Introduction

Tick-borne encephalitis is the most common tick-borne viral infection in humans. The disease occurs in North and Central Europe, Russia, Far East Asia, and Japan. In the last few decades, the number of reported cases increased within the endemic regions along with expanding of these areas.

Tick-borne encephalitis virus (TBEV) belongs to genus Flavivirus, family Flaviviridae like etiological agents of dengue, yellow fever, Zika infection, West Nile fever (WNF), and Japanese encephalitis. Three subtypes of the virus, European, Siberian, and Far Eastern, cause TBE with different severity and outcome of the disease.

Transmission routes of TBEV include bites of infected Ixodes ricinus ticks and consumption of row milk from infected goats, sheep, and cows. The incubation period is usually 7-14 days (between 2 and 28 days).

Like infections with other flaviviruses, most of the human infections with TBEV are asymptomatic (75-98%). Among the symptomatic patients infected with European subtype of TBEV, most develop unspecific febrile disease. In some cases only, infection of the central nervous system appears—meningitis (about 50% of the patients), meningoencephalitis (about 40%), and meningoencephalomyelitis (about 10%).

Tick-borne encephalitis is very unusual in Bulgaria. Over the past 40 years, only a few cases of TBE have been detected. Most of the TBE cases in the country are due to consumption of row goat milk. However, the tick vector, Ixodes ricinus, is widely distributed in Bulgaria, and Lyme borreliosis, transmitted by the same tick species, is endemic in the country.

Since 2009, reliable laboratory diagnosis of TBE, based on PCR and ELISA, was introduced, and the first three confirmed TBE cases in Bulgaria were identified: two cases in 2009 and one case in 2012 [26]. Two more TBE cases are identified in 2015.

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