Testing Goat Milk for Antibiotic Residues

Antibiotic residues in foods are of concern as they present potential health risks due to possible allergic reactions, carcinogenicity and promotion of the spread of bacterial resistance to antibiotics used in human medicine. In addition, starter cultures used in cheese and yogurt manufacture can also be affected by antibiotic residues in milk. This may result in considerable economic loss as product batches may be downgraded or discarded. Producers are required to offer an antibiotic free product, and are responsible to ensure illegal antibiotic residues are prevented. The loss of milks superior reputation could be an expensive price to pay.

Antibiotic Use in Food Animals

Antibiotics have been used in food animal production for the treatment and prevention of disease as well as growth promotion for nearly 50 years. Penicillin G was the first antibiotic introduced to veterinary medicine in 1947 for use in intramammary infusions. Since that time the use of antibiotics has become an integral part of managing animal health in agriculture.

Antibiotics are administered to animals by several different routes including injections, orally in the food and water, topically on the skin and by intramammary and intrauterine infusions. Theoretically, all of these routes may lead to residues appearing in foods of animal origin such as milk, meat and eggs if withhold times are not observed. It has been estimated that less than 1% of foods worldwide contain antibiotic residues at low levels.

The most commonly used antibiotics in food animals can be grouped into 5 major classes. These include the beta-lactams (ß-lactams) (e.g. penicillins and cephalosporins), tetracyclines (e.g. oxytetracycline, tetracycline and chlortetracycline), aminoglycosides (e.g. streptomycin and gentamicin), macrolides (e.g. erythromycin) and sulfonamides (e.g. sulfamethazine). A recent survey of veterinarians in the United States revealed that antibiotics were the drugs most often prescribed or used in the treatment of lactating dairy cows. Penicillin G was the most frequently used, and except for oxytetracycline, the five most prescribed drugs were all ß-lactams approved for use in lactating dairy cattle: penicillin G, ceftiofur sodium, cloxacillin, cephapirin and ampicillin. A similar pattern of drug usage would be expected for dairy goats.

Maximum Residue Limits and Milk Withhold Time

Antibiotic residues in milk are illegal when above maximum residue limits (MRL's). MRLs have been established by Health Canada to ensure public safety and are listed in the Food and Drugs Act. The term MRL may be defined as the maximum concentration of a residue resulting from the use of a veterinary drug, expressed in parts per million (ppm) or parts per billion (ppb) that is legally permitted or recognized as acceptable in or on food. In addition, all veterinary products which have been licensed for use in food animals in Canada must have a published withhold time. This time is the withdrawal period or the minimal period of time between the last recommended treatment and the time of slaughter or collection for use as food (eg. milk and eggs). This time allows the veterinary drug and it's residues to decrease to levels below the established MRL. Currently there are no products registered for use on dairy goats in Canada. Hence, there are no published milk withhold times for goat producers to follow. Therefore, it is very important that the milk from treated animals be tested with an appropriate test prior to going into the bulktank or if the animal has been treated by a veterinarian that an appropriate withhold time is given.

Antibiotic Residue Testing of Goat Milk in Ontario

The Ontario Ministry of Agriculture,Food and Rural Affairs has extensive programs in place for the testing of goat milk for antibiotic residues. Testing programs can be divided into two major areas.

1) All producers, have their bulk tank milk tested monthly on a random basis at the University of Guelph, Lab Services Division (LSD). The initial screening test is a microbial inhibition test whereby if an inhibiting agent is present in the milk being tested, growth of a bacteria in a test media is slowed or 'inhibited' and results in a 'positive' test result. The test has a very broad sensitivity range to various antibiotic families and has been adapted for screening large sample numbers at LSD. Suspicious samples are further characterized into either the ß-lactam family (penicillins and cephalosporins) or non-ß-lactam families (aminoglycosides, tetracyclines, macrolides) using the Standard Disk Assay method Positive samples are further classified into a specific family of antibiotics using rapid tests. Residue levels are quantified using High Performance Liquid Chromatography (HPLC).

2) Tanker truck loads of milk found positive at dairies are also confirmed at the LSD. Many processors are screening for ß-lactam antibiotics and some are also testing for sulfa drugs and tetracyclines. A positive screen test result at the plant results in a load sample and all associated producer samples being sent to LSD for confirmation using official tests. The truckload of milk cannot be unloaded prior to receiving confirmation that the official test is negative. If the load is found officially positive the milk must be disposed of properly. If the load is officially negative the milk may be marketed. A producer sample found to contain violative levels of antibiotic residues will have milk in the bulk tank placed under detention. If the detention sample test is negative the detention is lifted and the milk may be marketed. In addition the producer may be subject to a penalty by their marketer.

The official tests on which milk marketing and penalty decisions are based are the Standard Disk Assay for of ß-lactams and the HPLC for sulfamethazine. Load and producer samples which give a zone of inhibition greater in diameter than a sample containing 0.01 IU/ml penicillin G per millilitre of milk by the Standard Disk Assay procedure or whose milk tests more than 10 ppb sulfamethazine by the HPLC are considered violative. When an antibiotic is found in a producer's milk a follow-up is conducted by OMAF staff to pinpoint the cause of the inhibitor ending up in the milk and thus prevent future occurrences.

On Farm Test Kits

Currently, there are a variety of screening tests on the market suitable for on-farm testing. Each test has its own benefits and limitations in cost, speed and sensitivity. The major suppliers for the North American market are Idexx Laboratories, CHARM Sciences and DSM Food Specialties.

Test kits are available for the ß-lactam, tetracycline and sulfa families. An important point to remember however, is that most of the tests were developed for testing bovine milk and it has been reported that these tests may give false positive test results when testing goat milk. This is primarily due to the difference in composition between goat and bovine milk. For this reason any positive result on a screening test should be confirmed using official methods.

Preventative Measures by Producers

By far the majority of cases of positive inhibitors are traced back to improper use of antibiotics at the farm. Therefore the onus lies on producers to ensure they are taking all the necessary precautions when using antibiotics including the following standard recommendations:

• Only administer drugs on the advice of a veterinarian
• Follow label and veterinarian instructions including method of administration, dosage and dosing regime
• Identify treated animals with markings or bands and separate from milking herd
• Make a written and permanent record of the animal treated, dosages and dates given
• Ensure everyone involved in the milking is aware of animals that have been treated and special milking procedures are required.
• Milk treated animals separately
• Withhold milk from the bulk tank according to veterinarian or label directions
• Use an appropriate inhibitor test kit to test milk from treated does prior to re-entry into the milking line-up. Testing services are available from the Lab Services Division, (University of Guelph), veterinarian clinics and some milk processors.

Other things to consider that may cause a positive inhibitor test result:

• metabolism of the animal may be slower than expected thus slowing the rate of the drug leaving the animal's system
• using a combination of drugs to treat an animal may lengthen the withholding time for each individual drug
• 'off-label' usage can affect withdrawal rates. Off label usage refers to administration of drugs in a method different from that stipulated on the label. For example, administering intramammary rather than intramuscularly, or giving a higher dosage than recommended.
• withholding milk only from treated quarters
• animals drinking from medicated footbaths
• medicated feeds

For more information on what you can do to prevent antibiotics from ending up in your bulk tank of milk refer to the fact sheet Troubleshooting Antibiotic Residues in Goat Milk on the OMAF website at:

• Troubleshooting Antibiotic Residues in Goat Milk

For information on test kits available and ordering visit:

• CHARM Sciences
  
• Idexx Laboratories
  
• DSM Food Specialties