Investigation of Neorickettsia risticii and Neorickettsia findlayensis co-infection in Potomac Horse Fever clinical cases from Eastern and Western Canada

• Maria A. Papapetrou - Department of Pathobiology, Ontario Veterinary College, University of Guelph
• Luis G. Arroyo - Department of Clinical Studies, Ontario Veterinary College, University of Guelph
• Ashley Whitehead - Faculty of Veterinary Medicine, University of Calgary
• John D. Baird - Department of Clinical Studies, Ontario Veterinary College, University of Guelph
• Brandon N. Lillie - Department of Pathobiology, Ontario Veterinary College, University of Guelph

Potomac Horse Fever (PHF) is an acute colitis that affects horses seasonally. It causes fever, diarrhea, colic, laminitis, abortion, and death in 5-30 % of cases. Previously, Neorickettsia risticii was identified and widely accepted as the causative agent for PHF. A novel Neorickettsia spp., Neorickettsia findlayensis, with an 11.6 % genomic divergence from N. risticii, was shown to also be a causative agent for PHF in 2020. Molecular detection using PCR was performed on blood and fecal samples to identify the specific Neorickettsia spp. in 18 clinical cases from Eastern Canada (Ontario and Quebec), and 13 from Western Canada (Alberta). The goal was to identify the frequency of infection with N. risticii, N. findlayensis, or both, and compare the causative agent by location. qPCR was performed on DNA isolated from 31 blood samples and on DNA isolated from 26 fecal samples using a general Neorickettsia primer. If positive, additional PCRs were performed using primers specific to N. risticii and N. findlayensis. 12/31 blood samples were positive on qPCR using a general Neorickettsia primer; with 7/12 positive for only N. risticii, and 4/12 for only N. findlayensis, with 1 sample testing positive for both N. risticii and N. findlayensis. 8/26 fecal samples were positive using the general Neorickettsia primer; with 4/8 positive for N. findlayensis and 4/8 positive for both N. risticii and N. findlayensis. This study has established that co-infection with N. risticii and N. findlayensis is possible, and found in cases across Canada.