Objectives: The aim of our study was to establish clinical and laboratory differences between patients with acute human granulocytic ehrlichiosis (HGE) and patients with the initial phase of tick-borne encephalitis (TBE).

Methods: Clinical features and laboratory results of four patients with acute HGE (established by the presence of the specific DNA sequences of the HGE agent in whole blood by polymerase chain reaction and/or by seroconversion to the HGE agent by indirect immunofluorescence assay) and 12 patients with the initial phase of TBE (demonstrated by the presence of serum IgM antibodies to TBE virus) were compared. All these patients were uncovered at the Department of Infectious Diseases, University Medical Centre, Ljubljana, Slovenia, during 1995–1996, in a prospective study on the aetiology of febrile illnesses occurring within 6 weeks after a tick bite.

Results: Findings were similar for the majority of the examined parameters including severity of illness, level and duration of fever, presence of headache, leukopenia, thrombocytopenia, and liver – function test abnormalities. Statistically significant differences were found only for arthralgia (P =0.026) and elevated concentration of C-reactive protein (P =0.003); both variables were found more often in patients with acute HGE.

Conclusions: In a patient residing in the central part of Slovenia, who reports a tick bite followed by a febrile illness with leukopenia and/or thrombocytopenia, the presence of arthralgias and/or an elevated C-reactive protein value directs toward the diagnosis of acute HGE and against the initial phase of TBE.