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
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
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)