Because the human diet is so diverse, the type of matrices in which animal drug residues are found is quite varied. The following is a brief description of the food commodities commonly monitored for veterinary drugs.
The prevalent use of veterinary drugs in cattle, swine and poultry production has resulted in the need to monitor residues in the tissues of these animals. This includes muscle meat such as various cuts of beef and pork, as well as in poultry products. Initial sample preparation for meat products involves cutting the tissue into small portions and homogenizing the pieces with a homogenizer or similar apparatus. In some cases, residues can bind to the proteins in the muscle and hydrolysis is required to release the residues. Owing to typical animal excretion pathways, it is also quite common to find residues in organ meats such as liver and kidney. These tissues typically have a high fat content, which requires additional fat removal clean-up steps. The residues may also occur as conjugates, i.e., modified with glucoronide or acetyl groups, when present in these organ meats. The use and analysis of veterinary drugs in meat have been reviewed [18,19].
Dairy cows are susceptible to mastitis, a common bacterial infection that is effectively treated with antibiotics. Because milk is a commodity that is consumed in a higher proportion by children, the monitoring for residues in milk is of increased importance. Often the allowed amount of approved residues in milk is lower than that for edible bovine tissues. Sample preparation steps for veterinary drug residue methods for milk often involve defatting and protein precipitation, with acetonitrile or trichloracetic acid, prior to or concurrent with the extraction of residues. Information concerning the incidence of veterinary drug residues in other dairy products, such as dried or condensed milk or different kinds of cheeses, is fairly limited. Recent studies, however, indicate a growing interest in these types of food products [20-22].
Aquaculture, or fish farming, is a rapidly growing industry. Approximately 80% of the fish and shellfish consumed in the U.S. is imported from other countries, amounting to over 2 billion pounds of harvested seafood imported into the U.S. in 2004. A large percentage (approximately 40%) of that imported seafood is produced by aquaculture facilities . The use of antibiotics and antifungal drugs in aquaculture has also increased with the expansion of the industry . Fish raised in these high-density environments may experience higher stress and weakened immune systems requiring drugs to stem disease outbreaks. Methods for residue extraction and isolation in aquatic organisms are similar to those used for other animal tissues. The fat content of fish can vary significantly depending on the species and even the specific environmental growing conditions.
Veterinary drug residues can also be found in foods such as eggs and honey produced as animal by-products. The drugs of concern in eggs are similar to those monitored for in poultry tissue, such as coccidiostats. Residue-extraction procedures designed for tissue can be applied to eggs, but additional steps may be required to eliminate sample components that are unique to the egg matrix . Foul brood disease has become a global problem in the apiary industry and many drugs such as tetracyclines, macrolides and phenicols have been used to combat this threat to honey production. The high sugar content of honey complicates residue isolation and clean-up. Additional aqueous wash steps to eliminate the excess sugar, as well as the use of internal standards to compensate for any background interference or signal suppression, are often required .
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