Study on antigenic relationships and biological properties of swine influenza A/H1N1 virus strains isolated in Northern Kazakhstan in 2018

Swine influenza is a highly contagious acute disease characterized by pronounced fever, general weakness, and disorders of the respiratory system. Swine influenza virus can cause disease in humans and, on the contrary, swine may be infected by human influenza virus. In the pig’s organism, simultaneously infected with viruses of different origin, genetic reassortment takes place with the risk of occurrence of new dangerous highly pathogenic strains. The study of influenza viruses circulating in the pig population therefore plays an important role in preventing the development of dangerous outbreaks of the disease and planning preventive measures. In this work we studied the characteristics of the newly isolated strains of swine influenza virus which are epizootically relevant in the specified region at the present time. Our purpose was to identify the biological and antigenic characteristics of strains of swine influenza A/H1N1virus, circulating in the North Kazakhstan oblast of the Republic of Kazakhstan in 2018. Influenza A/H1N1 virus strains were studied including those isolated from pigs in pig farms of the North Kazakhstan oblast, the A/swine/Petropavlovsk/01/18, A/swine/Petropavlovsk/02/18, and A/swine/Petropavlovsk/03/18, and also the reference strains A/swine/Iowa/15/30, A/swine/USA/1976/31, and A/California/04/09 pdm. The strains were cloned in 10-day-old developing chicken embryo systems. The antigenic properties of surface glycoproteins of the strains were examined by cross-reactivity hemagglutination inhibition assay with rabbit immune sera. Infectivity was determined in chicken embryos (CE) and MDCK cell culture. The adsorption properties were studied on formalinized chicken red blood cells under constant stirring at 4 °С for 18 hours. Elution from red blood cells was determined after 30, 60, 120, 180, and 240 min in buffered saline at 37 °С. The heat sensitivity of hemagglutinin was assessed by the ability to agglutinate red blood cells after heating at 56 °С for 5, 10, 15, 30, and 60 minutes. The hemagglutinating activity of strains was assayed using 0.75 % suspensions of chicken, guinea pig, ram, horse, and blood group I(0) individual erythrocytes. The susceptibility of isolates to nonspecific inhibitors was determined in the hemagglutination inhibition assay with native and heated (30 min at 62 °С and 10 min at 100 °С) blood sera of guinea pig, chicken, and rabbit. The susceptibility of virus strains to different concentrations of antiviral drugs was evaluated by the level of reproductive suppression of lg 100 EID50/0.2 ml of virus in CE. The antigenic relationship of the examined variants of influenza A/H1N1 virus between each other and with the reference strains A/swine/USA/1976/31 and A/swine/Iowa/15/30 was revealed as well as their difference from the strain А/California/04/09 pdm. The studied strains in high titers agglutinated all types of red blood cells taken in the experiment. The infectious activity of swine influenza virus strains ranged within 6.5-7.9 lg EID50/0.2 ml in chicken embryos, and 3.5-4.3 lg TCD50/0.2 ml in MDCK cell culture. After heating at 56 °С, all strains agglutinated chicken erythrocytes in high titers (log2 = 6.3±0.6-9.6±0.8) and were characterized as thermostable. The isolated strains possessed good adsorption ability against chicken erythrocytes (90-100 %) and eluted from them after 30-60 min incubation at 37 °С. The strains revealed inhibitory resistance with native nonspecific sera and were suppressed by inhibitors of the heated sers only. The studied strains proved to be susceptible to Tamiflu and Remantadine (the inhibitory concentrations were 5.6-6.6 and 3.7-12.7 μg/ml, respectively). Viruses exhibited resistance to the drugs Arbidol and Ingavirin. Thus, the study revealed similarity of isolated and reference A/H1N1 strains in thermostability of the hemagglutinin, adsorption rate and susceptibility to antiviral drugs, as well as differences in infectious activity and the rate of elution from chicken red blood cells.