Case report: an investigation of chlortetracycline and oxytetracycline residues in suspect swine slaughtered in Manitoba, Canada, October 1987 to March 1988.

Tissue samples from 623 hogs suspected of having received antibiotic treatment were collected at federally-inspected abattoirs in Manitoba and submitted for analysis from October 1987 to March 1988. Initially, samples were screened by a multi-residue thin layer chromatography/bio-autography method, but, during the final two months of the investigation, analysis was restricted to a quantitative high performance liquid chromatography method for tetracycline antibiotics only. Sixteen animals (3%) were found with detectable tissue levels of chlortetracycline ranging from 0.10 to 1.38 ppm. One hundred thirty-eight animals (22%) were found with detectable tissue levels of oxytetracycline ranging from 0.05 to 2.07 ppm. Comparison of these levels with previously published values suggested that some producers were administering tetracyclines at therapeutic levels without observing the seven-day withdrawal period. The data generated by this study provided information for use by regulatory officials at Health and Welfare Canada in establishing specific tolerance levels for residues of the tetracycline antibiotics in animal tissue; previously, a zero tolerance had been in effect. During the study, only one hog had tetracycline residues in muscle which would have exceeded the current tolerance levels.