Biological properties of African swine fever virus ASFV/Kaliningrad 17/WB-13869

African swine fever (ASF) is a contagious viral disease of domestic pigs and wild boars of all ages and breeds. To date, the infection is endemic in many European and Asian countries including the Russian Federation. Previously ASF virus isolated and studied by Russian scientists were characterized as highly virulent, with 100 % mortality and assigned to genotype II. However, data on the detection of ASFV with reduced virulence and mortality were later reported, thus further analysis of modern isolates is of high importance. In this work, for the first time, we report the biological properties of the ASF virus (named ASFV/Kaliningrad 17/WB-13869) isolated on the territory of the Kaliningrad region of the Russian Federation. The bioassay was carried out on six large white breed pigs ( Sus scrofa domesticus L.) weighing 15-20 kg, the experiment was performed in a BSL-3 animal facility at the Federal Center for Animal Health (FGBU ARRIAH). Pigs Nos. 3-6 were infected intramuscularly with ASFV/Kaliningrad 17/WB-13869 (genotype II serotype 8) that was isolated from the tubular bone of a wild boar carcass (Bagrationovskiy District, Kaliningrad Province) at a dose of 10 HAD/head. Two uninfected pigs (Nos. 1, 2) were kept in-contact with the infected ones. Clinical signs and body temperature of experimental animals were registered daily. The presence and severity of clinical signs and pathological changes were expressed quantitatively (the sum of scores for a number of indicators). Clinical score was based on assessment of body temperature, weight, behavior, appetite and water consumption, the state of the digestive and respiratory systems, skin and mucous membranes, the presence of nasal discharge and vomiting. Pathological anatomical autopsy assessed changes in the organs of the spleen, kidneys, liver, lung, submandibular and mesenteric lymph nodes. Points were assigned on a scale of severity of recorded signs from 1 to 3 (the most severe). Blood sampling (5.0 cm3 from each animal) was carried out until the death of pigs on the 0th, 3rd, 6th, 10th, 13th and 19th days after the start of the experiment. Samples taken from dead animals included; spleen, kidneys, liver, lung, submandibular and mesenteric lymph nodes. Samples and blood were used to prepare 10 % suspensions in sterile saline using an automatic homogenizer, then centrifuged at 400 g (Sigma Laborzentrifugen GmbH, Germany) for 2 min. The resulting supernatant was used for DNA extraction. Blood serum samples were tested for the presence of Anti-ASFV antibodies using ELISA test systems INgezim PPA Compac (Ingenasa, Spain) and ID Screen (IDvet, France), and immunoperoxidase method (IPM). Real-time PCR detected ASFV genome starting from the 3rd day after infection, while IPM detected anti-ASFV antibodies 1-2 days before the death of infected animals, no anti-ASFV antibodies were detected in the serum of animals by ELISA test systems throughout the experiment. Maximum clinical score was registered in animals with a sub-acute form of the disease (21 and 35 points, respectively), while minimum with hyper-acute disease form (6 and 8 points). As a result, ASFV/Kaliningrad 17/WB-13869 virus isolate was characterized as highly virulent, capable of causing ASF in pigs in forms from hyperacute to subacute with mortality up to 100 % of infected and contact animals, which is similar to the clinical picture caused by ASF virus isolates from the Russian Federation in 2007-2018.