[Fever of unknown origin caused by anaplasmosis].

A 74-year-old man in good clinical condition presented with complaints of recurrent fever up to 38 degrees C and diffuse thoracic pain, both present for several weeks. Antibiotic therapy did not result in disappearance of the symptoms. Except for a picture of chronic inflammation and a positive Anaplasma IgM antibody titre, serology yielded no indication ofthe diagnosis. Further investigation, including imaging, showed no convincing cause. 'Anaplasma infection' remained the working diagnosis. One month after hospitalisation, the patient was free of fever without any type of therapy. Control blood tests revealed seroconversion to Anaplasma IgG antibodies, constituting serological evidence of a recent infection. The prevalence ofanaplasmosis is increasing. Mice are the principal reservoir for the intracellular bacteria and the infection is transmitted by ticks. In case of fever of unknown origin, since the transmission takes place in the same manner, one should consider both Borrelia and an infection with Anaplasma.