The application of molecular biology to determine the genomic structure, classification and pathogenesis of chicken disease-causing agents in poultry was a consistent theme of the presentations at the 2008 Southern Conference of Avian Diseases. This meeting in Atlanta formed part of the International Poultry Scientific Forum organised by the US Poultry and Egg Association.
Research has been intensified by the recognition that highly pathogenic avian influenza (HPAI) is endemic in Asia and West Africa. Sporadic cases of both low and high pathogenicity in Europe are an increasing cause of concern. Developing more effective vaccines to reinforce current inactivated products is a high priority of many research teams.
Scientists at the University of Arkansas described a modified Salmonella enteritidis (SE) strain as a vector of two M2e epitope sequences. This vaccine candidate was evaluated both by in-ovo and sub-cutaneous post-hatch administration. Within one day of administration, the SE vector could be isolated from liver, spleen and from caecal tonsils. After 28 days, it could not be isolated from liver and spleen tissues. Hatchability was unaffected by in-ovo administration into the air cell. The modified SE vector stimulated an antibody response detected at seven days but waned through the four-week post-vaccination study period.
Another recombinant AI vaccine was described by workers at Auburn University. They inserted genes from an H10N7 field isolate into Pichia pastoris. Preliminary results suggest this yeast vector may offer opportunities for commercial flocks.
The mode of action of HPAI viruses was investigated in a joint programme conducted at the Southeast Poultry Research Laboratory and Ohio State University. Reverse genetics was applied to evaluate the pathogenicity of a range of recombinant viruses. Single genes from a highly virulent virus isolated from egrets in Hong Kong were substituted in a chicken strain isolated in Indonesia in 2003. Substitution of genes coding for haemagglutinin proteins that are antigenic increased mortality, viral propagation and shedding. Increased virulence was noted with substitution of genes coding for NS, NP and N proteins as compared to the original virus.
Studies conducted at the University of Georgia, USA, established that infectious laryngotracheitis (ILT) strains can be classified into nine categories based on an analysis of four regions of the ILT genome. Biotype IV corresponds to Chick Embryo Origin (CEO) vaccine. Biotypes V and VI represent the majority of field isolates. This observation confirms the prevailing opinion that variants of CEO vaccines are responsible for field cases of LT in broilers in the south-eastern and Delmarva states of the USA. Biotypes VII through IX are mainly derived from backyard flocks and are rarely encountered in commercial broilers. Additional techniques to differentiate biotypes include clinical effects and the propensity to form plaques on avian cell tissue culture.
