Delvo-Ptest

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Rationale:

The milk of every cow that calves at the TMF and every cow that has been treated with an antibiotic that requires milk withhold must be tested for antibiotics before the cow is moved from Pen 41 and enters the milking pens. This is done to assure that no milk enters the bulk tank from a cow that may still be antibiotic positive. It is every persons responsibility at all times to assure that treated cows are kept only in PEN 41 , are properly leg banded as treated and proper withdrawal times are observed for all drugs, including but not limited to antibiotics. Treatment leg bands are removed only after a cow has tested negative.

How the test works:

The purple agar in the ampule contains Bacillus stearothermophilus, a bacterium that is exquisitely sensitive to some antibiotics, particularly the members of the Beta-lactam group (penicillin, ampicillin, amoxicillin, cephalosporins, etc.). If there is antibiotic in the milk, it will inhibit growth of the bacteria and prevent them from metabolizing the nutrient tablet and producing acid. If the bacteria can grow and produce acid, the acid will cause the agar to change from purple to yellow.

Milk sampling:

Each day, the herdsman will run a TXLIST command (in DairyCOMP: under Reports: daily TX review list) to generate the list of milking cows by pen that is used in the morning screening of cows. From that list, the herdsman should identify each cow in Pen 41 that is 1 or more days in milk (if not treated) or that has completed their required drug withdrawal period (if treated). A list of those cows is given to the morning milker, who should sample each cow into a vial labeled with her number.

The samples are brought into the laboratory area and the tests are set up as soon as possible after collection. Milk samples should not be frozen before testing.

Testing procedure:

Set up everything needed on a clean surface before you start. The test kit is in the white refrigerator. Turn on the incubation block to warm up. Wash and dry your hands thoroughly.

1. Label a test ampule containing purple agar medium for each cow being tested. You must cut (not tear) each ampule to separate it from the pack. If you tear the foil on any ampule, you must use it immediately or discard it.
2. Using the plastic forceps, poke a hole in the top of the ampule corresponding to the milk sample you are setting up. Be careful not to touch the end of the forceps with your hands or to touch the forceps to the agar. This avoids contaminating the test with substances that may interfere or create false positive tests.
3. Shake a few nutrient tablets into the bottle cap. Using the forceps, place one tablet into the ampule. If the tablet touches any surface, discard it and get a new tablet.
4. Attach a disposable pipette tip to the small spring syringe, being careful not to touch the end of the tip.
5. Shake the milk sample.
6. Depress the plunger of the syringe fully, dip the tip into the center of the milk sample, and allow the spring to suck up a sample of the milk. Avoid bubbles.
7. Squirt the sample in the test ampule.
8. Place the test ampule into the incubator block.

Repeat steps 1  8 for each sample.

Using the clock on the FAX machine as the timer, write the time the tests will be done (3 hours) on the white board above the incubator block.

Test interpretation:

If the bacteria can grow, they will metabolize the nutrient tablet and produce acid. The acid produced causes the agar to change from purple to yellow. Milk samples with enough growth inhibitor (antibiotics) to stop or slow bacterial metabolism will remain purple.

Tests that turn yellow are negative: no antibiotics (within the limits of the test and the antibiotic).

Tests that remain purple are positive: antibiotics are present and the cow should be kept in Pen 41 and retested.

If in doubt, keep the cow in Pen 41 and retest her the next day.