Principal investigator: Sophie St-Hilaire
Co-Investigators: Sandra McConkey & Derek Price

When bacterial disease occurs on a fish farm the entire population (>1 million) is usually treated. The logistics of treating this many animals are complicated, and the labels for many licensed antibiotics do not account for the complexity of treating large populations. Successful treatment becomes even more complicated when fish have chronic diseases and stop feeding, as the antibiotics are administered in the feed. The responses of fish to treatment vary, possibly due to decreased exposure to the antimicrobial in non-feeding fish.

We evaluated the concentration and the probability of finding tissue antibiotic concentrations above the MIC90 (Minimum Inhibitory Concentration) for the two most commonly used antibiotics in aquaculture, florfenicol and oxytetracycline. We found that the proportion of fish with tissue concentrations above the MIC90 varied, depending on the antibiotic product, species, day of the sample collection, prescribed dose, and size of fish. The proportion of fish above the MIC90 was lower in fish treated with florfenicol than in fish treated with oxytetracycline. We modeled the probability of tissue concentrations above MIC90 when fish were treated with florfenicol, and our model suggested lower probabilities of having concentrations above MIC90 1) in Atlantic salmon than in rainbow trout; 2) when samples were collected 14 days, rather than 7 days, after the treatment started; and 3) in small fish, compared with average and large fish.

Although we used Piscirickettsiosis (SRS) as our model disease, our findings are applicable to other chronic diseases in the Canadian aquaculture industry, such as bacterial kidney disease.