Experimental evidence for milk-borne transmission of tick-borne encephalitis virus to suckling lambs

Tick-borne encephalitis virus (TBEV) is a medically important orthoflavivirus endemic across Europe and Asia, transmitted primarily by Ixodes ticks but also through ingestion of unpasteurized milk from infected ruminants. While milk-borne human infections are well documented, experimental evidence for natural mother-to-offspring transmission in livestock remains scarce. Here, we experimentally assessed the potential for lactogenic transmission of TBEV in sheep. Eight ewes were infected subcutaneously with the European subtype TBEV strain Hochosterwitz and co-housed with 16 three-week-old lambs (two per ewe) for an 18-day observation period. Clinical parameters, hematology, and virological and serological profiles were monitored. All animals remained asymptomatic, with no hematological abnormalities. TBEV RNA was detected in ewes' serum and milk from day one post-infection, and in lamb serum from day five. Virus-specific antibodies emerged in ewes from day four and in lambs from day 13. These findings provide direct experimental evidence that TBEV can be transmitted via milk from ewes to their suckling lambs throughout the lactation period, underscoring a potential but underappreciated route for virus transmission in endemic areas.