Pathogenicity And Transmissibility Of Two Epidemic Strains Of Influenza Virus Isolated From Hebei Province

Flu is short for an acute respiratory disease caused by influenza virus,which occurs every year.The flu is one of the important zoonotic diseases,posing a threat to human health and veterinary public health security.The epidemiological characteristics of influenza in 2018 were different from those in previous years.There were not only influenza B infection,but also influenza A H1N1 infection.The number of influenza B patients was relatively large,but the symptoms were mild.The number of influenza A patients was small,but the symptoms may be more serious.In addition,a small proportion of influenza patients were co-infected with influenza A and B viruses,which had a more acute onset and were easy to develop into severe influenza.To provide theoretical and technical support for the prevention and control of influenza in Hebei Province,the study on the pathogenicity and transmissibility of epidemicstrains of influenza virusisolated from Hebei Provincein 2018 was conducted.In this study,two influenza strains were isolated in Hebei during the influenza epidemic season in 2018,which named A/Hebei/F076/2018(H1N1)(F076)and B/Hebei/16275B/2018(16275B),respectively.Genetic evolutionary analysis showed that F076 strain did not undergo cross-species recombination of influenza viruses.The HAgene and NA gene of F076 strain were both located in swine branch.The genomic fragments of 16275 B strain came from different strains.The HA gene of 16275 B strain was located in B Victoria branch,closest to B/Guangzhou/56/2016.The results of receptor-binding tests showed that F076 strain and 16275 B strain had only a binding capacity of alpha-2,6 sialic acid receptor,and it meant that the two strains have the potential to infect mammalsonly.The replication ability test of influenza virus showed that F076 and 16275 B strains had good proliferation ability on MDCK cells.The results of animal pathogenicity test showed that the mice infected with strain F076 and strain 16275 B showed clinical symptoms.During the experiment,the mortality rate of mice infected with F076 strain was 40%,and that of mice infected with 16275 B strain was 0.The distribution of influenza virus in different tissues of mice showed that the virus could be detected in the heart,liver,spleen,lung,kidney and brain of mice infected with F076 strain on 3 days postinoculation(dPI).F076 strain were detected in the liver,spleen,lungs and kidneys of mice on 5 dPI,and in the lungs of mice on the 7 dPI.The results of heart,liver,spleen,lung,kidney and brain of mice infected with 16275 B strain on 3,5 and 7 dPI were negative.Influenza-related pathological changes were observed in the lungs of mice infected with F076 and 16275 B strains on 3,5 and 7 dPI.Immunohistochemical staining results showed that the influenza virus antigen could be detected from the lungs of mice infected with F076 and 16275 B on 3,5 and 7 dPI.The results showed that both F076 strain and 16275 B strain had the ability to transmit among guinea pigs.The efficiency of direct contact transmissibility of F076 strain among guinea pigs was 66.7%,the transmissibility efficiency of aerosols was 33.3%.The efficiency of direct contact transmissibility of 16275 B strain was 100%,the transmissibility efficiency of aerosols was 33.3%.The results showed that F076 strain and 16275 B strain isolated from Hebei Province in 2018 had only the potential to infect mammals.F076 strain was more pathogenic to mice than 16275 B strain.The direct contact transmissibility efficiency of F076 strain was weaker than 16275 B strain,while the aerosol transmissibility efficiency of F076 strain and 16275 B strain was the same.