Simulating the effect of eliminating routine bacterial screening on the negative predictive value of the Ontario fish hatchery disease monitoring program.

We determined the impact of eliminating routine screening for Aeromonas salmonicida and Yersinia ruckeri on the efficacy of the Ontario Ministry of Natural Resources (OMNR) fish disease monitoring program, using Monte Carlo simulation. Because the main purpose of the program is to prevent transferring infected fish among OMNR hatcheries, or to wild fish populations through stocking waterways, the hatchery-level negative predictive value (HNPV) was used as an indicator of monitoring efficacy. The present program (which includes both routine screening of asymptomatic hatchery fish, and diagnostic testing of hatchery mortalities and clinically diseased fish) was confirmed to have a high median HNPV (0.999) for both study pathogens. Simulations suggested that the median probabilities that a hatchery would be pathogen-free if only diagnostic testing were continued (i.e. if no asymptomatic lots were screened), and all diseased lots tested negative for A. salmonicida and Y. ruckeri would be 0.994 for both pathogens (with <5% probability that HNPV would be less than 0.953 and 0.957, respectively) - indicating acceptable monitoring efficacy. However, limitations of the theoretical monitoring model must be considered.