Cross-sectional study on the prevalence of tick-borne encephalitis in horses in Lithuania

Tick-borne encephalitis virus (TBEV) is an emerging zoonotic pathogen of growing concern in Europe, particularly in endemic regions like Lithuania. While various animal species have been studied for their potential as TBEV sentinels, the role of equine as indicator of TBEV circulation remains incomplete. The present study evaluated TBEV prevalence dynamics in Lithuanian horses. A serosurvey targeting TBEV-specific IgG antibodies and direct virus detection analysis of selected horses (n=82) were performed in 7 districts in Lithuania. Results showed high seroprevalence, 54 (65.9%, 95% CI 55.1 – 75.2) horses from 7 districts of Lithuania were positive for TBEV IgG. To assess active infections, the whole blood samples were tested using PCR for the presence of TBEV RNA. This screening identified 7 horses (8.5%, 95% CI 4.2 – 16.6) as viremic. These positive samples were subsequently analyzed by digital PCR for precise quantification of viral load. The total viral load varied from 0.91 to 18.72 viral copies/μL. Additionally, comparative analysis was made to compare the results of TBEV antibodies distribution in 2019 and 2023 years. TBEV seroprevalence among horses significantly increased from 2019 (35.8%, 95% CI 26.2 - 46.7) to 2023 (71.4%, 95% CI 60.0 - 80.7), (p<0.01). This increase may indicate intensified TBEV circulation within equine environments in recent years due to climate change or more frequent contact between horses and ticks because of expanding tick habitats. Overall, the findings indicate widespread exposure of TBEV infections in Lithuanian horses. This highlights the potential of horses as sentinel animals for monitoring TBEV circulation in endemic areas. Regular surveillance in equine populations could strengthen early detection of TBEV activity, aiding public health efforts to mitigate transmission risks to both animals and humans.