Pharmacology of tetracycline water medication in swine

Medicating drinking water with tetracycline is commonly used in swine production systems to treat and prevent disease outbreaks. However, little information is known of the pharmacokinetics of this medication in water formulations. Twenty-four barrows, divided into 1 control group (of nontreated animals) and 3 equally sized treatments groups (n = 6/group), were treated with tetracycline water medication for 5 d at 125, 250, and 500 mg/L. Blood samples were collected at 0 (prestudy), 4, 8, 12, 24, 32, 48, 56, 72, 80, 96, and 104 h after exposure. Data analyses consisted of a noncompartmental pharmacokinetic analysis and statistical analysis of steady state concentrations with repeated measures ANOVA and multiple-comparison testing to determine whether plasma concentrations differed among groups. Derived pharmacokinetic parameters were consistent with previously published feed and intravenous data. Plasma tetracycline concentrations at steady state were 0, 0.33, 0.47, and 0.77 microg/mL for 0-, 125-, 250-, and 500-mg/L exposures, respectively. Treatment group steady-state plasma concentrations were significantly different from plasma concentrations in control animals (P < 0.0001); however, whereas the 125- and 250-mg/L groups were significantly different from the 500-mg/L group (P < 0.0001), their mean plasma tetracycline concentrations did not differ from one another. Furthermore, the study showed that tetracycline oral bioavailability is very small. The dose response curve also shows that concentrations of plasma tetracycline increase linearly, yet not in a 1 to 1 ratio, to the direct increase in water medication dose.