Kinetics of Viral Genome Distribution in Swine Peripheral Lymphoid Organs Following Oronasal Infection with Attenuated African swine fever virus strains.

African swine fever (ASF) continues to spread across the globe, causing a severe impact on the swine industry. Passive surveillance based on testing dead pigs is one of the most effective methods for early detection of ASF incursions. We have previously shown that the superficial inguinal lymph node (SILN) is a convenient and effective sample type for ASF virus (ASFV) genome detection in pigs succumbed to highly or moderately virulent ASFV infections. In this study, we explored the distribution kinetics of ASFV into SILN and other lymphoid tissues in pigs exposed to moderately virulent ASFV strains (ASFV Estonia 2014 and ASFV Malta'78), oronasally. The ASFV genome was detected in SILNs as early as 2-3 days post-infection (dpi), peaking around 5-9 dpi. The detection of ASFV Estonia 2014 started early, and the pigs succumbed to infection faster compared to the ASFV Malta'78 infected pigs that remained longer. All pigs that succumbed to ASF had comparable levels of ASFV genomic material in the spleen and SILNs. The levels of ASFV genomic material gradually started to decrease in pigs that did not succumb to ASF, indicating possible virus clearance. In contrast, ASFV genome levels in blood and spleen samples remained relatively steady during the study period. Immunohistochemistry and in situ hybridization of spleen and SILN samples supported real-time PCR results. This study demonstrates the distribution kinetics of moderately virulent ASFV in peripheral lymph nodes and highlights the utility of SILNs for dead pig screening.