Ineffective oral immunization of wild boar with the attenuated African swine fever virus NH/P68 grown in MA104 cell line

Abstract

African swine fever (ASF) is a highly contagious and lethal hemorrhagic disease affecting domestic pigs and wild boar, with serious consequences for animal health, the swine industry, and socio-economic stability. Given the lack of effective treatments and the limited success of conventional control measures, live attenuated vaccines (LAVs) have emerged as the most promising option, particularly for oral administration in free-ranging wild boar. The naturally attenuated genotype I isolate NH/P68 has shown protective potential in domestic pigs but retains residual virulence and persistence risks. To increase attenuation and facilitate large-scale production, the isolate was adapted to the MA104 continuous cell line. In this study, we assessed the safety and efficacy of NH/P68-MA104 in wild boar using an oral prime-boost immunization protocol (a primary dose of 10⁴ TCID₅₀/mL followed by two booster doses of 10⁵ TCID₅₀/mL), followed by a challenge with 10 HAD₅₀/mL of the virulent genotype II Armenia07 (Arm07) isolate. Oral administration of NH/P68-MA104 was safe, with no adverse effects observed throughout the vaccination period. Only one animal developed transient viremia, which was accompanied by clinical signs and a detectable antibody response. Two additional animals showed mild viremia, while the remaining animals displayed no evidence of infection or seroconversion before the challenge. After the challenge, vaccinated wild boar were not protected: all animals developed fever, viremia, and clinical signs consistent with acute ASF and succumbed at 14 ± 3 days post-challenge (dpc), comparable to in-contact and intramuscularly (IM) infected controls. These results demonstrate that oral immunization of wild boar with NH/P68-MA104 is safe but ineffective under the tested conditions. Continued research is needed to refine vaccine candidates and delivery strategies to achieve effective immunization of wild boar