Highly pathogenic avian influenza (HPAI) is a serious disease affecting multiple organ systems and resulting in high levels of mortality in domestic poultry and may also be a serious zoonotic condition. In July-August 2020, HPAI was confirmed on 3 egg-laying chicken farms in Victoria, Australia, while a further two turkey farms and one emu farm were diagnosed with low pathogenicity viruses. All six farms were depopulated and decontaminated by 26 November 2020 and Australia declared regained freedom from HPAI on 26 February 2021. As part of the follow-up surveillance in support of claiming HPAI freedom, a scenario-tree model was developed to estimate the population sensitivity of passive surveillance for the detection of HPAI in the Victorian commercial poultry industry, and to also estimate the confidence of freedom from HPAI provided by passive surveillance, predicted over a 2-year period. Risk factors included in the model were industry sector (breeder, broiler, layer and other), flock size: small commercial (50 ≤ 5000 birds) or large commercial (> 5000 birds) and housing type (cage, barn or free-range). A detection cascade was also modelled, with probabilities allocated, to estimate the flock sensitivity for flocks in each risk stratum. System sensitivity and confidence of freedom were then estimated across all flocks in the industry. Design prevalence was set at 1, 2, 5 or 10 infected flocks and prior confidence of freedom at 0.5. Other model inputs were entered as probability distributions and the model was simulated for 10,000 iterations. Outputs were expressed as median and 95% probability intervals (PI), with the time period for analysis set at 1 month. Median system sensitivity was 0.58 (95% PI: 0.47-0.69) per time period for a single infected flock, increasing to 0.81, 0.985 and 0.9998 for 2, 5 and 10 infected flocks respectively. Median confidence of freedom was > 0.7 (95% PI: 0.65-0.76) after one time period and exceeded 0.95 and 0.99 after 4 and 7 months, respectively for one infected flock and 2 and 3 months respectively for 2 infected flocks. These results support the conclusion that passive surveillance is a highly effective tool for the detection of HPAI in commercial poultry and add further weight to evidence that HPAI has been successfully eradicated from the Victorian poultry industry and that the industry has regained HPAI free status.
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