Excerpted from the thesis of PhD student Nguyen Đang Tho
Full name of PhD Student: Nguyen Dang Tho
Title of thesis: “Study on some biological characteristics of A/H5N6 virus in Vietnam”
Major: Veterinary Parasitology and Microbiology
Code number: 964 01 04
Supervisors:
1. Associate Professor, PhD. To Long Thanh 2. Associate Professor, PhD. Nguyen Viet Khong
Educational institution: National Institute of Veterinary Research
Content
Vietnam has a total poultry population of about 300 million poultry per year. Due to the characteristics of animal husbandry, avian influenza outbreaks has continuously occured since 2003 and caused much damage to poultry and public health. The prevention of avian influenza is very difficult because the avian influenza virus is constantly changing and has many sububtypes. Before 2014, avian influenza outbreaks were caused only by the H5N1 subtype virus. However, in 2014, H5N6 bird flu has occured for the first time and caused disease in chickens in Vietnam, with a death rate of over 50%. The new strain of avian influenza has posed many problems for disease prevention for managers, veterinarians and farmers. Is H5N6 virus different and different from H5N1 virus? Is H5N6 virus originating in Vietnam or originating from another country? Evolution of H5N6 influenza virus in Vietnam? Pathogenic properties and antigenic properties of H5N6 virus?
In this study, H5N6 virus samples were collected from outbreaks throughout the country in the period 2014-2017 to analyze genome sequencing, genealogy, clade classification, evolutionary research, and molecular characterization. Pathogenic properties include pathogenic index, disease duration, pathological symptoms, duration of viral excretion, viral distribution in tissues, oropharynx and cloacal are studied through induced challenging experiment on chickens. Antigenic properties of viruses are studied by comparing HI titres, r antigen relation values, antigen maps and vaccine evaluations.
Analysis results showed that H5N6 Vietnam influenza virus belongs to clade 2.3.4.4 and can be divided into 2 subclades 2.3.4.4a and 2.3.4.4b. These two subclades correspond to H5N6 Sichuan and Jiangxi lineages in China. H5N6 virus arises due to recombination of H5N2, H5N6 and H6N6 influenza viruses. For time, H5N6 2.3.4.4a viruses no longer appeared after 2007 but 2.3.4.4b viruses continue to evolve new branches. H5N6 influenza virus causes chicken disease similar to H5N1 viruses but manifests more slowly with symptoms that focus on respiratory and digestive tract. Chickens infected with H5N6 virus excrete virus arounf 24 hours post infection until death. Virus is distributed in all organs of infected chickens, especially liver, spleen, brain, lungs, and intestines. In terms of antigens, the H5N6 clade 2.3.4.4 viruses have very close antigen relations together but with viruses H5N1 clade 1 and 2.3.2.1c, antigenic relationship is significantly reduced and even to 0. Analysis of catography map shows that viruses belong to all three clades (including NIBR-14, Re-5, Re-6 vaccines) that form groups located saparately with distance of 2-4 antigen units. However, NIBR-14 and Re-5 vaccines still partially protect chickens against H5N6 viruses. But this incomplete protection will potentially increase the risk of new strains of influenza virus.
The research topic has contributed to making a deeper understanding of H5N6 avian influenza virus in Vietnam. The emergence of H5N6 virus is caused by the exchange of genes between the highly pathogenic avian inflluenza viruses of H5 Goose/Guangdong/96 lineages that have been around for years with low pathogenic influenza viruses. Control and prevention stragtergies against H5N6 viruses can be applied similarly to H5N1 viruses. However, H5N6 influenza surveillance programs need to be strengthened because their H5 (clade 2.3.4.4) gene is easily incorporated into the genes of other influenza viruses to create new strains. Using antigen maps to monitor antigenic changes of influenza viruses is essential to quickly update the virus strain as a diagnostic antigen in surveillance of bird flu as well as the selection of appropriate vaccines. Virus H5N6 clade 2.3.4.4 should be used for vaccine to improve protective efficiene.
