Acute clinical, hematologic, serologic, and polymerase chain reaction findings in horses experimentally infected with a European strain of Anaplasma phagocytophilum.
Authors: Franzén P, Aspan A, Egenvall A, Gunnarsson A, Aberg L, Pringle J
Journal: Journal of veterinary internal medicine
Summary
# Editorial Summary: European Anaplasma phagocytophilum in Horses Researchers experimentally infected six horses with a Swedish strain of *Anaplasma phagocytophilum* to characterise the clinical, haematologic and immunological progression of infection with this European variant, which had previously been poorly documented compared to the Californian strain affecting equine populations. Clinical presentation, fever patterns and blood abnormalities proved indistinguishable from the established North American form, with one horse dying during acute infection, though PCR detection of bacterial DNA emerged 2–3 days before clinical signs appeared and remained detectable 4–9 days after fever resolution—considerably extending the diagnostic window beyond the 2.6-day average required for inclusion bodies to become visible on blood smears. Antibody responses developed between 12–16 days post-infection, with titres peaking within 3–7 days of illness onset. These findings demonstrate that the genetic differences between European and Californian strains do not substantially alter disease pathogenicity or progression in horses, and suggest that PCR-based diagnostics offer significant advantages over microscopy during the critical early days of infection when clinical signs are present but traditional morphological markers remain absent. For practitioners, this underscores the value of molecular testing in accelerating diagnosis and initiating treatment promptly, particularly when fever and systemic signs warrant investigation but blood film examination proves inconclusive.
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Practical Takeaways
- •PCR testing should be considered early in suspected anaplasmosis cases, as it detects infection 2-3 days before clinical signs appear and before inclusion bodies are visible on blood smears
- •Serologic testing alone may miss acute cases since antibodies don't appear until 12-16 days post-infection; PCR is superior for early diagnosis during the critical initial disease period
- •Clinical signs and hematologic changes in equine anaplasmosis caused by European strains are indistinguishable from North American strains, suggesting consistent disease presentation regardless of strain origin
Key Findings
- •PCR detection preceded clinical signs by 2-3 days and persisted 4-9 days after clinical resolution
- •Diagnostic inclusion bodies appeared on average 2.6 ± 1.5 days after fever onset
- •Antibody response first detected 12-16 days post-inoculation with rapid peak within 3-7 days of clinical illness
- •European strain demonstrated similar pathogenicity to Californian A. phagocytophilum strain with one fatal case among six horses