Cognitive bias in veterinary clinical decision-making: Antemortem misdiagnosis of suspected giant kidney worm (Dioctophyme renale) in Australia.

A 10-year-old male neutered Rottweiler was referred for evaluation of suspected Dioctophyme renale infection following haematuria, lethargy, coughing and passage of a worm-like structure in the urine. The presumptive diagnosis was strongly influenced by the referring veterinarian's prior experience with D. renale in an endemic region. Diagnostic imaging identified left renal abnormalities including hydronephrosis, a dilated ureter containing a tubular intraluminal structure and multiple tubular structures within the urinary bladder; these observations further reinforced the initial clinical impression. However, several key findings, including the absence of ova on urinalysis and the presence of a discrete renal mass with disseminated nodules, were inconsistent with dioctophymosis; 'these were underweighted in clinical reasoning. Rapid clinical deterioration and evidence of extensive multisystemic disease led to euthanasia. Postmortem examination revealed metastatic renal haemangiosarcoma (epithelioid variant), and the presumed nematodes' were confirmed to be ureteral blood casts. This case illustrates how cognitive biases can influence diagnostic reasoning, particularly in unfamiliar epidemiological contexts. It highlights the importance of systematic reassessment of differential diagnosis, integration of local disease prevalence and critical appraisal of initial impressions to mitigate diagnostic error in veterinary practice.