Differential Effect of the Deletion of African Swine Fever Virus Virulence-Associated Genes in the Induction of Attenuation of the Highly Virulent Georgia Strain

African swine fever virus (ASFV) is the etiological agent of an often lethal disease of domestic pigs, African swine fever (ASF). The ASFV Georgia 2007 isolate (ASFV-G) is responsible for the current epidemic situation in Europe and Asia. Genetically modified ASFVs containing deletions of virulence-associated genes have produced attenuated phenotypes and induced protective immunity in swine. Here we describe the differential behavior of two viral genes, NL (DP71L) and UK (DP96R), both originally described as being involved in virus virulence. Deletion of either of these genes efficiently attenuated ASFV strain E70. We demonstrated that deletion of the UK gene from the ASFV-G genome did not decrease virulence when compared to the parental virus. Conversely, deletion of the NL gene produced a heterogeneous response, with early death in one of the animals and transient fever in the other animals. With this knowledge, we attempted to increase the safety profile of the previously reported experimental vaccine ASFV-GΔ9GL/ΔUK by deleting the NL gene. A triple gene-deletion virus was produced, ASFV-GΔ9GL/ΔNL/ΔUK. Although ASFV-GΔ9GL/ΔNL/ΔUK replicated in primary cell cultures of swine macrophages, it demonstrated a severe replication deficiency in pigs, failing to induce protection against challenge with parental ASFV-G.