New Tick-Borne Disease in Dogs Mimics Rocky Mountain Spotted Fever

Veterinary testing has confirmed a new tick-borne bacterium in U.S. Dogs whose illness can closely resemble Rocky Mountain spotted fever (RMSF). The discovery of Rickettsia finnyi helps explain why some sick dogs exhibit symptoms similar to RMSF but don’t test positive for the typical markers of the disease.

What is Rickettsia finnyi?

Rickettsia finnyi is a newly recognized species of bacteria belonging to the spotted fever group of Rickettsia. Researchers at North Carolina State University (NC State) first identified the novel species in dogs in 2018 and formally confirmed it as a distinct species in December 2025 [1]. The bacterium causes illness in dogs that shares symptoms with RMSF, including fever, lethargy, and signs of vascular inflammation like swelling, rash, and pain [1].

How Was Rickettsia finnyi Identified?

The identification of Rickettsia finnyi was a multi-step process. Initial reports came from veterinarians in the Midwest and Southeast who noticed dogs presenting with symptoms suggestive of RMSF but testing negative for the disease [1]. Dr. Barbara Qurollo, associate research professor at NC State, tracked a recurring pathogen that standard tests missed [1]. Genetic analysis confirmed it as a new species. To officially name the species, researchers successfully cultured the bacteria, sequenced its genome, and deposited samples in biobanks for further study [1], [2].

Symptoms in Dogs

Across 17 dogs confirmed to be infected with Rickettsia finnyi, common symptoms included fever and fatigue [1]. The bacteria invade cells lining blood vessels, potentially causing swelling, pain, and a spotted rash. Blood tests often reveal thrombocytopenia, a low platelet count that increases the risk of bruising and bleeding [1].

Transmission and Ticks

While the exact tick species responsible for transmitting Rickettsia finnyi hasn’t been definitively confirmed, researchers suspect the lone star tick (Amblyomma americanum) may play a role [1], [2]. Researchers in Oklahoma have found R. Finnyi DNA within lone star ticks [1]. Ticks transmit the bacteria through their saliva when feeding on a host.

Treatment and Prognosis

Veterinarians typically treat Rickettsia finnyi infections with doxycycline, an antibiotic that inhibits bacterial protein synthesis and reduces inflammation [1]. Follow-up tests on treated dogs have shown the bacterium’s genetic material disappearing after treatment, suggesting the antibiotic is effective.

Implications for Human Health

As of December 2025, no human infections with Rickettsia finnyi have been reported [1]. However, because dogs and humans share similar environments and can be bitten by the same ticks, monitoring this new pathogen in canine populations is important for public health surveillance. Spotted fever cases in the U.S. Have been increasing, rising from 486 in 2000 to 6,248 in 2017, highlighting the growing threat of tick-borne diseases [2].

Future Research

Ongoing research will focus on determining the prevalence of Rickettsia finnyi in dogs and ticks, expanding geographic surveillance, and refining diagnostic tests. The availability of cultured R. Finnyi strains through CDC reference collections will facilitate these efforts [3], [4].