Contribution of Canine‐Based Factors to the Spread of RMSF in Arizona and Mexico
Authors: Madsen Soren , Oliva Alec , Maag Christopher , Schaefer Charles , Finley Abigail , Hernandez Jose , Kreisler Rachael , Lee Jung Keun , Quinlan Michael , VandenBrooks John 
Published: April 2020

Prior to 2003, the number of annual cases of Rocky Mountain spotted fever (RMSF) reported in Arizona was reliably close to zero. However, since an initial RMSF outbreak in 2003, case rates have rapidly increased in Arizona and continue to rise. This outbreak has been tied to a novel vector, Rhipicephalus sanguineus, the brown dog tick. Since then, the vectoring capacity of the brown dog tick is seemingly spreading as Northern Mexico across the border from Arizona has had over 4000 reported cases since 2008, with mortality rates as high as 38%. This discovery was somewhat unsettling considering the continental‐wide distribution of these ticks. Given this is a zoonotic disease, canines act as an amplifier for the bacterial agent Rickettsii rickettsii and then brown dog ticks pass it on to humans as an incidental host. Therefore, the need to understand the canine‐based factors that drive the spread of RMSF in this region has become more urgent.

Previously, we have identified three distinct lineages of R. sanguineus within the region with variation in rickettsial infection rates between clades. This could lead to the distribution of ticks on a canine patient, the tick infection rate, and canine seroprevalence driving the human case rate that we see across Arizona and New Mexico. To test this hypothesis, we have focused on the canine as the unit of interest in our epidemiological approach. To this end, we collected all of the ticks and serum from a series of canine patients from five locations across distributed across Arizona and New Mexico. These ticks were phylogenetically categorized based on sequencing of mitochondrial genes and tested for the presence of rickettsial DNA. The serum was tested for rickettsial antibodies using immuno‐fluorescence assay.

The results of this study show that the population of ticks found on an individual dog tend to be monophyletic belonging to a single clade, meaning ticks taken off the same canine are more closely related to one another than to ticks taken off other canines. This implies that the distribution of tick clades across the region plays a fundamental role in the transmission of RMSF. Moreover, while the rickettsial prevalence differs between these different tick lineages, we have found that the likelihood of one tick being infected on a canine is directly correlated to the number of other ticks infected on the canine. Therefore, transmission among feeding groups and co‐localized ticks is a major determinant of canine infection rates. Given these two factors drive the canine infection rate, it is likely they drive the human rate as well. These data were then used to evaluate the feasibility of a canine‐based vaccine by calculating the basic reproduction rate among other variables, which allows us to assess the plausibility of achieving herd immunity through vaccination campaigns. These methods will help to control the spread of RMSF to humans as well as prevent future outbreaks.

Support or Funding Information

MWU One Health Award to JVB, MWU REAP Award to JVB, MWU Kenneth A. Suarez Award to SM, MWU CGS Funding to AO