2012 Annual Report
1a.Objectives (from AD-416):
Characterize avian influenza viruses circulating in Vietnam, select representative isolates, and perform vaccine efficacy studies with these isolates. The goal is to provide scientific data to allow the determination of the best vaccine and vaccination protocols to be used as part of the vaccine program to be used in the country. The specific objectives are:.
1)Sequence analysis of recent avian influenza (AI) viruses from Vietnam to identify what is circulating in the country;.
2)Antigenic analysis of selected viruses to provide information for vaccine selection;.
3)Vaccine efficacy studies using available commercial vaccines with representative strains to identify vaccines that provide clinical protection and reduce viral shedding to the greatest degree in chickens and ducks.;.
4)Studies evaluating the effects of maternal immunity on vaccination. This is an important issue, especially in ducks, since vaccination at the hatchery would be an ideal solution for increasing vaccination of all birds, as opposed to the twice yearly campaigns that leave large gaps in coverage.
1b.Approach (from AD-416):
Representative isolates will be sent from the National Center of Veterinary Diagnosis in Hanoi. The viruses will be fully sequenced and based on the phylogenetic analysis, isolates will be selected for biological and antigenic comparison. The hemagglutinin gene will be cloned and placed in either deoxyribonucleic acid (DNA) vaccine vectors or in a reverse genetics constructed virus and inoculated in chickens to get immune serum that will be compared with H5 standards by hemagglutination inhibition analysis. This information will provide a prediction of antigenic differences among the Vietnamese isolates. Additionally, selected isolates will be used for biological assessment in ducks for pathogenic potential. Using the sequence, antigenic and biological information, two or more isolates will be selected as challenge strains for vaccine efficacy studies. Available commercial vaccines and autogenous vaccines made from the challenge viruses will be used to vaccinate both chickens and ducks. After 3 weeks the antibody response will be measured and the birds challenged with the Vietnamese highly pathogenic avian influenza viruses. The vaccine efficacy will be evaluated by measuring morbidity, mortality, and virus shedding. The serologic response before and after challenge will also be evaluated using both homologous and heterologous antigen to evaluate the immune response in conjunction with the challenge data. The effect of maternal immunity on the response to vaccination in chickens and ducks will be evaluated by passive antibody transfer studies.
This project is related to objective 1 and 5 of the in-house project: Characterize variant and emerging avian influenza viruses in lilve poultry markets and commercial production systems; 5. develop new vaccine platforms designed to control and prevent avian influenza virus outbreaks.
The pathogenicity of two recent H5N1 highly pathogenic avian influenza (HPAI) viruses circulating in Vietnam was evaluated in domestic ducks. One of the viruses, A/duck/Vietnam/NCVD-672/2011 (clade 2.3.2B), was highly virulent for ducks but the other virus, A/chicken/Vietnam/NCVD-675/2011 (clade 2.3.2A) was moderately pathogenic, with some ducks surviving infection. Vaccine failures have been reported in Vietnam when attempting to protect poultry against infection with these HA clade 2.3.2 viruses. The protection conferred by two commercial inactivated vaccines currently used in Vietnam (Re-1 and Re-5) to protect ducks against the HA clade 2.3.2 viruses was also evaluated. Both vaccines protected ducks against mortality for one of the viruses (HA clade 2.3.2A), but the Re-1 vaccine did not protect ducks against the HA clade 2.3.2B virus. All ducks vaccinated with either one of the vaccines shed virus for more than eight days after infection, indicating that the vaccines are not protecting ducks adequately against the HA clade 2.3.2 H5N1 viruses. This is a serious concern for the control of H5N1 in Vietnam considering the important role of domestic ducks in the epidemiology of H5N1 HPAI.