Pathogenicity and pathobiological characterization of a recombinant genotype I/II African swine fever virus in pigs

African swine fever virus (ASFV) is a highly contagious pathogen affecting domestic pigs and wild boar, causing substantial economic losses. Recently, a highly virulent recombinant ASFV strain combining genotypes I and II (rASFV I/II) emerged in Asia. Genotype II ASFV vaccine candidates have failed to effectively protect against rASFV I/II infection, presenting a critical challenge. Here, we investigated the pathobiological characteristics of rASFV I/II in domestic pigs. Ten healthy 7-week-old female pigs were intramuscularly inoculated with an rASFV I/II strain. Clinical signs, rectal temperatures, and samples were collected daily, and necropsies were conducted postmortem. All inoculated pigs succumbed to infection within 5-7 days post-inoculation (dpi), with a mean of 5.5 ± 0.7 dpi. High fever ( > 40.5°C) was observed at an average onset of 2.6 ± 0.8 dpi, and the incubation period averaged 3.0 ± 1.1 dpi. Viral DNA was first detected in blood at 2 dpi, with viral genome copies increasing steadily. Postmortem analysis revealed severe congestive splenomegaly and hemorrhagic lymphadenopathy in all pigs. A preliminary comparison of histopathological lesions in early phase of viral infection showed that all three rASFV I/II-infected pigs showed endothelial injury lesions, while only one genotype II ASFV-infected pig showed this lesion. High viral DNA copy numbers were detected in all organs, particularly the liver, lymph nodes, spleen, lungs, and kidneys. These findings demonstrated distinct pathobiological features of rASFV I/II that warrant further investigation for their implications on disease control. The results also emphasized the urgent need for enhanced surveillance in Asia, rapid diagnostic methods, and effective vaccines targeting emerging ASFV variants.