[TICK-BORNE ENCEPHALITIS VIRUS: EPIDEMIOLOGICAL AND CLINICAL PICTURE, DIAGNOSIS AND PREVENTION].

Tick-borne encephalitis virus (TBEV) is a small, enveloped virus that belongs to the family Flaviviridae, genus Flavivirus, tick-borne encephalitis serocomplex. There are three subtypes of TBEV: European, Far-Eastern and Siberian subtypes, which differ in geographical distribution, tick vector and clinical manifestation of disease in humans. TBEV is endemic in a wide geographic area ranging from Central Europe and the Scandinavian Peninsula to Japan. The virus is maintained in nature in so-called natural foci in cycles involving ticks and wild vertebrate hosts (mainly small rodents). The principal vector for the European subtype is Ixodes (I.) ricinus tick, whereas for Far-Eastern and Siberian subtypes it is I. persulcatus. In the Baltic States and Finland, co-circulation of two or all three subtypes was documented. Several animals, principally small rodents, serve as virus reservoirs. In the tick population, TBEV is transmitted by feeding/co-feed ing on the same host, transovarially (from infected females to their eggs) and trans-stadially (from one development stage to the next). An infected tick remains infected for life. While most TBE infections in humans occur following a tick bite, alimentary routes of TBEV transmission (consumption of unpasteurized milk/milk products from infected livestock) have also been described. All three tick stages can transmit the infection to humans. In the last decade, an increase of TBE incidence has been observed in some endemic areas. This could be due to a number of interacting factors such as changes in the climatic conditions affecting tick habitats, improvements in the quality of epidemiological surveillance systems and diagnostics, in landscape resources and their utilization and more outdoor recreation activity. In addition, the endemic area of TBEV has expanded to higher altitudes (up to 1500 m), apparently influenced by climatic changes. The typical clinical picture of infection with European subtype TBEV is characterized by a biphasic course (50%-77%). The first phase is characterized by nonspecific, flu-like symptoms followed by an asymptomatic interval of about one week. In 20%-30% of persons who develop symptoms, the second phase occurs with symptoms of central nervous system involvement (meningitis, encephalitis, myelitis, radiculitis). The mortality rate for European subtype is 1%-2%. Diagnosis is usually based on detection of specific antibodies (enzyme immunoassay, indirect immunofluorescent assay, plaque reduction neutralization test). From 1993 to 2013, a total of 777 cases of TBE were reported in Croatia. Endemicity is highest in north-western counties (mean incidence 3.61-6.78/100,000 inhabitants). The majority of patients were older than 20 years (88%). Most cases (73%) were reported from May to July.