Rickettsial Pathogens in Dogs and Ticks During an Epidemic of Rocky Mountain Spotted Fever in Ensenada, Baja California, México

A Rocky Mountain spotted fever (RMSF) epidemic has spread through the state of Baja California, Mexico over the last decade and a half, beginning in Mexicali, and subsequently to Tijuana and to Ensenada by 2018. In October of 2022, we surveyed dogs and homes in randomly selected Áreas Geoestadisticas Básicas (AGEBs) with and without reported human cases. Brown dog ticks (Rhipicephalus sanguineus sensu lato) were found on 33.9% of dogs and in the yards of 23.6% of homes. Homes from AGEBs with cases had over a 6-fold increased odds of ticks being present in the yard than AGEBs without reported cases. Both dogs that were permitted to roam and the presence of roaming dogs in the neighborhood were strongly associated with tick infestation of dogs and homes. No ticks or blood samples were polymerase chain reaction-positive for Rickettsia (R.) rickettsii, the causative agent of RMSF, although 54.6% of dogs were seropositive for spotted fever group rickettsiae, and 17.4% were seropositive for typhus group rickettsiae. R. massiliae and R. felis were detected in eight (1.3%) and 29 (4.8%) ticks, respectively; and R. felis was also detected in eight (4.6%) dog blood samples. Although the pathogenic potential of these other rickettsial species and their role in RMSF transmission remains unclear, our data on tick burdens in dogs and homes as risk factors for RMSF exposure provide further support to the pivotal need to reduce tick burdens and the numbers of roaming dogs to successfully manage the RMSF epidemic in northern Mexico.