First detection and characterization of a non-haemadsorbing African swine fever virus (genotype II) isolated from a wild boar in Russia.

In 2023, a non-haemadsorbing African swine fever virus (ASFV) of genotype II (isolate ASFV/Kirov/WB2023/1448) was recovered from a wild boar in the Kirov Region of Russia-the first such case during the current epidemic (2007-present). Whole-genome analysis of the non-haemadsorbing ASFV revealed a large deletion (~1,300 base pairs) in the EP153R and EP402R genes, and a smaller deletion (~50 base pairs) in the MGF 110-1 L gene. Phylogenetic analysis confirmed that ASFV/Kirov/WB2023/1448 belongs to the genetic lineage «II/EU1/RU7», which is typical for the European part of Russia. Following intramuscular inoculation of eight domestic pigs with isolate ASFV/Kirov/WB2023/1448 at a dose of 103 TCID50 moderate virulence was observed, with the development of acute and subacute forms of ASF. Furthermore, the non-haemadsorbing virus was shown to be contagious, as two pigs maintained in direct contact with the inoculated animals developed ASF. The mortality rate among pigs infected via the intramuscular and contact routes was 60%. Two out of ten infected pigs recovered from ASF; persistence of the non-haemadsorbing virus was not detected, as complete elimination of the virus from the host organism was observed within 28 days post-infection. Upon challenge of the recovered pigs with the highly virulent haemadsorbing isolate ASFV/Kirov/WB2023/1447-which had been recovered from the same wild boar population as ASFV/Kirov/WB2023/1448-the animals exhibited no clinical signs of disease, viraemia, virus shedding, or virus dissemination in organs and tissues. To investigate the possible circulation of the non-haemadsorbing virus in wildlife, 122 field samples collected from hunted and found-dead wild boars within a single population in Kirov Region and adjacent regions of the Russian Federation during the period 2021-2023 were examined. Two hunted animals sampled in 2023 in the Republic of Mari El (bordering Kirov Region) were found to be seropositive with no detectable ASFV. The present study may indicate the emergence of natural mechanisms of virulence attenuation of ASFV genotype II in the European wild boar population over a relatively short period of pathogen circulation (15-16 years).