Pathobiological characterization and estimation of the basic reproduction number of a recombinant african swine fever virus in contact-exposed pigs in Vietnam.

African swine fever virus (ASFV) causes a highly lethal disease in domestic pigs, resulting in severe economic losses. This study examined the clinical course, viral dynamics, immune response, and pathological changes in pigs infected with a recombinant ASFV strain of p72 genotypes I/II (rASFV I/II) via direct inoculation or natural contact. The directly infected pig developed persistent fever (> 40 °C), anorexia, lethargy, and respiratory distress soon after the onset of viremia, and died six days post-inoculation (dpi). Contact-exposed pigs showed similar clinical signs and died between 12 and 21 days post-exposure (dpe). Real-time PCR revealed rapid viral replication in blood and secretions, with peak viral loads detected at or just before death. No detectable antibody response was observed in either group, indicating acute disease progression and high virulence. Gross pathological examination revealed severe lesions in the spleen, lymph nodes, gastrointestinal tract, and lungs. Epidemiological analysis using the susceptible-infectious-removed (SIR) model estimated the generation time (mean ± SD) as 1.101 ± 0.507 days. The basic reproduction number (R₀) was 1.201 (95% CI: 0.628-2.302) using the exponential growth method and 1.248 (95% CI: 0.310-3.236) using the maximum likelihood method. These findings demonstrate the high virulence and transmission potential of the rASFV I/II strain, highlighting the need for early detection and rapid containment measures.