An outbreak of infectious bursal disease (IBD) in two California layer flocks resulted in the isolation of two infectious bursal disease viruses designated rA and rB, according to Jackwood and co-authors of a paper in the current issue of Avian Diseases. Increased mortality plus gross and histopathology in the layer flocks suggested rA and rB could be very virulent infectious bursal disease virus (vvIBDV).

Preliminary studies indicated that high mortality resulted when bursa homogenates from the layer farms were used to inoculate specific-pathogen-free (SPF) chicks. In addition, rA and rB contained VP2 amino acid sequences typically seen in vvIBDV.

Molecular and in vivo studies were conducted to more thoroughly identify and characterize the rA and rB viruses.

Nucleotide sequence analysis demonstrated that rA and rB had identical sequences across the hypervariable VP2 (hvVP2) and segment B regions examined, and their amino acid sequences in the hvVP2 region were identical to the vvIBDV type strains UK 661, OKYM and Harbin.

Furthermore, the genome segment B nucleotide sequences examined for rA and rB were a 98.1 per cent match with vvIBDV and only an 88.0 per cent match with classic IBDV strains.

Phylogenetic analysis placed the rA and rB viruses with other vvIBDV and confirmed these viruses were close genetic descendants of vvIBDV seen around the world.

Pathogenicity studies in four-week-old specific pathogen-free (SPF) chicks demonstrated that at a high dose (105.5 50 per cent egg infective dose [EID50]) and a low dose (102.0 EID50) of rA and rB, mortality ranged from 91 to 100 per cent. A pathogenic classic virus, standard challenge (STC), at similar doses did not cause mortality in the SPF chicks. In addition, mortality occurred in three out of four SPF birds exposed by direct contact to rA and rB inoculated chicks.

Serum from convalescent birds inoculated with rA had high titres to IBDV but were negative for antibodies to infectious bronchitis virus, avian influenza virus, chicken anemia virus, Newcastle disease virus, Mycoplasma gallisepticum and Mycoplasma synoviae. Virus isolation attempts on the rA and rB bursa homogenate inocula also indicated that no contaminating microorganisms contributed to the high mortality and pathology observed in the SPF chicks.

In one experiment, broilers with maternal immunity to IBDV were protected from infection and disease when they were challenged with 102 EID50 and 105 EID50 of the STC virus. When challenged with 102 EID50 of the rA virus, the maternally immune broilers were protected from disease but not infection as evidenced by a positive reverse transcription–polymerase chain reaction (RT-PCR) assay for the virus.

When the broilers were challenged with 105 EID50 of the rA virus, they had typical gross and histopathologic signs of IBD but no mortality by seven days post inoculation.

Jackwood and co-authors concluded that the rA and rB viruses meet the genotypic and phenotypic characteristics of a vvIBDV.

Reference

Jackwood D.J., S.E. Sommer-Wagner, S.T. Stoute, P.R. Woolcock, B.M. Crossley, S.K. Hietala and B.R. Charlton. 2009. Characteristics of a very virulent infectious bursal disease virus from California. Avian Diseases 53 (4): 592–600. doi: 10.1637/8957-061109-Reg.1

Further Reading

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Further Reading

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Find out more information on infectious bursal disease (IBD) by clicking here.