Virus strains from a flock exhibiting unusually high mortality due to infectious bursal disease

To characterise infectious bursal disease viruses (IBDVs) isolated from commercial broiler flocks exhibiting unusually high mortality due to infectious bursal disease (IBD).

An IBD outbreak occurred in mid 1999 on two broilers farms (A and B) in northern New South Wales amongst chickens 28 to 38 days of age, with a sharp rise in mortality of 2.5%. Initial histopathological diagnosis indicated acute IBD. Since acute IBD caused by classical pathogenic and very virulent (vv) IBDVs is exotic to Australia, samples from both farms A and B were obtained and used for virus characterisation.

Tissue homogenates were made from six bursae collected from farm B. One histological sample from farm A was also used. Nucleotide sequencing of the hypervariable region (HVR) within the VP2 gene of IBDVs was determined and the deduced amino acid sequences compared with previously characterised Australian and overseas IBDVs. The phylogenetic relationship between IBDVs from farm B and IBDVs from Australia and overseas was then determined. Pathogenicity of one isolate, N2/99 from farm B, was compared with 3 other local IBDVs, as well as with three pathogenic overseas strains in 3-week-old specific pathogen-free (SPF) chickens.

Initial histopathological characterisation of a sample of bursa from a bird on farm A showed widespread acute lymphoid necrosis, follicular haemorrhage and stromal oedema, indicative of acute IBD. Subsequent analysis using reverse transcriptase polymerase chain reaction (RT-PCR), followed by nucleotide sequencing of the same bursal sample, as well as 6 samples from nearby farm B, showed that the IBDVs involved were similar in sequence to Australian vaccine strains and not to classical pathogenic or vvIBDVs. One isolate, N2/99 from farm B, was only marginally more pathogenic than other local IBDVs. It induced mild clinical signs in 30% of chicks and no mortality. In comparison, vvIBDV CS89 and classical pathogenic 52/70 strains induced severe clinical signs in 100% and 80% of chickens, respectively with mortalities of 27% and 12%, respectively.

The results illustrated the value of nucleotide sequencing as a method for discrimination of local and exotic types of IBDV.