Although the availability of tetracycline as a water medication for swine has been available for quite some time, there has been limited pharmacokinetic information available. Investigators from North Carolina State University and Pfizer Animal Health recently published an assessment of tetracycline water medication in swine.

Tetracycline is an antibiotic labeled or the control and treatment of salmonellosis, some enteric bacteria, and susceptible respiratory diseases in pigs. The focus of the experiment was to measure tetracycline concentrations in swine plasma. This study allowed for group comparisons of four doses, including a control. These studies also assessed whether tetracycline concentrations in plasma consistently reached minimum inhibitory concentrations of quality control (sensitive) bacteria.

Twenty-four Yorkshire-Landrace cross barrows were housed in individual pens on concrete. Barrows, approximately eight weeks-of-age and weighing 35-40 pounds at the initiation of the study, were assigned to one of four treatment groups, six animals per group. One group received 0.5 times the label dose (125 mg/L) of tetracycline hydrochloride; the second, a label dose (250 mg/L); the third, two times the label dose (500 mg/L); and the control group received water without tetracycline. Treatments of 125, 250, and 500 mg/L were achieved by dissolving a preweighed amount of an approved tetracycline water medication into 19.2 L of water in individual carboys for each animal.

All animals were given five days to acclimate to the facility and adapt to the carboy drinking system before the start of the trial. Water flow rates varied between 500 and 800 mL/min which is consistent with flow rates needed to prevent dehydration in adult pigs. Blood samples were collected just prior to medicating the water with tetracycline and subsequently at 4, 8, 12, 24, 32, 48, 56, 72, 80, 96, and 104 hours after dosing.

Results from this study indicate that tetracycline water medication concentrations as dosed were found to be consistent with tetracycline feed additive concentrations used in other studies; thus, would likely have similar efficacies. This study also assessed antibiotic sensitivity assays using sensitive control bacteria. Results indicate that the minimum growth inhibition concentration of 1 μg/mL was seldom reached or exceeded by any animal or group.

Based on this study and corroboration with previous work on tetracycline oral medications in pigs, tetracyclines should not be used orally to treat salmonellosis and respiratory disease. There may still be positive gastrointestinal effects from using oral tetracycline for prevention of scours and other enteric disease, but this needs to be assessed further.