Release of fragmented host, cell-free, genomic DNA into the circulation of pigs during infection by virulent African swine fever virus

Abstract

African swine fever virus (ASFV) causes a severe hemorrhagic disease in domestic pigs resulting in high case fatality rates. The virus replicates in circulating cells of the monocyte-macrophage lineage and within lymphoid tissues, e.g. tonsils, spleen and lymph nodes. The infection results in high fever and a variety of clinical signs from about 3 days post infection. In this study, it was observed that one of many changes resulting from ASFV-infection within pigs was a large (>1000-fold) increase in the level of circulating cell-free DNA (cfDNA), including the beta-actin gene, derived from the nuclei of host cells, in the serum. This change occurred in parallel with the increase in circulating ASFV DNA. In addition, elevated levels (about 30-fold higher) of host mitochondrial DNA (mtDNA) were detected in serum from ASFV-infected pigs, but with a much higher baseline level of mtDNA in sera from uninfected pigs. The host derived cfDNA is derived from dead cells which may, or may not, have been infected. For comparison, the release of the cellular enzyme, lactate dehydrogenase (LDH), a commonly used marker of cellular damage, was also found to be elevated during the infection. The cfDNA is readily detected in serum and is a more sensitive host marker of ASFV infection than the release of mtDNA or LDH. In addition, sera from pigs infected by classical swine fever virus (CSFV), which causes a clinically similar disease as ASFV, were also tested but this infection did not result in the release of cfDNA, mtDNA or LDH.