Evaluation of tetracycline in milk following extra-label administration of topical tetracycline for digital dermatitis in dairy cattle.

Digital dermatitis (DD) is a painful infectious foot lesion commonly treated topically with extra-label tetracycline. Our objectives were to determine the concentrations of tetracycline in milk and plasma and to calculate a withdrawal interval following topical application at various doses. Another objective was to evaluate agreement between tests for measuring tetracycline in milk. A randomized block trial was conducted on 2 farms, where 50 cows with active DD lesions on 2 feet were allocated to 1 of 5 treatment groups (n = 10 cows per group). Treatment groups consisted of topical applications of tetracycline hydrochloride, in a paste or as a powdered form under a bandage, at 3 different dosing levels (2, 5, and 25 g) on each of the 2 affected feet. Following enrollment and treatment, samples were collected from milk, teat skin, and blood every 8 to 24 h for up to 7 d postdosing. Concentrations of tetracycline were measured by liquid chromatography-mass spectrometry and milk samples were further tested using the Charm ROSA TET test (Charm Sciences Inc., Lawrence, MA). Tetracycline was present in milk, plasma, and teat skin from all treatment groups. Tetracycline concentrations varied depending on time of sampling, method of application, and dosing level. At 8 h post-treatment, 11% of cows had tetracycline present in milk higher than 100 ng/mL (ppb) but none higher than 300 ng/mL. The 25-g treatment group had the longest estimated withdrawal interval, the highest observed concentrations (210-244 ng/mL) of tetracycline present in milk, and the longest observed consecutive period of tetracycline presence (from 8 to 72 h) among all treatment groups. Compared with liquid chromatography-mass spectrometry, the Charm test had a sensitivity of 77 and 100% for measuring tetracycline in milk at ≥30 and ≥100 ng/mL, respectively. Post-treatment samples of the teat skin were taken from 15 cows on 6 occasions, and every cow had tetracycline present in at least 1 of those 6 samples. This confirms an association between topical DD treatment with tetracycline and contamination of the teat. A total of 22% of blood samples had detectable tetracycline, and the majority (63%) occurred at 8 h post-treatment. At 100 ng/mL, the estimated cow-level milk withdrawal interval ranged from 0 to 70 h. At 300 ng/mL, the estimated cow-level withdrawal interval ranged from 0 to 34 h, and was 0 h at the bulk tank level. We recommend that conservative measures be adopted after extra-label use of topical tetracycline for DD treatment, including using a low dose and strategic post-treatment testing for tetracycline-class drugs in milk.

Serum haptoglobin concentrations in dairy cattle with lameness due to claw disorders.

In cattle, elevated blood serum concentrations of haptoglobin, an acute phase protein, have been demonstrated in association with several diseases, but not with lameness. Serum haptoglobin was measured in 60 Holstein dairy cattle diagnosed with lameness due to four claw disorders, pododermatitis septica (PS; n=41), pododermatitis circumscripta (PC; n=8), interdigital necrobacillosis (IN; n=7), papillomatous digital dermatitis (PDD; n=4). Haptoglobin was measured on day 1 (0-3 days after lameness was observed but before treatment) and on days 3 and 5. A total of 10 healthy cows served as controls (haptoglobin values <1.0 mg/dL). Each of the claw disorders was associated with elevated haptoglobin on day 1 (PS, PC, IN and PDD: 65.9%, 37.5%, 71.4% and 25.0%, respectively). Trimming and antibiotic treatment led to a reduction in the number of PS and IN cows with increased haptoglobin concentrations, respectively (P<0.05), but trimming did not lead to any reduction in cows with PC. The study showed that lameness due to claw disorders can be associated with a systemic acute phase response and elevated serum haptoglobin in dairy cattle. Based on the course of haptoglobin, treatments seemed effective for all claw disorders except for PC.