Objective—To develop epidemiologic cutoff values by use of frequency distributions for susceptibility to 4 antimicrobial agents when tested against a representative population of a major aquaculture pathogen, Aeromonas salmonicida.
Sample Population—217 typical and atypical A salmonicida isolates obtained from 20 states and 12 countries.
Procedures—Species identification of A salmonicida isolates was confirmed by detection of specific nucleotide sequences by use of a PCR assay. Minimal inhibitory concentration (MIC) and diameter of the zone of inhibition for oxytetracycline, ormetoprim-sulfadimethoxine, oxolinic acid, and florfenicol were determined for each isolate in accordance with standardized antimicrobial susceptibility testing methods that have been approved by the Clinical and Laboratory Standards Institute for bacterial isolates from aquatic animals. Susceptibility data were tabulated in a scattergram and analyzed by use of error rate bounding.
Results—Susceptibility tests for oxytetracycline, ormetoprim-sulfadimethoxine, and oxolinic acid revealed 2 distinct populations of bacteria. Isolates tested against florfenicol clustered into a single population. Oxolinic acid susceptibility data revealed higher MICs in the non–United States A salmonicida isolates. Slow-growing (atypical) A salmonicida isolates were generally more susceptible than typical isolates for all antimicrobials, except oxolinic acid.
Conclusions and Clinical Relevance—Use of frequency distributions of susceptibility results to develop epidemiologic cutoff values appears to be applicable to aquatic isolates. Frequency distributions of susceptibility results for A salmonicida revealed clear divisions between isolate susceptibilities. This type of data, considered in conjunction with pharmacokinetic and efficacy data, may be useful for developing clinical breakpoints for use in aquaculture.
Objective—To determine whether renal crystals can be experimentally induced in animals fed melamine or the related triazine compound cyanuric acid, separately or in combination, and to compare experimentally induced crystals with those from a cat with triazine-related renal failure.
Animals—75 fish (21 tilapia, 24 rainbow trout, 15 channel catfish, and 15 Atlantic salmon), 4 pigs, and 1 cat that was euthanatized because of renal failure.
Procedures—Fish and pigs were fed a target dosage of melamine (400 mg/kg), cyanuric acid (400 mg/kg), or melamine and cyanuric acid (400 mg of each compound/kg) daily for 3 days and were euthanatized 1, 3, 6, 10, or 14 days after administration ceased. Fresh, frozen, and formalin-fixed kidneys were examined for crystals. Edible tissues were collected for residue analysis. Crystals were examined for composition via Raman spectroscopy and hydrophilic-interaction liquid chromatography–tandem mass spectrometry.
Results—All animals fed the combination of melamine and cyanuric acid developed goldbrown renal crystals arranged in radial spheres (spherulites), similar to those detected in the cat. Spectral analyses of crystals from the cat, pigs, and fish were consistent with melamine-cyanurate complex crystals. Melamine and cyanuric acid residues were identified in edible tissues of fish.
Conclusions and Clinical Relevance—Although melamine and cyanuric acid appeared to have low toxicity when administered separately, they induced extensive renal crystal formation when administered together. The subsequent renal failure may be similar to acute uric acid nephropathy in humans, in which crystal spherulites obstruct renal tubules.