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Monensin, salinomycin and narasin were detectable in six, two and one, respectively, out of 161 eggs surveyed in Northern Ireland in 1994. In all cases, the concentrations detected were less than 2.5 ng/g. Lasalocid was detectable in 107 eggs at concentrations ranging from 0.3 to 129 ng/g. Cross-contamination of unmedicated feeds with monensin during feed manufacture (up to eight batches of unmedicated feed contaminated with monensin) was similar to that previously observed for lasalocid (up to nine batches contaminated). Therefore differences in the incidence in eggs could not be explained by differential carry-over during feed manufacture. In a feeding trial it was shown that the relative ability of monensin, salinomycin and lasalocid to accumulate in eggs was in the ratio 0.12:3.3:63 ng/g egg per mg/kg feed, respectively. This indicated that the potential for monensin and salinomycin to cause residues in eggs was very low, by comparison with lasalocid. In 1995, a granular formulation of the lasalocid premix was introduced into the United Kingdom that decreased the carry-over of this drug from medicated to unmedicated feed. Six months after the introduction of this formulation, the incidence of lasalocid residues in eggs (21%) was lower than that found (66.5%) in an earlier survey (1994) carried out, and published, by this laboratory.

Lasalocid is a coccidiostat licensed for use in poultry, but not for use in egg-laying birds. Lasalocid residues were determined in an egg sample from each of 161 egg producers in Northern Ireland using liquid chromatography-electrospray ionization mass spectrometry, following reports from the Veterinary Medicines Directorate concerning the incidence of lasalocid residues in the United Kingdom. Approximately 66% of the eggs contained lasalocid residues at concentrations in excess of 0.3 ng/g. There was no apparent difference in the incidence of lasalocid residues between free range and battery eggs. Carry-over of lasalocid from medicated to unmedicated batches of both premix and feed, during milling processes, was identified as a possible cause of contamination. Subsequently, egg-laying birds were fed meal containing a range of lasalocid concentrations, similar to those found as a result of unintentional contamination at a feed mill (0.1-5.0 mg/kg). The concentrations of lasalocid, measured in their eggs, were similar to that found in the survey. Lasalocid persisted in eggs for 10 days after withdrawal of medicated feed and replacement with lasalocid-free feed.