African swine fever virus NL gene is not required for virus virulence

Previously, we described a highly conserved non-essential African swine fever virus (ASFV) right variable region gene, NL. Deletion of NL from the European pathogenic isolate E70 resulted in almost complete attenuation of the virus in domestic swine. To study gene function further, NL gene deletion mutants were constructed from two pathogenic African ASFV isolates, Malawi Lil-20/1 (Mal) and Pretoriuskop/96/4 (Pr4). Unexpectedly, both Mal (Mal-DNL) and PR4 (Pr4DNL) null mutants remained highly virulent when inoculated in swine. Mal-DNL exhibited a disease and virulence pheno-type indistinguishable from its revertant, Mal-NLR, which caused 100 % mortality. Mortality among Pr4DNL-infected animals was also high; however