Summary

IBDV is continuing to evolve around the world. Since the emergence of very virulent IBDV (vvIBDV) and antigenic variants in the mid 80’s, genetically distinct genotypes of IBDV have continued to be identified in different countries. To date, 8 genogroups of IBDV (A1 to A8) have been identified worldwide, with Australian strains comprising genogroups A7 and A8. It is therefore crucial that we continue to monitor strains circulating worldwide to ensure that the current tests being used here in Australia are able to rapidly detect all exotic strains of IBDV and differentiate them from endemic IBDV strains. Currently, a single qPCR test is used to detect any circulating IBDV strains in Australia, followed by nucleotide sequencing of the viral VP2 gene to determine the identity of the IBDV isolate. This need to sequence samples to determine the type of IBDV involved limits both the sensitivity and number of isolates that can be sequenced on any given day (currently 48 samples per 8 hr work period). Hence, we aim to develop new qPCR tests which can differentiate all exotic IBDV strains (genogroups A1 to A6) from endemic Australian strains (genogroups A7 and A8) to improve the sensitivity and turnaround time in identifying exotic strains of IBDV. To assist with the design of primers and probes required for qPCR tests, whole genome sequencing of Australian IBDV will also be undertaken. Once developed, these qPCR assays will greatly enhance our ability to quickly differentiate exotic strains from endemic strains in the event of an incursion, thus improving outbreak response times and the chances of eradication.

In Australia, we have been pathotyping field isolates of IBDV using the protocol based on OIE guidelines, which recommends the use of 3-6 week old chickens and a virus dose of 105 50% egg infectious dose (EID50). This high dose was originally recommended to define what constitutes a “vvIBDV phenotype”. Strains producing greater than 50% mortality were classified as vvIBDV. The protocol was not designed to depict the disease that is typically observed in the field. Similarly, it was not designed to detect any pathotypic differences amongst strains that do not induce mortalities in SPF birds. This protocol was chosen to confirm that Australian strains not cause any mortalities in SPF chickens using a dose of 105 EID50. Unfortunately, due to the high dose of virus given, it is often difficult to detect differences amongst the Australian strains which are statistically significant. Variants typically produce slightly greater histopathological changes in the bursa, with average lymphocyte depletion scores reaching the maximum score of 4 (on a scale of 1-4) compared to classical strains which average scores closer to 3. Recently however, we have been observing more classical strains reaching lymphocyte depletion scores approaching 4. Hence our scale of comparison has effectively reached its peak. The significance of these high lymphocyte depletion scores observed in the bursa in terms of flock performance is uncertain as we have never tested the immunosuppression of any Australian strains.

Although Australian strains induce no mortalities in SPF chickens using the protocol recommend by the OIE above, Australian variants do typically induce mild to moderate clinical signs in chickens. We believe the appearance of clinical signs is dose dependent. We recently tested a Victorian variant at a lower dose of 104 EID50 in SPF chickens and observed no clinical signs (AgriFutures report PRJ-010978). Several studies have reported that overseas strains of IBDV (genogroups A2, A4 and A6) induce no clinical signs in SPF chickens, hence Australian variants may be perceived to be more virulent in comparison. Laboratories in other countries often utilize different parameters when assessing the pathotype of IBDV strains which makes it difficult to directly compare the pathotype of Australian strains to that of overseas strains. A more standardized approach to pathotyping of different IBDV strains worldwide was recently published in 2020 by Saif and Fletcher (Avian Diseases 64, pp 241-242) which recommends using 2-week-old SPF chickens and a lower dose of virus containing 102 EID50. The authors claim that using higher doses of virus produces “exaggerated unrealistic results”. They also point out that a dose of 102 EID50 gives a better representation of the disease observed in the field and is the challenge dose of IBDV used to ascertain the effectiveness of different IBDV vaccines. As part of this new protocol, they also recommend that the immunosuppressive potential of strains be tested by measuring the antibody response to Newcastle Disease Virus (NDV) in SPF chickens previously challenged with IBDV. We propose to evaluate this new model developed by Saif and Fletcher (2020) using a selection of both Australian and overseas strains, thus confirming any immunosuppressive or pathotypic differences. This will help strengthen Australia’s argument that’s overseas strains of IBDV are more immunosuppressive and/or virulent than local endemic strains, thus justifying current quarantine laws which stipulate that all chicken meat entering Australia is heat treated at 80oC for 125 mins (or equivalent time and temperature) to inactivate exotic IBDV.

Program

Chicken Meat

Research Organisation

Commonwealth Scientific and Industrial Research Organisation (CSIRO)