Molecular evidence for persistence of Anaplasma phagocytophilum in the absence of clinical abnormalities in horses after recovery from acute experimental infection.
BACKGROUND
Anaplasma phagocytophilum infects several mammalian species, and can persist in sheep, dogs, and calves. However, whether this organism persists in horses or induces long-term clinical abnormalities is not known.


OBJECTIVE
To evaluate whether A. phagocytophilum can persist in horses and to document clinical findings for 3 months after complete recovery from acute disease.


METHODS
Five clinically normal adult horses that had recovered spontaneously from experimentally induced acute disease caused by a Swedish equine isolate of A. phagocytophilum.


METHODS
Horses were monitored for up to 129 days post inoculation (PI) by daily clinical examination and at least alternate day blood sampling for evidence of A. phagocytophilum on polymerase chain reaction (PCR) and blood smears. All horses were euthanized and underwent postmortem examination.


RESULTS
All horses were periodically PCR positive after recovery from acute infection. Before day 66 PI 2 horses were persistently PCR negative whereas 3 horses were intermittently PCR positive. Subsequently, 4 of 5 horses were intermittently PCR positive, particularly after stress mimicking interventions. One animal was positive immediately before postmortem examination. Clinical abnormalities related to persistence of anaplasma were not observed. No specific changes were found at postmortem examination, and all sampled tissues from all horses were negative on PCR for A. phagocytophilum.


CONCLUSIONS
Infection with A. phagocytophilum can persist in the horse for at least 129 days. However, the continued presence of the organism is not associated with detectable clinical or pathological abnormalities.
Franzén
P
P
Department of Clinical Sciences, Swedish University of Agricultural Sciences, Uppsala, Sweden. vet.franzen.@gmail.com
Aspan
A
A
Egenvall
A
A
Gunnarsson
A
A
Karlstam
E
E
Pringle
J
J
D041081:Anaplasma phagocytophilum / Q000302:isolation & purification*; D000712:Anaplasmosis / Q000382:microbiology*; D000818:Animals; D002908:Chronic Disease; D004269:DNA, Bacterial / Q000302:isolation & purification; D006734:Horse Diseases / Q000382:microbiology*; D006736:Horses; D025202:Molecular Diagnostic Techniques
D004269:DNA, Bacterial