives in a region with complex relationships among ticks, hosts, and <i>Rickettsia</i> species. In particular, the finding of an apparently novel pairing of an unknown <i>Argas</i> sp. tick and <i>R. tarasevichiae</i>-like organism suggests that argasid species are an important target for future research. In addition, the results-both tick species submitted and resulting <i>Rickettsia</i> spp. identified-highlights the strengths and potential biases associated with a partner-based sampling method for tick surveillance.