From the October 2001 Pork News & Views Pork News and Views is prepared by the Swine Advisory Team of the Ontario Ministry of Agri- culture, Food and Rural Affairs Editor: John Bancroft Clinton OMAFRA, 519-482-3133,

Drug residues are the drugs and/or its metabolites that remain in animal tissues such as the muscle, liver and kidney. They are food safety hazards because some people are allergic to them and some drug residues are carcinogenic to humans.

Drug resistance is a bacteria’s tolerance to drugs, which make drugs ineffective in killing bacteria. Drug resistant bacteria poses a potential threat to public health because in case of a human bacteria infection there would be less or no drug effective for the treatment. Drug residue and drug resistance are two separate issues and the focus here is on drug residue prevention.

Drug residue prevention continues to be a challenge to the pork industry due to its complex nature. Studies have found that the problem cannot be solved by only following the drug withdrawal time. There are many other sources that may cause a residue problem such as contaminated feeds, contaminated water, contaminated environment and equipment, intentional misuse and many others. The main source, however, is cross-contamination of non-medicated feed with medicated feed associated with on-farm-feed-mixing and handling. Following are points to check and tips for residue prevention.

Adopting a quality assurance program
Implementing an effective quality assurance program, such as the Canadian Quality Assurance (CQAÒ ) program is the key to the production of high quality and safe pork. The CQAÒ program is a preventive approach to pork safety where potential food hazards are identified and, then, measures are taken to minimize or eliminate the hazards.

Using medications requiring no withdrawal time
There are drugs approved for use in Canada, which require no withdrawal time. Consider using these drugs first, if possible, to reduce the residue incidence.

Choosing a right form of sulfa drugs
Studies show that Sulfamethazine is more prone to drug residue than sulfathiazole. A very small amount of sulfamethazine in the feed could cause a residue problem in pork. For example, a study at University of Kentucky showed that as little as 1 gram of sulfamethazine per tonne of feed (1 ppm) would cause a high incidence of residues in the liver. Compared with sulfamethazine, higher amounts of sulfathiazole are required to cause the same results. Emptying and cleaning feeders
When switching pigs from medicated feed to non-medicated feed, feeders must be emptied and cleaned if the same feeders are used to finish out pigs. Most commonly used feeders have areas where the residual feed is likely to remain and contaminate the next batch Using granular form of sulfa drugs
The powder form is extremely electrostatic and dusty. Drug powder could remain on the surfaces of feed bins, feed carts, floors and equipment and in the air. It is, therefore, practically impossible to use them without a risk of cross contamination and carryover. A study conducted at the University of Kentucky showed that a sulfa level in feed dust taken from the inside surface of a mixer was 276 ppm when powdered sulfamethazine was used as compared with only 59 ppm when granulated sulfamethazine was used. Using proper feed mixing sequence
Producers who mix their own feeds on the farm should have a written procedure for the sequence of mixing feeds. A proper mixing sequence will reduce drug residue risk. For example, after mixing medicated feed, prepare non-medicated feed for animals that are not intended for slaughter, such as sows, replacement gilts and growers before mixing feed for finishers. Finisher feeds should never be mixed immediately following a feed medicated with a drug requiring a withdrawal time. Making sure correct dosage and uniform mixing
Drugs must be used according to their instructions. Using higher dosages than the approved level is illegal and contributes greatly to carryover. Scales must be accurate and calibrated regularly to ensure the proper dosage level is in the feed.

Uneven drug dispersal in feeds due to poor feed mixing is often a potential cause of drug residue. Sufficient mixing is a key to uniform dispersal of drugs and other micro-ingredients. Higher than intended dosages and insufficient mixing contribute to drug residue problems even if the approved withdrawal time is followed.

Preventing drug carryover
Drug carryover in feeds can occur in a number of ways. Feed manufacturing, delivering and storing equipment such as mixers, augers, storage bins and feeders can all harbor dust residual feed, which can cause cross contamination with non-medicated feed. Failure to clean out the residual feed will cause the next batch to be contaminated. For example, a 2 kilogram carryover of medicated feed into 100 kilograms of non-medicated feed will produce a detectable sulfamethazine level.

Therefore, a thorough clean-out of all equipment, such as conveyors, augers, feed bins, feed carts is essential in reducing the chance of carryover. Mixers should be flushed with clean feed (at least 5% of the mixer’s capacity) after making medicated feeds. If feasible, using a separate set of equipment for mixing clean finisher feed should be considered.

Flushing drinking water system
Like a feed delivery system, a drinking water system can also harbor drugs and be a potential cause of drug residues in pigs. If a drug requiring withdrawal time is administered through drinking water, the system should been thoroughly cleaned by flushing with clean water. Also, the same drug should not be medicated in both the feed and water at the same time because it could cause high intakes of the compound and could result in a residue. Water medicators should be calibrated on a regular basis to ensure correct dosage delivery. Identifying individually drug treated animals
Residue risk can be high if a whole group of pigs are fed with medicated feed. However, one or more animals treated individually with drugs requiring withdrawal time can also cause a residue problem due to fecal contamination or withdrawal time not followed. Treated animals should be marked and recorded or separated from the rest of group if it is in a final finishing stage. Treatment records should be reviewed before shipping for slaughter to ensure the drug withdrawal time is completed. Cleaning manure containing drug residues
Studies repeatedly show that manure left in pens from pigs fed medicated feeds or treated with drugs could cause residue positive tests. Hogs housed on solid floors are more likely to recycle drugs than hogs housed on slotted floors. Following sulfa or other drug withdrawal, manure must been removed and cleaned if the same pens will be used for finishing out hogs. Avoid using recycled lagoon water
If lagoon water is recycled, don’t use it in finishing areas because the recycled water could contain residual drugs and cause a positive drug residue test.

Avoid extra label use of drugs
Extra-label uses or off label use of drugs can occur in many ways. Any use of a drug not according to the label is extra label use and illegal without a veterinarian prescription. For example, using a combination of drugs must be according to product labels because this could alter the dosage and withdrawal time. An example is if water-soluble sulfa drugs are used, do not use Aureo SP-250 G medicated feed. Following drug withdrawal time
Drug withdrawal time must be followed all the time. If you are not sure about the withdrawal time consult your with your veterinarian or other swine professionals. Making sure no drugs in meat and bone meals
Meat and bone meals could be a source of drug residues. If meat and bone meals are used, measures should be taken to ensure that they are drug or residue free, such as asking the supplier for clarification or testing them for drug residues. Preventing delivery errors
Delivery trucks can also be a source for drug residues. Delivering a wrong feed, or to the wrong feed bin, can sometimes happen. Feed bins should be labeled clearly so feed truckers know exactly which feed bin the feed should be delivered in. Bulk delivery trucks are also potential source for drug residues. Drug carry-over in feeds can occur if medicated and non-medicated feeds are hauled at the same time or if the conveying system on these trucks is not cleaned out properly between deliveries of the medicated and non-medicated feeds.

Finally, for successful drug residue elimination, hog producers must pay attention to all the possible causes of drug residues, have a written drug avoidance protocol and ensure that it is carried out effectively.

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Further reading in the October 2001 Pork News and Views:

Finishing $ense Swine Enterprise Budgets - August 2001 Ontario Monthly Hog Market Facts United States Hog Market Facts Job Satisfaction Transportatin Regulations - Research Briefs Tips for Drug Residue Prevention in Pork Ready for Validations Information Tidbits What Kind of Injectable Tetracycline Is It? Upcoming Events