Deletion of the H108R Gene Reduces Virulence of the Pandemic Eurasia Strain of African Swine Fever Virus with Surviving Animals Being Protected against Virulent Challenge.

African swine fever virus (ASFV) is the etiological agent of African swine fever (ASF), a devastating disease affecting domestic and wild swine and currently causing a global pandemic, severely affecting swine production. Here, we demonstrate that the deletion of the previously uncharacterized ASFV gene, H108R from the highly virulent ASFV-Georgia2007 (ASFV-G) genome strain, reduces virulence in domestic swine. ASFV-G-ΔH108R, a recombinant virus with the H108R gene deleted, was used to evaluate the involvement of the H108R gene for ASFV replication and virulence in swine. ASFV-G-ΔH108R showed a delayed replication in swine macrophage cultures. A group of five pigs, intramuscularly inoculated with 102 HAD50 of ASFV-G-ΔH108R, was observed over a 28-day period and compared with a similar group of animals inoculated with similar doses of the parental virulent virus. While all animals inoculated with ASFV-G developed an acute fatal disease, ASFV-G-ΔH108R inoculated animals, with the exception of one animal showing a protracted but fatal form of the disease, all survived the infection, remaining clinically healthy during the observational period. The surviving animals presented protracted viremias with lower virus titers compared with those of animals inoculated with the parental virus, and all of them developed a strong virus-specific antibody response. Importantly, all animals surviving ASFV-G-ΔAH108R infection were protected when challenged with the virulent parental strain, ASFV-G. This report constitutes the first evidence that the H108R gene is involved in ASFV virulence in swine and that the deletion of this gene may be used as a tool to increase the attenuation of currently experimental vaccines to improve their safety profiles. IMPORTANCE Currently, there is no commercial vaccine available to prevent ASF. ASFV-Georgia2007 (ASFV-G) and its field isolate derivatives are producing a large pandemic which is drastically affecting pork production in Eurasia. We present here the discovery of a novel virus determinant of virulence, the H108R gene, which, when deleted from the ASFV-G genome, significantly reduces virus virulence in domestic swine. Additionally, animals that survive the inoculation with a recombinant virus harboring a deletion of the H108R gene, ASFV-G-ΔH108R, are protected against a challenge with the virulent parental virus. Although presenting residual virulence, ASFV-G-ΔH108R confers protection even at low doses (102 HAD50), demonstrating its potential to be used as an additional gene deletion to increase the safety profile of the preexisting vaccine candidate.