The narrow window of protection: protective efficacy of maternally derived antibodies against virulent classical swine fever virus in Japan.

This study investigated the protective efficacy of maternally derived antibodies (MDAs) against the currently circulating virulent classical swine fever virus (CSFV) strain in Japan (JPN/1/2018). Thirty-eight piglets from guinea-pig exaltation of Newcastle disease virus-negative phenomenon (GPE^-)-vaccinated (Groups 1-2) or unvaccinated sows (Group 3) were divided into three groups: Group 1 (n = 16, MDA titres < 2 to 5.6 against JPN/1/2018), Group 2 (n = 16, MDA titres 45-362), and Group 3 (n = 6, no MDAs). All piglets were orally challenged with the JPN/1/2018 strain and monitored for clinical signs, viremia, viral shedding, and viral tissue distribution. Groups 1 and 3 showed similar clinical manifestations, viremia, viral shedding and viral distribution patterns, whereas Group 2 piglets generally remained clinically normal with limited viral detection in clinical samples and organs. On the basis of these results, an MDA titre of 45 against JPN/1/2018 was determined as the approximate protective threshold. The use of antigenic relatedness (r₁) values between the JPN/1/2018 and GPE^- strains (mean r₁ = 0.43) corresponds to an MDA titre of approximately 105 against the GPE^- strain. However, two Group 2 piglets whose MDA titres were above the threshold died during the experiment, suggesting that individual factors such as body size, social hierarchy, and health status may influence protection. The narrow window between the protective threshold (MDA titre against the GPE^- strain of 105) and the titre that interferes with the vaccine-induced antibody response (MDA titre against the GPE^- strain of ≥ 128) presents challenges for farm management, highlighting the importance of combining vaccination with strict biosecurity measures.