Canine Spotted Fever Group Rickettsial Seroprevalence as an Indicator for Human Rocky Mountain Spotted Fever Case Rates in Arizona, USA

Abstract

Introduction: Rocky Mountain spotted fever (RMSF), a rapidly progressing febrile disease caused by the bacterium Rickettsia rickettsii, is the deadliest tick-borne disease in the world. Human infection initially results in non-specific symptoms and, if untreated, can result in death in up to 35% of cases. Rhipicephalus sanguineus (brown dog tick) was discovered to spread RMSF in Arizona and Northern Mexico in the early 2000s, and the disease is now considered endemic in areas of the Southwestern United States. This study investigates the relationship between canine spotted fever group rickettsial (SFGR) seroprevalence and human RMSF case rates in Arizona. Methods: Canine serum samples were opportunistically collected from 12 counties in Arizona between February 2018 and November 2022. Immunofluorescence assays were employed to determine the anti-SFGR antibody titers in each of the samples, with dilutions ≥ 1:64 considered positive. An exponential nonlinear regression was used to determine the association between canine SFGR seroprevalence and human RMSF case rates per 100,000 as reported by the Arizona Department of Health Services. Results: Of the 423 dogs sampled, 21% (89/423) were found to be positive for anti-SFGR antibodies. Eight of the 12 counties had seropositive dogs. Geometric mean titers ranged from 64 to 464, with a median of 179. A nonlinear regression model demonstrated a strong association between canine SFGR seropositivity and human RMSF case rates, with the best-fitting model employing RMSF case rates lagged by 1 year. A similar model also showed a significant association between canine SFGR geometric mean titers and RMSF case rates. Conclusion: Canine SFGR seroprevalence correlates with human RMSF case rates, with RMSF risk rising exponentially as canine SFGR seroprevalence increases. This lends support to the potential use of canine SFGR serology as an epidemiological tool for forecasting RMSF.