A systematic review on the prevalence of tick-borne encephalitis infection in milk and milk products in Europe

Introduction: Tick-borne encephalitis is a vector-borne disease caused by a Flavivirus, the tick-borne encephalitis virus (TBEV). Although TBEV is mainly transmitted to humans by tick bite, the alimentary route is also possible. Objectives: This systematic review (SR) was conducted to assess the prevalence of TBEV in milk and milk products from domestic ruminants in Europe. Materials & methods: A comprehensive search was performed in three databases (Medline, Embase and CAB Abstracts, from 1980 to November 2022). Screening, data extraction and critical appraisal were conducted independently by two reviewers. TBEV prevalence was calculated using the number of milk or milk product samples tested (RNA or specific anti-TBEV antibodies), and number of positive samples. A narrative synthesis was performed. Results: 396 articles were identified from the searches, 58 were selected for full-text screening, and 15 were included. Data from 32 data collections were extracted (27 on milk and 5 on cheese), of which the sample size ranged from a single sample (9 studies) to 1363 samples in one study. Regarding animal species, data collections focused on cow milk (n=14), goat milk (n=10), sheep milk (n=2), goat cheese (n=4), cow and goat cheese (n=1) and sheep and goat milk (n=1). Fifteen studies on milk were carried out on individual animal samples and 12 on bulk-tank milk. Studies with larger milk samples (N≥29; corresponding to the sample size needed to detect at least one positive sample, with an expected prevalence of 10%, and 95% confidence level) had a median prevalence of infection of 4.5% (range 0% to 20.7%). Overall, 10.1% of cheese specimens were positive (all studies combined). Conclusion: Our SR confirmed that unpasteurised milk or milk products from domestic ruminants can be a vehicle of TBEV transmission to humans. Therefore, surveillance on these products could be a valuable tool for studying the epidemiological situation in a geographic area.