Antibiotic residues in milk and meat will continue to be a hot topic of discussion in 2012. Dairy producers who have not done so already should review their residue avoidance programs to ensure compliance. The use of antibiotic test kits by dairy farms is an important part of a comprehensive residue avoidance program.

In this article, we review the basic technologies used by the test kit manufacturers and how the differences affect the use of the kits.

On-farm antibiotic test kits are available from multiple suppliers. They are all based on two basic technologies: antibody-binding reactions and bacterial growth inhibition.

Antibody-binding:
Narrow-spectrum tests
Antibodies are one of the main defenses animals have against infection. They are produced as a reaction to foreign biological material entering the body, but they can also be employed as an important tool for detection of molecules, like antibiotics in milk.

Certain types of antibodies, referred to as monoclonal antibodies, can be very specific in binding to molecules like proteins, sugars or even smaller components of these materials. An easy way to visualize how antibodies bind a target molecule is to think of an antibody as a key and the foreign body it attacks or binds to as the lock.

Antibodies, similar to a lock and key set, can be very specific, where only one key will work in the lock; or they can be less specific, similar to how a skeleton key can work in more than one lock. There is even the case where antibodies function like a key that fits into a lock but will not turn in the lock.

A test that employs a highly specific antibody is called a narrow-spectrum test. The test will detect only what it is specifically designed to detect.

An additional feature of these types of tests is that the reaction can take place quickly, which means that the test can be run in a matter of minutes rather than hours.

Most of the antibody-binding test kits on the market today are designed to look for the specific beta-lactam ring that is present in penicillin-class antibiotics and other related antibiotics like cephalosporin.

These antibiotics share the same basic chemical ring structure, as shown in red in Figure 1 . As you can see, they have slightly different molecules attached to the basic structure.

Now compare this to the structure of tetracycline in Figure 2 . As you can see the chemical structure is very different.

An antibody that binds to penicillin would not react with tetracycline, just as a key from one lock type will not fit into or open the lock of another lock type.

This is why a test designed to detect beta-lactam drugs will not detect a tetracycline-based drug.

Bacteria growth:
Broad-spectrum tests
This type of test uses a bacterial culture that is extremely sensitive to antibiotics and prefers to grow at relatively high temperatures.

The test comes in a small tube that has a growth medium, the fast-growing indicator bacteria and a pH color indicator that helps to detect the growth of the bacteria.

As the bacteria grow, they produce an acid that lowers the pH of the growth medium. The acid changes the color in the tube, in the case of a test kit, from purple to yellow.

When an antibiotic is present at a high enough level, the bacteria is inhibited from growing and cannot produce the acid to produce the color change, and the media in the tube stays purple.

This type of test is called a broad-spectrum test because almost any type of antibiotic present will prevent the growth of the indicator organism and the color change from happening. This type of test takes a couple of hours to run since the bacteria need a little time to grow.

There are important points to remember when using any test kit. They include:

• Carefully following the manufacturer’s instructions.

• Store the test according to the manufacturer’s directions. Generally, test kits need to be stored under refrigeration conditions, and bacterial growth tests should never be frozen. Make sure that the refrigerator is in good working order, too. Many older refrigerators will have large swings in their temperature range, often going below 32 degrees for a short period of time. Also remember a refrigerator kept outside in an environment that is below 32 degrees will not keep the contents of the refrigerator above 32 degrees.

• Isolate any incubators, timers or test-reading equipment from the rest of the electrical system by using a surge suppressor or similar device. Most of these newer machines contain parts that can be harmed by “dirty power.”

• Keep antibiotics away from the test kit. Do not handle antibiotics before running a test. Remember that these tests detect antibiotics in the parts per billion range. This is like finding five specific pennies in a swimming pool full of pennies.

• Maintain accurate records. This applies to everything you do when treating animals.

• Use the resources that are available to you. The people you sell your milk to often have programs to help you buy and use test kits. There are producer education materials available like the National Dairy Farm Program’s 2012 Milk and Dairy Beef Drug Residue Prevention manual.

• When in doubt about a test result, rerun the test. If the test is run on an individual cow, wait 24 hours and rerun the test.

Using the test that is right for you and how you treat your cows
There are many tests available today for milk testing. Picking the right one is not always easy. Here are some basic guidelines for picking the right test:

• Ask yourself if you need to do any testing at all. This would be an easy answer if you had complete control over all aspects of your operation or you did not use antibiotics. Most producers should consider using antibiotic test kits since most cannot answer no to this question. Many of the companies that buy milk have programs in place to help their patrons test milk for antibiotic residues.

• Make sure that you are using a test that will detect the antibiotics currently in use on the farm. For instance, if you are treating cows with tetracycline, you need to make sure you are using a test that detects tetracycline. Use either a broad-spectrum test or a narrow-spectrum test specifically designed to detect tetracycline. Remember that most fast tests are designed to detect the six beta-lactam drugs that are covered under Appendix N from the NCIMS / FDA. (Amoxicillin, Ampicillin, Ceftiofur, Cephapirin, Cloxacillin and Penicillin).

• How much time do you have to run the test? Broad-spectrum tests are not an option when the milk hauler is a few minutes from your farm. On the other hand, a broad-spectrum test can be run overnight and read the next morning.

• It is important to remember that there are two separate tests run on milk that are mandated by the FDA. One is the Appendix N testing, which is done on every tanker of milk at receiving stations, usually using a fast test for the six beta-lactam antibiotics. The other is Section 6 testing, where samples are taken at random from individual farms and tested. Most milk tested under Section 6 is tested using the broad-spectrum, bacterial growth tests.

• Consider the cost of the test and any associated equipment needed. The cost of a violation in dollars and reputation is very large. The cost of having a good program in place for testing is small in comparison.