Authors
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Abstract
Objective—To determine whether, and at what time, penicillin enters milk at a concentration that is detectable following bulbar subconjunctival injection in lactating dairy cows.
Design—Randomized clinical trial.
Animals—66 Holstein cows that were at least 2 weeks past calving and had not been treated with antibiotics in the preceding 30 days.
Procedure—Cows were randomly assigned to receive a treatment of 1 ml (300,000 units) procaine penicillin G by bulbar subconjunctival injection or remain untreated. Composite milk samples were collected immediately before treatment and 4, 10, 16, 22, 28, and 40 hours after treatment. Milk samples were tested by use of a commercial test for β-lactam antibiotics.
Results—Among penicillin-treated cows, the first positive test results were observed 4 hours after treatment, and the last positive result was observed 22 hours after treatment. The percentages of positive test results before treatment and at 4, 10, 16, 22, 28, and 40 hours after treatment were 0, 9, 87, 42, 8, 0, and 0%, respectively. None of the untreated cows had positive test results for β-lactam antibiotics at any sampling time.
Conclusions and Clinical Relevance—Penicillin was detected in milk for up to 22 hours after a single subconjunctival injection of procaine penicillin G in cows. This result should be considered when recommending milk withholding periods following the administration of penicillin by this route in lactating dairy cows. (J Am Vet Med Assoc 2000;217:369–371)
