Molecular Evidence for Persistence ofAnaplasma phagocytophilumin the Absence of Clinical Abnormalities in Horses after Recovery from Acute Experimental Infection

Abstract

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

Anaplasma phagocytophilum DNA amplified from lesional skin of seropositive dogs

“…Persistent A. phagocytophilum infection has been described in dogs, sheep, and horses (Egenvall et al, 2000;Stuen and Bergström, 2001;Franzén et al, 2009). The diagnosis of persistent granulocytic anaplasmosis is based on intermittently positive PCR results, reoccurrence of morulae in peripheral blood smears, and fluctuating antibody titers (Egenvall et al, 2000;Franzén et al, 2009;Scorpio et al, 2011).…”

“…Despite the active research, there are several key mechanisms still not clear, such as the site of the persistence of the organism. To date, A. phagocytophilum DNA has been amplified from several sites in different animal species: circulating granulocytes in sheep and horses (Franzén et al, 2009;Stuen et al, 2008), poorly vascularized connective tissue in horses (Chang et al, 1998), and skin in sheep (Granquist et al, 2010). The above-mentioned sites have also been listed as possible sites of persistence of the organism in animals without typical clinical signs of granulocytic anaplasmosis.…”

“…The diagnosis of persistent granulocytic anaplasmosis is based on intermittently positive PCR results, reoccurrence of morulae in peripheral blood smears, and fluctuating antibody titers (Egenvall et al, 2000;Franzén et al, 2009;Scorpio et al, 2011). Since clinical signs are absent or vague and serial PCR and serological testing are frequently difficult to obtain, the persistent granulocytic anaplasmosis might be underdiagnosed in a clinical setting (Stuen and Bergström, 2001;Woldehiwet, 2010).…”

Borrelia burgdorferi and Anaplasma phagocytophilum in ticks and their equine hosts: A prospective clinical and diagnostic study of 47 horses following removal of a feeding tick

“…Although four to five horses seroconverted during the examination period, none came up positive in either of the infectious agent detection tests (blood smear; PCR-RLB). This is most probably explained by the relatively long intervals between sampling; infected horses have been reported to clear the parasite within weeks, although Franzen et al (2009) did find A. phagocytophilum DNA intermittently in blood, but not in tissues, months after an experimental infection. In summary, even if horses do become persistently infected, their role as agent reservoir should be comparatively minor.…”

“…In contrast to B. burgdorferi, diagnosis of clinical A. phagocytophilum infections in horses is straightforward and based on a combination of clinical signs and the detection of the agent in the blood, either microscopically in smears or via molecular tests (Butler et al 2008, Franzen et al 2005, Franzen et al 2009). While serological assays are commonly used for prevalence studies, there is as yet no established gold standard test.…”