Diagnosis of Potomac horse fever (syn. equine neorickettsiosis) in 2 foals in southwestern Ontario.
Authors: Rosalie Fortin-Trahan, E. Sjolin, A. Lack, Cristina Lopez de Arbina, Amy McFadden-Bennett, Lidan Wang, John D. Baird, Y. Rikihisa, L. G. Arroyo
Journal: The Canadian veterinary journal = La revue veterinaire canadienne
Summary
Potomac horse fever (PHF), caused by *Neorickettsia risticii* and *N. findlayensis*, has traditionally been considered a disease of adult equines, presenting with fever, depression, anorexia, ileus, diarrhoea, and occasionally laminitis—yet its occurrence in foals remains poorly documented in the literature. This case report from southwestern Ontario describes two foals diagnosed with PHF via faecal PCR testing, which identified *N. risticii* in the first case and *N. findlayensis* in the second; both animals recovered following hospitalisation and intensive supportive care. The findings highlight an important diagnostic gap: veterinarians working in endemic regions should maintain a higher index of suspicion for PHF in young stock presenting with compatible clinical signs during the summer months (July–September), rather than assuming the disease is limited to mature horses. Practitioners should submit both blood and faecal samples for *Neorickettsia* PCR testing when PHF is suspected, as early diagnosis enables timely treatment and improves outcomes. These cases expand our understanding of PHF epidemiology and reinforce that age should not be used to exclude this differential diagnosis in foals presenting with acute gastrointestinal and systemic signs during the peak transmission season.
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Practical Takeaways
- •During summer months (July-September) in endemic areas, include PHF in your differential diagnosis for foals presenting with fever, depression, anorexia, diarrhea, and ileus—not just adult horses
- •Submit both blood and feces for PCR testing of Neorickettsia spp. to confirm diagnosis, as clinical signs alone are non-specific
- •PHF in foals is responsive to treatment with appropriate hospitalization and intensive care, so early diagnosis and supportive therapy are warranted
Key Findings
- •Potomac horse fever, caused by Neorickettsia risticii and N. findlayensis, can affect foals in endemic areas such as southwestern Ontario
- •Both affected foals presented with fever, depression, anorexia, ileus, and diarrhea, successfully diagnosed via PCR testing of feces and blood
- •Both foals recovered following hospitalization and intensive care, indicating PHF is treatable in the young equine population