Viral Distribution of Wild Boar Exposed to Low (Vaccine Candidate) and High Virulence African Swine Fever Virus Isolates: Immunohistochemical Characterization

Although several biosecurity and control measures are currently in place to mitigate the African swine fever (ASF) epidemic, vaccination is being explored as a potential long-term strategy. However, standardized guidelines for evaluating the safety and efficacy of ASF vaccines are not yet fully established. Understanding infection dynamics in wild boar is crucial, as they play a key role in the spread and persistence of the virus. This work aims to provide comprehensive information on viral distribution through immunohistochemical analysis (p72) with histopathologic assessment in wild boar. The study design comprises animals: (i) intramuscular infected with the high-virulence genotype II isolate Arm07 (highly virulent isolate [HVI]; <i>n</i> = 6); (ii) orally vaccinated with the low virulence isolate Lv17/WB/Rie1-<i>Δ</i>CD (low virulent isolate [LVI]; <i>n</i> = 6); and (iii) orally vaccinated with Lv17/WB/Rie1-<i>Δ</i>CD, either with a single dose (LVI-HVI1; <i>n</i> = 6) or repeated doses (LVI-HVI2; <i>n</i> = 6), followed by intramuscular challenge with Arm07. Clinical monitoring, viral load quantification in blood and tissues via real-time quantitative PCR, and virus viability in tissue cultures using peripheral blood mononuclear cells were performed. HVI animals had hemorrhagic and inflammatory lesions, along with generalized lymphoid depletion, correlated with widespread viral dissemination. LVI animals rarely showed mild lymphoid depletion of the lymph nodes; minimal immunostaining was observed in macrophages of the tonsils and lymph nodes, typically restricted to the oral entry point. A few LVI-HVI1 cases had infected resident sinus macrophages related to necrotic lesions at tonsils and lymph nodes, preventing the virus from disseminating to vital organs. No viral immunostaining or associated histopathologic lesions were observed in LVI-HVI2 animals, indicating that revaccination enhances safety against virulent challenges. Observed changes following vaccination do not reflect chronic infection but rather a transient one, followed by lymphoid system recovery. Immunohistochemical and histological evaluation has proven valuable in advancing our understanding of ASF pathogenesis in wild boar, contributing to improved vaccination safety and disease management strategies.