Elucidating the Role of Rhipicephalus sanguineus (the Brown dog tick) as a Vector for Rocky Mountain Spotted Fever (RMSF) Transmission in Arizona

Objectives and Hypotheses The objective of this study was to understand why the brown dog tick acts as a vector for Rocky Mountain Spotted Fever (RMSF) in the Arizona region, but not elsewhere in the United States even though the brown dog tick is endemic throughout the entire country. We have proposed four possible hypotheses: 1) the Arizona population of ticks is a distinct species with a morphological or genetic feature that makes it more suitable as a vector for RMSF than other populations, 2) the pathogen ( Rickettsia rickettsii) that causes RMSF is a different strain or maintains a reservoir more favorably within the populations of R. sanguineus in Arizona, 3) the high density of ticks and dog keeping practices in some locales increases the prevalence of RMSF transmission due to increased exposure of domestic dogs, which act as amplifiers for the disease, and 4) the changing climate and habitat has become more favorable within Arizona allowing the population of R. sanguineus to act as a vector. Methods Several thousand ticks have been collected from six counties in Arizona, two counties in New Mexico, and one state in Mexico. This includes regions where human outbreaks of RMSF have been recently reported and regions where few or no cases have been reported. A subset of the male tick specimens representative of all regions were selected and then bisected so that both morphological and molecular data can be obtained for each individual. Morphologic measurements have been taken on 200 of these ticks using a combination of light microscopy and SEM. Genetic isolation, PCR, and sequencing for Cox‐1, 12s, and 16s genes had been conducted on these same ticks. Additionally, a preliminary screen for whether the ticks were carrying the rickettsial bacteria has been carried out and this data has been mapped on a climate model. Results Our study has confirmed the presence of multiple genetically distinct populations of R. sanguineus within Arizona and the surrounding localities. Based on their geographic distribution and climate mapping, these results are consistent with the identification of separate warmer (tropical) and cooler (temperate) climate lineage within the state of Arizona. However, we have found that these different populations occur sympatrically in some locations, and were even found feeding on the same dog. A comparison of morphological measurements and ratios showed some significant variations between the different genetic groups. Preliminary rickettsial testing has demonstrated that different tick populations are capable of hosting the bacteria and this may play a role in disease transmission. Conclusions Our results support the idea that distinct genetic populations of ticks in Arizona affect the epidemiology of Rocky Mountain Spotted Fever within the region. This study provides a better understanding of the relationship between dog and human rickettsial infections and the brown dog tick vector. It is likely that the geographic range of tropical lineages is spreading north due to more favorable environmental conditions associated with warming climates. This research may help predict the potential of the brown dog tick to transmit RMSF in states outside Arizona. Support or Funding Information This project was supported in part by the Midwestern University One Health Center, Midwestern University, Glendale, AZ. This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .