Tick-borne encephalitis foci in northeast Italy revealed by combined virus detection in ticks, serosurvey on goats and human cases

Abstract

Tick-borne encephalitis (TBE) is a severe zoonotic neurological disease endemic in northeast Italy since 1992. In the Province of Trento, a sharp increase in TBE incidence has been recorded since 2012, despite the vaccination efforts. To assess current TBE infection hazard in this area, we applied an integrated approach combining the distribution of human cases, the seroprevalence of tick-borne encephalitis virus (TBEV) in sentinel hosts and the screening of questing ticks for TBEV. A total of 706 goat sera from 69 farms were screened for TBEV-specific antibodies resulting in 5 positive farms, while the location of human cases was provided by the local Public Health Agency. Tick sampling was concentrated in areas where TBEV circulation was suggested by either seroprevalence in goats or human cases, resulting in 2,410 Ixodes ricinus collected and analyzed by real-time RT-PCR. Four tick samples from 2 areas with record of human cases were positive to TBEV corresponding to a 0.17% prevalence in the region, while risk areas suggested by serology on goats were not confirmed by tick screening. Our results revealed an increase in TBEV prevalence in ticks and the emergence of new active TBE foci, compared to previous surveys, and demonstrated the importance of an integrated approach for TBE risk assessment. A phylogenetic analysis of the partial E gene confirmed that the European TBEV subtype is circulating in northeast Italy and suggested that the different Italian TBEV strains originated independently as a result of different introductions from neighbouring countries, presumably through migratory birds.

Keywords: Italy; Ixodes ricinus; Tick-borne encephalitis; goats; human cases; phylogenetic analysis; prevalence.

Figure 1.

(A) Map of the Province of Trento reporting the locations of (i) the tick sampling sites; (ii) of the farms providing goat serum samples; (iii) of the human TBE cases. Thicker lines mark the borders between valleys. (B) Map of Italy showing the location of the Province of Trento within Triveneto.

Figure 2.

Human TBE cases reported by valley in the Province of Trento from 1992 to 2019. The incidence rate (IR) was calculated as the number of cases/resident population per 100,000 inhabitants.

Figure 3.

Variable sites in the E protein gene sequences between the TBEV strains from this study and the reference Neudoerfl strain. Nucleotide (upper) and amino acid (lower) positions are related to the complete polyprotein sequence. Nucleotide and amino acid substitutions are highlighted by colours.

Figure 4.

Phylogenetic tree of the partial E gene sequences generated in this study together with 67 TBEV strains retrieved from GenBank. Sequences obtained in this study are shown in bold, while those identified in the Province of Trento in 2006 (Italy-TN) and in Friuli Venezia Giulia in 2013 (Italy-FVG) are underlined. Only bootstrap values exceeding 70% are shown. The scale bar indicates 0.05 nucleotide substitutions per site.