Introduction
Crimean-Congo hemorrhagic fever (CCHF) is a tick-borne human viral disease with a fatality rate up to 30%. It is characterized by a sudden onset of fever and muscular pain, often progressing to hemorrhagic manifestations [7].
Various wild and domestic mammals are natural reservoir hosts of CCHF virus (CCHFV). Ticks of the genus Hyalomma are primary vector and serve also as reservoir hosts. Rhipicephalus and Dermacentor ticks may play an additional role in maintaining the circulation of CCHFV in an enzootic cycle between vertebrates and ticks. The vector-competent ticks stay infected through the molt (“transovarial transmission”) [7]. Humans are infected by infected ticks and also by contact with tissues or body fluids of infected animals or patients. Nosocomial outbreaks are described [8]. A case of probable airborne transmission is reported [9].CCHF is spread in over 50 countries in Africa, Southern Asia, the Middle East, and Southeastern Europe, including the Balkan Peninsula. Cases have been reported in Bulgaria, Turkey, Kosovo, Albania, and Greece. CCHFV (genus Orthonairovirus, family Nairoviridae) is a negative-stranded RNA virus with a three-segmented genome: large (L), medium (M), and small (S) segments. CCHFV strains belong to seven genetic lineages. Lineage V, also named Europe 1, contains pathogenic CCHFV strains. Lineage VI, called Europe 2, contains genetically different Greek AP92 strains and recently reported similar strains from Turkey, Greece, Kosovo, and Algeria. Besides the detection in ticks, CCHFV lineage Europe 2 has been detected in a mild CCHF case in Turkey [10]. A fatal case due to an AP92-like strain has been recently reported in Iran [11].
With sporadic cases or small outbreaks, CCHF appeared in Bulgaria every year since 1950s. CCHFV was first detected in 1952 in Stara Zagora region [12]. Over 1500 cases have been reported in the country since then. For the last 10 years, 2009-2018, a total of 60 CCHF cases are officially recorded in the country. Case fatality rate of CCHF was an average 15.0%. There is a tendency for CCHF to spread in previously non-endemic areas. The strains causing CCHF in the country are closely related to others in the Balkan peninsula, belonging to lineage Europe 1 [13].
2.2