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Veterinarians are facing a newly identified bacterial threat to dogs across the Midwest and Southeast, one that mimics the symptoms of Rocky Mountain spotted fever (RMSF) but doesn’t register on standard diagnostic tests. The culprit, dubbed Rickettsia finnyi, is a tick-borne pathogen that has now been formally recognized as a distinct species, offering a clearer target for diagnosis and treatment.
The discovery of R. Finnyi helps explain puzzling cases where dogs presented with fever, lethargy, and other signs of spotted fever, yet tested negative for the more common Rickettsia rickettsii, the bacteria responsible for RMSF. Researchers at North Carolina State University (NC State) have been investigating this anomaly for years, and the formal naming of the fresh species is a crucial step toward better understanding and managing the illness in canine populations.
Identifying a New Threat
The story began in 2018 with reports of unusual cases in dogs. Blood samples sent to NC State from veterinary clinics revealed a recurring, unidentified pathogen. Dr. Barbara Qurollo, associate research professor at NC State, led the effort to pinpoint the source of these infections. “We first reported the novel species of Rickettsia in a 2020 case series involving three dogs,” Qurollo said. Further investigation, involving samples from an additional 16 dogs, confirmed the presence of the same unique bacteria.
Establishing R. Finnyi as a new species required a rigorous process: culturing the bacteria, sequencing its entire genome, and depositing samples in secure biobanks for verification by other researchers. This process ensures that other labs can compare their findings and confirm the presence of the new species in their own cases. The research was published in Emerging Infectious Diseases in November 2025.
Symptoms and Diagnosis
Across 17 dogs confirmed to be infected with R. Finnyi, common symptoms included fever and fatigue. The bacteria invade cells lining blood vessels, potentially causing swelling, pain, and a spotted rash – symptoms mirroring those of RMSF. Blood tests often revealed thrombocytopenia, a dangerously low platelet count that increases the risk of bruising and bleeding.
One of the challenges in diagnosing R. Finnyi is that standard tests for RMSF often produce false positives due to cross-reactivity with other Rickettsia species. Antibodies created in response to one species can sometimes bind to others, leading to inaccurate results. To address this, Qurollo’s team developed a specific screening test for R. Finnyi, improving diagnostic accuracy.
Tick Transmission and Treatment
While the exact tick species responsible for transmitting R. Finnyi remains unconfirmed, researchers suspect the lone star tick, Amblyomma americanum, may play a key role. A research group in Oklahoma has already detected R. Finnyi DNA within lone star ticks, lending weight to this theory, according to Qurollo. The lone star tick is prevalent throughout much of the South and Midwest. The American Kennel Club Canine Health Foundation highlights the importance of understanding tick-borne diseases in dogs.
Fortunately, R. Finnyi appears to respond to doxycycline, a common antibiotic used to treat RMSF and other tick-borne illnesses. In most cases, veterinarians initiated treatment with doxycycline before receiving definitive diagnostic results, and follow-up tests indicated the bacteria were cleared from the bloodstream in several dogs.
Implications for Public Health
Although no human infections with R. Finnyi have been reported to date, the discovery raises concerns about potential spillover to humans. Dogs often share outdoor spaces with people, and can serve as sentinels for emerging infectious diseases. The Centers for Disease Control and Prevention (CDC) emphasizes the importance of early treatment for suspected cases of Rocky Mountain spotted fever, and the ability to accurately identify R. Finnyi in dogs could help prevent misdiagnosis and delayed treatment in humans.
Spotted fever cases in the United States have been on the rise, increasing from 486 in 2000 to 6,248 in 2017, highlighting the growing threat of tick-borne diseases. Further research is needed to determine the prevalence of R. Finnyi in both canine and human populations, and to assess the risk of transmission in different geographic areas.
The identification of Rickettsia finnyi represents a significant advancement in our understanding of tick-borne diseases in dogs. Continued surveillance, improved diagnostic tools, and ongoing research will be crucial in protecting both animal and human health.
Disclaimer: The information provided in this article is for general knowledge and informational purposes only, and does not constitute medical advice. This proves essential to consult with a qualified veterinarian for any health concerns or before making any decisions related to your pet’s health or treatment.
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