Surveillance for veterinary drug residues

2 The UK national surveillance programmes for residues of veterinary drugs in meat and animal products 

The Statutory Surveillance Programme implements European Union (EU) legislation in accordance with the provisions set out in Directive 96/23/EEC. This Directive sets out the sampling regime and the veterinary drugs that must be monitored by each EU member state. 

In other nations similar bodies exist for the surveillance of veterinary drug residues in food. Any country that is not a member of the EU, wishing to export meat or animal products into the EU, must have a residues surveillance programme that is compatable with EU legislation. 

Surveillance for veterinary drug residues is undertaken with more than one objective in mind. These objectives are  

These objectives are not comprehensive but the approach taken in surveillance will depend largely upon the desired objective. 

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Between May, 1986, and December, 1990, over 35,000 analyses were conducted in the United Kingdom for surveillance for veterinary drug residues in food. The results drawn from these analyses allow the incidence and concentrations of drug residues in the national food supply to be assessed (29). Although a very large number of samples were analyzed, a rather small number of samples were found to contain violative residues that mainly concerned sulfonamides in swine, and oxolinic acid and oxytetracycline in farmed fish. However, the incidence of sulfonamide residues in a range of swine tissues from home-produced and imported sources showed a significant decline over the period 1986-1990 (Table 13.21). 

Like pesticides, veterinary drugs have an important role to play in reducing disease and suffering but their use has been brought into question by concerns about residues of them in food. The fact that their use can lead to residues in the food supply was recognised rather later than for pesticide residues. Nevertheless, methods of analysis and surveillance for veterinary drug residues are now well established in many parts of the world. 

The most recently reported UK results on surveillance for veterinary drug residues in meat and animal products show that traces of these compounds can, and sometimes do, arise in food. As all of these compounds are biologically potent in order to be effective in use, it is necessary to ensure that any residual activity in a food product does not present a risk to the consumer. The use of veterinary medicines inevitably leads to the presence of trace residues in food and the purpose of toxicological safety evaluation is to determine at what concentration the residues of a particular compound becomes a cause for concern with regard to human health. Thus, dose-response relationships have to be established and used to determine the concentration of a dmg at which the risks to human health become acceptable and are outweighed by the benefits from the use of the drug. This is in essence the process involved in the setting of Acceptable Daily Intakes (ADIs) and... 

Over a period of about twenty years the results of surveillance for veterinary drug residues in meat and animal products in the UK have provided reassurance that residues of veterinary drugs occur at very low concentrations and generally at low frequencies. Over this period repeated surveillance and improved communication and awareness of the producer and feed industry has resulted in a downward trend in the level and incidence of residues detected. However, contamination hot spots remain (e.g. animal medicated feed) and will continue to be monitored in the future. In addition, as new products are introduced into the market they will be evaluated during the licensing process to establish any risk from their residues present in human food. The UK also imports a significant amount of animal-derived food and the presence and risk to the UK consumer of residues in these products must also be considered and appropriate measures taken to protect the UK consumer. 

It is the responsibility of the licence applicant to demonstrate that residues concentrations have declined, through the processes of metabolism and excretion, to a safe concentration within the proposed withdrawal period. The product data sheet, which accompanies the veterinary product, is required to state the withdrawal period clearly. Veterinarians and farmers must observe these withdrawal periods if safe concentrations of residues are not to be exceeded. The withdrawal periods are binding in law and observation of them will ensure that the MRLs for veterinary drugs are not exceeded. This, together with residues surveillance, provides a further safeguard for the consumer. 

The implementation of legislation in the UK has ensured more effective control over residues than has previously been possible. This will help to deter an isolated number of farmers who do not observe the recommended conditions of use for veterinary products and who are therefore probably responsible for the low number of residue violations recorded. Refinements in analytical methodology, to improve the speed and reliability of residue detection, will help assist the expansion of the UK veterinary drug residues surveillance programme. 

RICO, A. G. (ed.) Drug residues in animals. Academic Press, London (1986). Veterinary Medicines Directorate Annual Report on Surveillance for Veterinary Residues in 1998. Veterinary Medicines Directorate 1999. ISBN 0-9531234-2-1. 

The correct implementation of surveillance and residue monitoring programs involves the development of robust and sensitive analytical methods to provide control authorities with effective tools. Since veterinary drugs are generally quite polar and thermolabile compounds, liquid chromatography coupled to mass spectrometry (LC—MS) has become the predominant methodology for determination of these substances in foods [4]. 

The safe concentration of drug-related residue must be known in order to determine the withdrawal period for a veterinary product. Often the toxicity data is incomplete and an estimate must be made to progress with requisite residue studies. One approach is to conduct a total residue study with sufficiently widely-spaced sacrifice intervals to assess the rate of depletion of total residue over the projected range of probable safe concentrations. A zero-withdrawal sacrifice interval should be included. The target tissue and marker residue are identified and surveillance/confirmatory assays developed. If a major portion of residue is non-extractable (bound) and the marker is undetectable at times when total residue is still significant, a residue bioavailability study may be necessary. To complete the data package, final residue and comparative metabolism studies are conducted. Studies on the metabolism of flunixin in cattle will illustrate this approach. 

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