European study finds Klossiella equi in one-third of sampled equids

A new European surveillance study suggests Klossiella equi may be far more common in equids than many clinicians appreciate. In the Frontiers in Veterinary Science paper, investigators reported PCR detection of K. equi DNA in 94 of 284 equids, or 33%, using kidney and urine samples collected across five countries. The study also found low genetic diversity among European isolates, pointing to a relatively conserved parasite population in the sampled regions. (frontiersin.org)

That matters because K. equi has long sat at the margins of equine parasitology. The parasite is described in the paper as a renal apicomplexan of equids whose distribution, pathogenicity, and genetic diversity remain poorly defined. Historically, most diagnoses have come from histopathology, necropsy, or incidental identification of sporocysts in urine, which helps explain why the organism may be underdetected in routine practice. Earlier case reports and reviews have linked K. equi to acute kidney injury, interstitial nephritis, or heavy renal infection in some horses, but the broader clinical picture is still unclear. (frontiersin.org)

In the new study, the team analyzed opportunistically collected material from the Czech Republic, Italy, the Netherlands, Romania, and Spain. Kidney samples were positive in 31.0% of animals, while urine samples were positive in 25.6%. Among 82 paired kidney and urine samples, the agreement was moderate, with a Cohen’s kappa of 0.5. Using a composite reference standard, urine testing reached an apparent specificity of 100% but an apparent sensitivity of 65.6%, suggesting that a positive urine PCR is informative, but a negative result does not exclude infection. (frontiersin.org)

The genetic findings were narrower than the prevalence findings. Sequencing of a geographically representative subset of 38 PCR-positive samples showed just two observed haplotypes for cytB and four for COI, with limited divergence across European isolates. The most genetically distinct haplotypes were found in Italian mule and donkey samples, which may point to host or regional variation worth exploring in future studies. That interpretation is inferential, but it is consistent with the authors’ observation that broader epidemiologic and evolutionary questions remain open. (frontiersin.org)

The study also lands alongside new work suggesting the parasite is not just a European curiosity. A 2026 report on equids in the Americas described K. equi as underreported and found an overall prevalence of 45% across three equine populations in Canada, the United States, and Argentina, with no infected animals younger than 10 months in that dataset. Taken together, the European and American findings support the idea that K. equi may be a common but overlooked parasite of adult equids rather than a rare incidental finding. (sciencedirect.com)

Why it matters: For equine veterinarians, pathologists, and diagnostic laboratories, the immediate takeaway is not that every PCR-positive horse has clinically important renal disease. It is that K. equi deserves a place on the differential list when urine sediment, renal pathology, or unexplained urinary findings raise questions, especially in adult animals. A non-invasive urine PCR could make prevalence studies and targeted screening more feasible, but the test’s limited sensitivity means it is better suited to surveillance or case support than to confidently ruling infection out. (frontiersin.org)

The bigger gap is clinical interpretation. If K. equi is common in apparently normal equids, veterinary teams will need better data to distinguish incidental detection from clinically meaningful infection. That will likely require prospective studies pairing urine PCR with urinalysis, renal biomarkers, histopathology where available, and clinical follow-up. Until then, the parasite’s apparent prevalence may outpace certainty about what a positive result means for an individual patient or for herd-level management. (frontiersin.org)

What to watch: The next phase will likely focus on validating urine-based testing in live horses, defining age, geography, and species risk patterns, and clarifying whether PCR positivity correlates with renal lesions, shedding dynamics, or measurable clinical disease. (frontiersin.org)