AFRICAN SWINE FEVER VIRUS NL GENE IS NOT REQUIRED FOR VIRUS VIRULENCE

Previously, we described a highly conserved nonessential African swine fever virus (ASFV) right variable region gene, NL, Deletion of NL fro m the European pathogenic isolate E70 resulted in almost complete atte nuation of the virus in domestic swine. To study gene function further , NL gene deletion mutants were constructed from two pathogenic Africa n ASFV isolates, Malawi Lil-20/1 (Mal) and Pretoriuskop/96/4 (Pr4). Un expectedly, both Mal (Mal-Delta NL) and PR4 (Pr4 Delta NL) null mutant s remained highly virulent when inoculated in swine. Mal-Delta NL exhi bited a disease and virulence phenotype indistinguishable from its rev ertant, Mal-NLR, which caused 100% mortality. Mortality among Pr4 Delt a NL-infected animals was also high; however, a significant delay in o nset of fever and viraemia and in time to death was observed. These da ta indicate that NL gene function is not required for ASFV virulence a nd that other yet-to-be identified viral determinants perform signific ant virulence functions in these African field isolates.