Analysis of veterinary residues in foods

Abstract

Increasing world population, and particularly the urbanisation of that population, has meant that efforts are constantly being made to increase food production. This enhanced productivity, especially using intensive rearing practices as in poultry, eggs and veal production, has been accompanied by, and in many cases made possible by, the use of chemicals (Fig. 1). These may take the form of drugs to prevent or cure disease, feed additives or chemical implantation for growth promotion, tranquillisers to reduce stress before slaughter and oestrous-synchronising agents to control breeding or milk production. Consequently, worries have arisen about chemical residues in food and their possible toxic effects. The problem was addressed as early as 1969 in the U.K. with the publication of the Swann report’ in which the use of antibiotics in food production was investigated following fears of bacterial resistance building up in the animal’s gut flora possibly passing antibiotic resistance onto bacteria either indigenous to man or, perhaps more importantly, pathogenic to man. Also, antibiotic residues present in animal products could result in hypersensitivity in some people, and the development of a reservoir of antibiotic resistance in man’s natural bacterial flora. Many countries now have laws governing the use of veterinary drugs and drug residues in foods. Regulations vary from country to country reflecting differences in disease patterns and animal husbandry practices. A recent report2 of a joint FAO/WHO expert consultation on veterinary drug residues stresses the need for the establishment of internationally agreed maximum residue limits (MRLs) so that food products are not excluded from world markets. Commodities for which MRLs are needed must be identified and agreements on the residue data for establishing MRLs made. Further problems include the difficulty of identification of drug residues and their metabolites in animal tissues, their quantitation at levels well below the MRLs, and the need to establish and control withdrawal periods before slaughter. Drug residues can occur in all edible products of treated animals and analytical methods must be able to cope with different levels of fat and other interfering compounds which may be present. Recent pharmacokinetic studies2 have shown that a rapid initial fall in applied drug concentration is followed by a much slower phase of depletion, emphasizing the importance of withdrawal periods and the need for extremely sensitive and selective techniques for the identification and quantitation of residues. Two types of analytical approach are required. The first, the most cost effective, consists of rapid screening techniques for presumptive evidence of the nature and levels of substances present. Screening methods should be amenable to both multiresidue analysis and use outside the laboratory. Unequivocal identification and quantitation of suspected positive samples using more expensive instrumental techniques forms the second approach.