TBE in Bulgaria

Iva Christova

E-CDC risk status: endemic

(lack of consistent testing and reporting)
(data as of end 2022)

History and current situation

First cases of probable tick-borne encephalitis (TBE) were reported in 1961 by Andonov et al. in eastern regions of Bulgaria.1 Possible TBE cases with the typical two-wave fever, originating from consumption of raw goat milk, were described back in 1953 by Vaptzarov et al. in southern Bulgaria.2 Investigations in the 1960s were able to isolate 3 tick-borne encephalitis virus (TBEV) strains from Haemaphysalis punctata and 1 from Dermacentor marginatus ticks from goats and sheep in the district of Plovdiv.3 The antigenic properties of these 4 virus strains were identical to the highly virulent strain “Hypr” of the European subtype of TBEV (TBEV-EU).3

Laboratory diagnosis of TBE, based on serology using complement fixation assay, was introduced in Bulgaria in the 1970s. Since then single case reports of presumed TBE have been reported, but these lack reliable microbiological confirmation.4-5 However, investigations of ticks between 1974 and 2002 resulted in the isolation of 8 TBEV strains among 6,849 ticks investigated.6

Beginning in 2009, the National Reference Laboratory of Vector-Borne Pathogens introduced reliable laboratory diagnosis methods for TBE, based on polymerase chain reaction (PCR) and enzyme-linked immunosorbent assay (ELISA), and identified the first 3 confirmed TBE cases in Bulgaria: 2 cases in 2009 and 1 case in 2012.7 Two more TBE cases were identified in 2015 and one case was reported in 2017, one case in 2019, two cases in 2020, and one case in 2021.

Nationwide seroprevalence survey on circulation of TBE virus in Bulgaria found an overall seroprevalence of 0.6% (Fig. 4). However, district analysis showed TBEV seroprevalence to be up to 4.0%–4.8%, indicating that the TBEV infection seems to be more widespread in the country than previously described.8 Though TBE cases are reported sporadically, TBEV circulates in Bulgaria, causing human cases, associated with either tick bites or consumption of unpasteurized milk.

Overview of TBE in Bulgaria

Table 1: Virus, vector, transmission of TBE in Bulgaria
Viral subtypes, distribution European subtype of TBEV (TBEV-EU)3
Reservoir animals Not known
Infected tick species (%) Dermacentor marginatus, Haemaphysalis punctata
Dairy product transmission Yes

Table 2: TBE-reporting and vaccine prevention in Bulgaria
Mandatory TBE reporting TBE reporting is mandatory since 2014.Both physicians and laboratories report.   

 

EU case definition for confirmed, probable, and possible TBE case are accepted.
Other TBE-surveillance No
Special clinical features Bi-phasic disease
Available vaccines No information available
Vaccination recommendations and reimbursement No
Vaccine uptake by age group/ risk group/ general population Information not available
Name, address/website of TBE NRC National reference laboratory of vector-borne pathogens at the National Center of Infectious and Parasitic Diseases, Sofia, Bulgaria
www.ncipd.org

Figure 1: Burden of TBE in Bulgaria over time

Click the image above to enlarge

Source data
YearNumber of CasesIncidence / 10 5
20092n.c.
20100n.c.
20110n.c.
20121n.c.
20130n.c.
20140n.c.
20152n.c.
20160n.c.
20171n.c.
20180n.c.
20191n.c.
20202n.c.
20211n.c.
20220n.c.
Case reporting is sporadic and thus incidences cannot be calculated. Here only microbiologically confirmed cases are mentioned.
Case numbers by year are listed in the appendix. n.c. = not calculable

 

Age and gender distribution of TBE in Bulgaria

No table can be provided, the number of cases is too low to give any meaningful interpretation.

 

Figure 3: Place of residence of reported TBE cases in Bulgaria, 1953–2021

Click the image above to enlarge

 

Figure 4: Seroprevalence in Bulgaria, 2015

Click the image above to enlarge

Contact:

iva_christova@yahoo.com

Citation:

Christova I. TBE in Bulgaria. Chapter 12b. In: Dobler G, Erber W, Bröker M, Schmitt HJ, eds. The TBE Book. 6th ed. Singapore: Global Health Press;2023. doi:10.33442/26613980_12b5-6

References

  1. Andonov P, Rusakiev M, Hristova T. Natural outbreak of TBE in Eastern Balkan mountain. Hygiena. 1961;6:23-26.
  2. Vaptzarov I, Turpomanov A, Spasov Z, Nikov D, Dragiev M. Recurrent viral meningitis in South Bulgaria. Suvr Med (Sofiia). 1954;5(2):86-103.
  3. Pavlov P, Daniel M, Georgriev B, Kolman JM, Rashev KH, Arnaudov D, Ignatov D. The natural focus of tick-borne encephalitis of sheep and man in the Rhodope mountains (Bulgaria). Folia Parasitol (Praha). 1972;19(1):33-40.
  4. Georgiev B, Rosický B, Pavlov P, Daniel M, Arnaudov D. The ticks of the natural focus of tick-borne encephalitis of sheep and man in the Rhodope Mountains (Bulgaria). Folia Parasitol (Praha). 1971;18(3):267-73.
  5. Kaneva Z. Infectious diseases. Sofia: Znanie; 2006, pp. 261-2.
  6. Dikov I, Gacheva N, Kamarinchev B. Tick-borne encephalitis.In: Serbezov V, Kalvatchev Z, editors. Arboviral infections. Sofia: Iztok-Zapad Ltd; 2005, pp. 92-106.
  7. Mohareb E, Christova I, Soliman R, Younan R, Kantardjiev T. Tick-borne encephalitis in Bulgaria, 2009 to 2012. Euro Surveill. 2013;18(46). pii:20635.
  8. Christova I, Panayotova E, Tchakarova S, Taseva E, Trifonova I, Gladnishka T. A nationwide seroprevalence screening for West Nile virus and Tick-borne encephalitis virus in the population of Bulgaria. J Med Virol. 2017;89(10):1875-1878. doi:10.1002/jmv.24855

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