Testing a multi‐factorial model of the role of Rhipicephalus sanguineus in the spread of Rocky Mountain Spotted Fever in the Arizona region

Abstract

Rocky Mountain Spotted Fever (RMSF) is the deadliest tick‐borne disease in the United States. RMSF is spread from a canine host to humans through incidental contact with a tick vector. Historically, transmission of R. rickettsii, the causative agent, has been associated with the American dog tick (Dermacentor variabilis) and the Rocky Mountain wood tick (Dermacentor andersoni). However, in 2003, an outbreak of RMSF in Arizona was associated with a novel vector, the brown dog tick (Rhipicephalus sanguineus). Since then, the vectoring capacity of the brown dog tick has spread across the American Southwest and into parts of Mexico. Currently, the brown dog tick is not known to vector the bacteria outside of this area despite brown dog ticks being present in every state in the U.S. If this vectoring capacity continues to expand, many parts of the country will be at risk due to the brown dog tick’s ubiquitous nature. This is especially alarming as failure to treat RMSF in humans within the first five days of infection can lead to mortality rates as high as 80%. Therefore, this research was aimed at determining the factors that are currently limiting the vectoring capacity of the brown dog tick to Arizona and the surrounding region in order to prevent further spread of RMSF.The results of our research show that the current spread of RMSF in the Arizona region is due to a combination of four factors: (1) the presence of a distinct population of brown dog ticks, (2) variation in the genetic strain of the causative agent R. rickettsii, (3) distribution of canine seroprevalence across the region, and (4) geographic and climatic factors. In support of this, we have identified three genetically distinct clades of brown dog tick across the region that vary in the R. rickettsii infection rate. In sequencing intergenic regions of the R. rickettsii isolated from these tick populations, we have identified variation in the rickettsial strain across the region. Using immunofluorescence assays, we determined that canine seroprevalence rates are rising and seropositive dogs were found in regions that have previously not reported any human cases including regions as far north as Petaluma, CA. Even more concerning, 14 of 16 counties surveyed were at medium to high risk for human outbreak. These data were then analyzed at a county level and the relative role of each of the factors varied by geographic location. Together, this multi‐factorial model is a first step towards understanding the epidemiology of RMSF in the Arizona region and provides insights into ways to improve human and canine health by stopping future spread of RMSF.