Molecular evidence for persistence of Anaplasma phagocytophilum in the absence of clinical abnormalities in horses after recovery from acute experimental infection.
Authors: Franzén P, Aspan A, Egenvall A, Gunnarsson A, Karlstam E, Pringle J
Journal: Journal of veterinary internal medicine
Summary
# Editorial Summary: Persistence of Anaplasma phagocytophilum in Horses Whether *Anaplasma phagocytophilum* establishes chronic infection in horses—as it does in sheep, dogs, and cattle—remained unclear, prompting Franzén and colleagues to investigate bacterial persistence and long-term clinical consequences in five horses experimentally infected with a Swedish equine isolate. Following recovery from acute disease, horses underwent intensive clinical monitoring and blood sampling via PCR and microscopy for up to 129 days post-inoculation, with postmortem tissue examination at study conclusion. All five horses demonstrated intermittent PCR positivity after clinical recovery, particularly following stress-mimicking interventions, yet showed no associated clinical signs, haematological abnormalities, or pathological findings; notably, all tissue samples tested negative post-mortem, suggesting circulating organism persistence rather than tissue localisation. This work establishes that *A. phagocytophilum* can persist asymptomatically in equine circulation for extended periods—a finding with significant implications for understanding disease epidemiology and the potential role of apparently healthy horses as reservoirs within populations. Clinicians and stud managers should recognise that whilst recovered horses may pose transmission risk via arthropod vectors, persistent infection alone does not predict ongoing clinical disease or performance impairment.
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Practical Takeaways
- •Recovered horses may carry A. phagocytophilum asymptomatically for extended periods; PCR testing may be needed to identify carrier status if epidemiologically important
- •Stress events can trigger reactivation or amplification of organism in carrier horses, so monitoring previously infected animals during stressful situations may be warranted
- •Absence of clinical signs in recovered horses does not guarantee pathogen elimination; serological or molecular surveillance may be appropriate depending on herd management goals and tick exposure risk
Key Findings
- •All 5 horses demonstrated PCR evidence of A. phagocytophilum persistence for up to 129 days post-inoculation after clinical recovery from acute infection
- •Persistence pattern showed 2 horses persistently PCR negative before day 66 PI, then 4 of 5 horses intermittently PCR positive, particularly after stress-mimicking interventions
- •No clinical abnormalities or pathological changes were detected on postmortem examination despite molecular evidence of persistent infection
- •All tissue samples from all horses were PCR negative for A. phagocytophilum at postmortem examination, indicating organism presence primarily in circulation