Implications for safety: ASFV-G-ΔI177L vaccine compromises health and semen quality in adult breeding boars.

African swine fever poses an ongoing threat to Eurasian wild pigs and the associated global swine production, with no internationally licensed vaccine available. Live attenuated vaccines like ASFV-G-ΔI177L demonstrated promising safety and protection in young domestic pigs, but comprehensive safety evaluations in breeding animals are necessary for regulatory licensing in the EU. This study investigated the safety of intramuscular immunization with ASFV-G-ΔI177L in adult breeding boars (Sus scrofa domesticus). Eight healthy boars were intramuscularly vaccinated and monitored for clinical manifestation, viral genome and infectious virus in blood, oral fluid, semen, and tissues. Furthermore, ASFV-specific antibodies and effects on semen quality were assessed over an observation period of 28 days. Clinical signs appeared in all boars at 6 days post vaccination (dpv), including high fever, lethargy, and anorexia in most animals by 8 dpv. Four animals developed severe disease and were euthanized at humane endpoints or succumbed between 11 and 15 dpv, while four animals recovered. Viral genome was detected in blood, oral fluid, and semen of all animals, with higher loads in succumbed individuals. Infectious ASFV-G-ΔI177L was found in reproductive tissues, spleens, salivary glands, and semen samples from acutely affected boars and one recovered boar. All boars developed ASFV-targeting antibodies, but titers remained low. Semen quality was significantly reduced after vaccination and failed to recover in surviving boars by 28 dpv. These findings raise concerns regarding vaccine safety in reproductively active boars and highlight the risk of virus transmission through semen.