Contamination of animal feedingstuffs with undeclared antimicrobial additives

A study was carried out on 397 feedingstuffs and 11 premixes on which 161 (39.5%) were declared free of medication and 247 (60.5%) were medicated. These were subjected to analysis for the presence of antibiotics. Of the 247 medicated feeds 87 (35.2%) contained undeclared antimicrobials of which 59 (23.9%) were at a concentration sufficient to allow quantification by HPLC. Of the 161 unmedicated feeds 71 (44.1%) were shown to contain detectable antimicrobials of which 42 (26.1%) contained concentrations which could be quantified by HPLC. The most frequently identified contaminating antimicrobials were chlortetracycline (CTC) (15.2%), sulphonamides (6.9%), penicillin (3.4%) and ionophores (3.4%). Four samples (ionophores--3, sulphadimidine--1) contained therapeutic concentrations and one sample a supra-therapeutic concentration (monensin). The remainder were sub-therapeutic. All the contaminating concentrations of sulphadimidine detected were sufficient to cause violative tissue residues if fed to animals immediately prior to slaughter.