The role of the phylogeographic distribution of Rhipicephalus sanguineuson the spread of Rocky Mountain spotted fever in Arizona and Mexico

Abstract

Rocky Mountain spotted fever (RMSF) is a rapidly‐progressing, tick‐borne, zoonotic disease caused by the bacterium Rickettsia rickettsii. Most of the symptoms of RMSF are flu‐like in nature; including fever, nausea, fatigue, and muscle weakness. The one non‐specific symptom is the presence of petechiae, but by the time they appear it is often too late. Even with treatment, RMSF can have a fatality rate of up to 35% and if left untreated it can rise to as high as 80%. In 2003, a new vector for RMSF was identified; Rhipicephalus sanguineus, or the brown dog tick. Unlike its Dermacentor counterparts, R. sanguineus is found nationally, yet is only known to act as a vector in the Arizona region. The number of cases in Arizona has been on the rise year over year and, perhaps more troubling, just south of the Arizona border, the number of cases in Mexico has surpassed 4000 in the last three years, with an annual fatality rate as high as 47%. Recently, the brown dog tick was indicated as the vector for this outbreak in Mexico. Therefore, understanding why R. sanguineus can act as a vector for RMSF in the Arizona region, but not elsewhere has become more urgent. We propose three hypotheses for the recent emergence of R. sanguineus as a vector for RMSF: (1) there exists a distinct sub‐population of R. sanguineus which better transmits R. rickettsii, (2) a distinct strain of R. rickettsii maintains more favorably in the R. sanguineus populations in this region, and (3) changing environmental factors and dog‐keeping practices in the Arizona region have contributed to the spread of RMSF. To test these hypotheses, we have partnered with the Midwestern Veterinary Mobile Clinic as well as the Arizona Animal Welfare League to collect brown dog tick and serum samples from 16 locations spanning Arizona, New Mexico, and Mexico. We isolated and sequenced 12S, 16S, and COX‐1 tick mitochondrial genes and used a maximum likelihood phylogenetic method on the concatenated sequences. To determine rickettsial prevalence, we tested for the rickettsial‐specific, rOmpA gene and sequenced it. Furthermore, immunofluorescence assay (IFA) was conducted on canine serum samples to evaluate canine anti‐rickettsial antibody seroprevalence in these locations. Finally, all these data were mapped on a geographic climate model. Results indicate variation in tick phylogeographic distribution, rickettsial prevalence rates ranging from 18% to 64% among sub‐populations of R. sanguineus, and seroprevalence rates as high as 33%, indicating areas with endemic levels of RMSF (>15%). These data indicate that no single factor, but rather a confluence of factors drive the spread of RMSF in the region. With this data we can begin to predict and prevent future outbreaks of RMSF and improve canine and human health. Support or Funding Information MWU One Health Award to JVB, MWU REAP Award to JVB, MWU CGS Funding to AO, Kenneth A. Suarez Award to SM