Abstract

An outbreak of Rocky Mountain spotted fever (RMSF) transmitted by the brown dog tick (Rhipicephalus sanguineus sensu lato) has emerged as a major human and animal health concern in Mexicali, Mexico. Due to high rates of brown dog tick infestation, susceptibility, and association with humans, dogs serve as sentinels and have a key role in the ecology of RMSF. A cross-sectional household questionnaire study was conducted in six rural and urban locations to characterize dog ecology and demography in RMSF high-and low-risk areas of Mexicali. In addition, we tracked movement patterns of 16 dogs using a GPS data logger. Of 253 households, 73% owned dogs, and dog ownership tended to be higher in high-risk areas, with a mean dog:human ratio of 0.43, compared with 0.3 in low-risk areas. Dogs in high-risk areas had higher fecundity and roamed more, but the dog density and numbers of free-roaming dogs were comparable. There was a higher proportion of younger dogs and lower proportion of older dogs in high-risk areas. The high proportion of immunologically naïve puppies in high risk areas could result in a lack of herd immunity leading to a more vulnerable dog and human population. The marked increase of space use of free-roaming dogs in high-risk areas suggests that unrestrained dogs could play an important role in spreading ticks and pathogens. As means to limit RMSF risk, practical changes could include increased efforts for spay-neuter and policies encouraging dog restraint to limit canine roaming and spread of ticks across communities; due to dog density is less impactful such policies may be more useful than restrictions on the number of owned dogs.

Highlights

  • Rocky Mountain spotted fever (RMSF), a potentially fatal disease of dogs and people caused by the bacterium Rickettsia rickettsii, is the most important rickettsiosis in North America [6, 7]

  • Over the last two decades, eastern Arizona and northwestern Mexico have experienced emergence of RMSF vectored by brown dog ticks [16, 24]

  • Parasitism by brown dog ticks and transmission risk of R. rickettsii are associated with an increase of free-roaming dogs and the presence of highly infested dogs in the environment [13, 14]

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Summary

Introduction

Dog ecology in a rickettsia spotted fever epidemic region in Mexico. Dogs share pathogens and parasites with humans, in some cases bridging disease from other animals including wildlife to people, serving as reservoirs for shared disease agents, and acting as sentinels for human risk [3]. Dogs with greater roles in human disease ecology include those that interact with wildlife or roam free, those in populations characterized by rapid turnover (due to high mortality or fecundity rates), and those subject to low standards of veterinary care including vaccination. Rocky Mountain spotted fever (RMSF), a potentially fatal disease of dogs and people caused by the bacterium Rickettsia rickettsii, is the most important rickettsiosis in North America [6, 7]. Rickettsial infections in dogs influence prevalence in ticks and serve to amplify the presence of the pathogen [8, 15]

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