To date, only isolated incidences of human granulocytic anaplasmosis (HGA) have been reported in Europe. However, entomological studies in Bavaria, Germany showed a prevalence of Anaplasma phagocytophilum of between 2 and 9.5% in the tick vector Ixodes ricinus. In this study we assessed the risk of pathogenic A. phagocytophilum infection after a tick bite in Bavaria. The seroprevalence of anti-Borrelia burgdorferi sensu lato (s.l.) antibodies was investigated as an indicator of past exposure, seroconversion as actual exposure of participants to ticks. Patients with a tick bite in the preceding four weeks were recruited by participating doctors. Questionnaires on demographics, tick exposure and clinical signs were completed by patients and doctors, respectively. Two blood samples, taken at an interval of two weeks, were tested for antibodies against A. phagocytophilum and B. burgdorferi s.l. One of 107 recruited patients showed serological evidence of an acute infection of A. phagocytophilum but had no clinical signs. Four out of six patients with serological evidence of an acute B. burgdorferi s.l. infection, presented with erythema migrans. A seroprevalence of 7.5% for A. phagocytophilum and 13.1% for B. burgdorferi s.l. was detected. The comparatively high seroprevalence of B. burdorferi s.l. and A. phagocytophilum antibodies indicate frequent past exposure of participants to ticks. The finding of one acute infection of A. phagocytophilum in the absence of clinical signs, supports entomological evidence that the strains of A. phagocytophilum predominantly present in the Bavarian tick population may cause transient infections but are of low pathogenicity in humans.
Keywords: Anaplasma phagocytophilum; Borrelia burgdorferi; Ixodes ricinus; Risk assessment; Tick bite.
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