Variation in the Geographic Distribution and Rickettsial Infection Rates of Rhipicephalus sanguineus Contributes to the Spread of RMSF in Arizona and Mexico

Abstract

Rocky Mountain Spotted Fever (RMSF) is a rapidly progressing disease with a human mortality rate of up to 80%. The causative agent of RMSF is the bacteria Rickettsia rickettsii. Tick‐borne vectors can transmit this bacterium to both canines and humans. Traditionally, the major tick vectors for R. rickettsii in the United States have been identified as Dermacentor variabilis and Dermacentor andersoni. However, a previously unknown vector, Rhipicephalus sanguineus (the brown dog tick), was identified during a recent outbreak of RMSF in Arizona where there have been over 380 cases to date. However, while the brown dog tick is present in every state in the United States, currently it is only known to act as a vector for RMSF in Arizona and more recently Northern Mexico. Therefore, we investigated why R. sanguineus can act as a vector in Arizona, but not elsewhere. Here we propose three hypotheses to explain this:1) there is a unique population of R. sanguineus, 2) there is a distinct strain of R. rickettsii that maintains more favorably in R. sanguineus and/or 3) changing climate, habitat, and dog‐keeping practices in the region contribute to the spread of RMSF. To test these hypotheses, we have collected brown dog ticks and canine serum samples from 16 locations across Arizona, New Mexico, and Mexico. Using phylogenetic analysis of three mitochondrial genes, we have shown that there are several distinct sub‐populations of R. sanguineus that coexist within Arizona. Additionally, these sub‐populations have distinct morphologic features and geographic/climatic distributions. We then tested the tick samples for R. rickettsii using polymerase chain reaction to probe for a rickettsial specific gene (OmpA). All positive samples were gene sequenced to confirm the identification of the R. rickettsii species. Based on our analysis, rickettsial prevalence ranged from 10% to 23% among sub‐populations of R. sanguineus and from 0% to 43% among geographic locations. We have identified four hotspots of tick infection in the region: Yuma, AZ (31%), Scottsdale, AZ (38%), Hopi, AZ (36%) and San Carlos, Mexico (43%). To test whether this variation in tick infection may be contributing to differences in incidence rates of RMSF, we conducted immunofluorescence assay to test for the presence of antibodies against the bacteria R. rickettsii in canine serum samples collected from these locations. Ultimately, this data will be used to determine the feasibility of a canine‐based vaccine in an effort to help prevent the future spread of RMSF both within and outside of Arizona.Support or Funding InformationThis project was supported in part by Midwestern University's College of Graduate Studies, College of Veterinary Medicine, and the One Health Center.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.