Chemical Residue Testing in Beef

Manufacturers of drugs must submit drug depletion data to the Bureau of Veterinary Drugs in Ottawa. This data is used to establish a drug withdrawal period that permits residues to decrease below a previously established tolerance concentration before animals are slaughtered for use as food. Tolerance is the amount of residue permitted in foods obtained from treated animals, and it is an estimate of the concentrations of drugs in meats of animal origin that can be consumed safely by humans. Tolerances are calculated from safety data obtained in toxicity trials, acceptable daily intakes of the drug, and a food factor that modifies the value of daily intake by the percentage of meat consumed in the diet.

Violative drug residues occur when withdrawal periods are not followed, either due to inadequate records, hired labour or occasional unconcern. Drugs that are used extra-label (e.g. in a species they are not approved for) may also cause violative drug residues because their withdrawal periods are not known with certainty. Animals that are sick and have been treated may not excrete drugs from their body in a normal fashion and this may result in drug residues even when the withdrawal period has been adhered to. Occasionally feed mills mix or deliver the wrong feed or bins and feed trucks are inadequately cleaned resulting in violative drug residues.

The average occurrence of violative residues for all chemicals in Canadian domestic beef in 1997/98 was below 1.3% (CFIA). For antibiotics, the compliance rate in Canadian domestic beef in 1997/98 was 99.96%, and for sulfonamides, it was 99.6%. Previous reports suggest that chemical residues cause approximately 4% of food-borne disease in humans.

The current meat inspection system routinely monitors randomly selected slaughtered animals for residues using the STOP (Swab Test on Premises) or the CAST (Calf Antibiotic Sulfa Test). The CAST was initially developed for use in calves < 21 days of age or < 150 lbs body weight, but it is now used for all species of any age.

In addition to the randomly sampled carcasses which are monitored, other animals are tested including nonambulatory cattle, carcasses with visual injection sites, carcasses condemned for pathology, suspects (pneumonia, mastitis), and animals from listed violators. Kidney samples are first tested at the plant using one of the two screening tests. The screening tests take about 24 hours to complete and those carcasses that test positive are retained. Muscle, kidney, and injection site (if available from the animal) are sent to the Centre of Veterinary Drug Residues, Canadian Food Inspection Agency, Saskatoon for confirmation and identification of tissue residues using a battery of analytical methods and test kits.

The muscle and injection site are tested upon receipt and the results are sent to the Veterinarian in Charge at the plant and the Regional Office, usually within 1-3 days. Testing of the muscle is discontinued when violative residues are detected or when all tests have been completed. Disposition of the carcass is determine by residues found (or not found) in the muscle (or if chloramphenicol is found in the injection site). Kidneys are directed to a second program in which all available antimicrobial tests are applied to the tissue in order to provide a comprehensive list of drugs used on the animal, for information purposes.

Prevention of violative drugs begins at the farm. All withdrawal periods for drugs must be strictly adhered to and all extra-label use of drugs should be avoided. Suspect animals such as those chronically ill, should be: 1) held at the farm for longer periods than the stated withdrawal periods to help ensure they are free of violative residues, 2) an on-farm residue test of urine can be used to test for violative residues (LAST (Live Animal Swab Test)) after the withdrawal period and prior to slaughter, or 3) the inspector at the plant can be informed that a suspect animal is coming in and take the necessary precautions to ensure that violative residues do not enter the food chain.

The LAST is similar to the STOP and CAST that are used in slaughter plants. Additional equipment, such as an incubator, is needed to run the test. The test may not test for the antibiotics which you treated the animal and it may result in false positives or negatives if the test is not used properly, stored correctly, or is outdated. These limitations, including the lower limit detection ability of the test (sensitivity), are common for screening tests. For further information on the usefulness of the LAST for on-farm residue testing in beef, consult your local veterinarian.

Additional information on residues is available from FARAD (Food Animal Residue Avoidance Databank) which is a computerized databank of residue avoidance information on approved drugs for food animals, pesticides, and environmental contaminants. A Canadian database is currently under development. This service is available to producers, extensionists, and veterinarians. Contact your herd veterinarian for information on how to access this database.