In Slovenia, tick-borne encephalitis and Lyme borreliosis are both endemic diseases with high incidence rates and they are both transmitted by a bite of infected Ixodes ricinus tick. In clinical practice, tick-borne encephalitis is confirmed by demonstration of tick-borne encephalitis antibodies in serum of a patient with compatible clinical presentation and cerebrospinal pleocytosis. Patients with Lyme meningitis or meningoradiculitis also have cerebrospinal pleocytosis, however the presence of borrelial antibodies in serum does not attest Lyme neuroborreliosis.

Patients with tick-borne encephalitis and positive borrelial antibodies in serum, but not fulfilling criteria for Lyme neuroborreliosis, are often being treated with antibiotics in several European countries due to the possibility of double infection. The investigators hypothesise that such patients do not benefit from antibiotics. Such an approach may appear safe regarding the possibility of borrelial infection, however it can also be associated with detrimental consequences such as antibiotic related adverse reactions, negative epidemiological impact on bacterial resistance, and intravenous catheter related complications.

Inclusion Criteria:

• 18 years or older
• clinical picture compatible with tick-borne encephalitis,
• clear cerebrospinal fluid,
• cerebrospinal pleocytosis (leucocytes in cerebrospinal fluid >5 x 106/)L,
• positive serum immunoglobulin M (IgM) and immunoglobulin G (IgG) antibodies against tick-borne encephalitis virus,
• positive serum IgG antibodies against Lyme borreliae.

Exclusion Criteria:

• isolation of B.burgdorferi sensu lato from cerebrospinal fluid,
• positive intrathecal borrelial antibody production index,
• seroconversion of borrelial IgG antibodies,
• presence of erythema migrans and/or borrelial lymphocytoma in the last month,
• Bannwarth syndrome.