Infectious Diseases

Two species of Neorickettsia likely leading to more Potomac Horse Fever cases

By Marie Rosenthal MS

Although Neorickettsia risticii accounts for most Potomac Horse Fever (PHF) cases, Neorickettsia findlayensis has been identified in multiple locations across the United States. However, this causative agent often goes unrecognized, according to Virginia Logue, DVM, a third-year large animal internal medicine resident at the University of Pennsylvania College of Veterinary Medicine. A recent study found that nearly one-third of positive cases were caused by N. findlayensis rather than N. risticii.

“Neorickettsia findlayensis accounted for approximately one-third of PHF-positive cases, suggesting that it is not a rare cause of disease. Horses infected with Neorickettsia findlayensis exhibited clinical signs consistent with classic Potomac horse fever, and would likely be clinically indistinguishable from horses infected with N. risticii,” Dr. Logue said at the ACVIM Forum 2026 in Seattle.

In addition, “these findings suggest that N. findlayensis is likely under-recognized and highlight the importance of using diagnostic assays capable of detecting this organism when evaluating horses that are suspected to have PHF.”

Dr. Logue and her colleagues conducted a retrospective study evaluating 3,160 equine PHF submissions received by Cornell University’s Animal Health Diagnostic Center between January 2023 and September 2025 that were tested using polymerase chain reaction (PCR) assays. All the samples were initially tested with a real-time PCR assay that detects the Neorickettsia genus but does not differentiate species.

They retested the 218 PHF-positive cases with multiplex real-time PCR assays to differentiate between the 2 species that can cause disease—N. risticii and N. findlayensis. When available, they also noted the geographic origin, signalment, specimen type and submission type.

“Of the 218 positive cases that underwent species differentiation, Neorickettsia risticii remains the predominant species, accounting for approximately 69% of cases. However, Neorickettsia findlayensis accounted for nearly one-third of all PHF-positive cases, demonstrating that it represents a substantial proportion of PHF infections within this population,” Dr. Logue said.

Although PHF occurs more commonly in the Northeast, 16 states spanning multiple geographic regions, including the Northeast, Midwest, Northwest and Southwest, reported cases. “New York had the highest proportion of Neorickettsia findlayensis infections with approximately 61% of typed PHF-positive cases attributable to Neorickettsia findlayensis, compared with 13% in Wisconsin and 6% in Pennsylvania,” she said.

PHF samples were submitted for testing throughout the year, but most cases were identified in July and August—the seasonal distribution of PHF that most veterinarians would expect. And this season distribution was seen for both Neorickettsia.
Most often, whole blood was submitted for analysis, but fecal samples also were submitted in some cases. However, “neither sample type appeared to be uniformly superior, supporting the continued submission of both whole blood and fecal samples,” Dr. Logue said.

“Overall, no obvious clinical pattern emerged that reliably distinguished between the species,” she said. The most frequently reported clinical signs included fever, diarrhea, lethargy and inappetence, with laminitis or foot pain being infrequently reported regardless of infecting species. In addition, age, breed and sex distributions were similar between species.

The 1 vaccine against PHF is only partially protective, and Dr. Logue and her colleagues wondered whether this was because it was designed to protect against N. risticii rather than N. findlayensis infection. However, they did not have vaccination histories for all the horses, so they could not definitively evaluate this question. “Vaccine efficacy against PHF has long been recognized as variable, and while strain variation has often been discussed as a contributing factor, our findings raise the question of whether species differences may also play a role.”

She said more studies would be needed to evaluate this question.

A limitation of her study was that the data originated from a single diagnostic laboratory in the Northeast, so there could be a geographic submission bias, with Northeast cases likely overrepresented relative to other regions of the country.

“While PHF was once considered a relatively geographically restricted disease, its recognized distribution has expanded substantially over the past several decades, with cases now reported throughout much of North America,” Dr. Logue said. “For decades, Neorickettsia risticii was considered the sole cause of Potomac Horse Fever, and as a result has been the primary target of both diagnostic testing as well as vaccine development.”

She said 1 challenge in identifying N. findlayensis is that some PCR assays either do not differentiate Neorickettsia or are designed to detect N. risticii. “As a result, Neorickettsia findlayensis infections may go unrecognized, potentially impacting diagnosis, treatment decisions and our understanding of the organism’s epidemiology,” Dr. Logue explained.

She suggested that clinicians who suspect PHF verify that their diagnostic laboratory uses an assay capable of detecting Neorickettsia findlayensis.

“In conclusion, Neorickettsia findlayensis appears to be an important and geographically widespread cause of Potomac Horse Fever in the United States, and although clinical cases had previously only been reported in Ohio and Canada, Neorickettsia
findlayensis was identified in 16 US states in this study, substantially expanding its recognized geographic distribution.

“These findings have practical implications. Clinicians should recognize that Neorickettsia findlayensis represents a substantial proportion of PHF infections and should be considered in horses with compatible clinical signs,” Dr. Logue said. MeV