Tick-borne encephalitis virus in cows and unpasteurized cow milk from Norway

Tick-borne encephalitis virus (TBEV) is recognized as the most important zoonotic tick-transmitted virus in Europe. TBEV is mainly transmitted to humans through bites from TBEV-infected ticks (Ixodes ricinus and Ixodes persulcatus). However, alimentary infection after consumption of unpasteurized milk and cheese from domestic ruminants has been reported. There is little information about TBEV in ruminants in Norway. The objectives of this study were to analyse unpasteurized cow milk for TBEV RNA and to study the presence of IgG antibodies to TBEV in the same animals. A total of 112 milk and blood samples were collected from cows from five different farms spread from southern to northern Norway. The milk samples were analysed by an in-house reverse transcription (RT) real-time polymerase chain reaction and confirmed by pyrosequencing. Serum samples were screened by a commercial enzyme-linked immunosorbent assay and verified by a TBEV-specific serum neutralization test. We found TBEV RNA in unpasteurized milk collected from farms in the municipalities of Mandal, Skedsmo and Brønnøy in 5.4% of the tested animals. Specific antibodies to TBEV were only detected in Arendal, where 88.2% of the tested animals were positive. Further studies on milk containing TBEV RNA should be performed to conclude if TBEV found in unpasteurized milk in Norway is infectious, which could be of great importance in a One Health perspective.