Evaluation of a modified thrombelastography assay initiated with recombinant human tissue factor in clinically healthy horses

Background: Thrombelastography (TEG) is used to evaluate the viscoelastic properties of blood during clotting and provides a global assessment of hemostasis and clot lysis. TEG analysis initiated with recombinant human tissue factor (TF) has not been evaluated in clinically healthy horses. Objectives: The purpose of this study was to determine whether TEG results are affected by the time elapsed between sampling and analysis (storage time) of equine blood samples and to establish a preliminary equine reference interval for a modified TEG assay, using recombinant human TF to initiate coagulation. Methods: Citrated blood samples were obtained from 20 clinically healthy adult horses. Thirteen samples were stored for 30, 60, and 120 minutes at room temperature before TEG analysis. Coagulation was initiated by adding 20 μL of CaCl₂ to 330 μL of blood and 10 μL of diluted recombinant TF for a final dilution of 1:3600. Reaction (R) and clotting (K) times, angle (α), and maximum amplitude (MA) were compared between time points. A preliminary reference interval (minimum–maximum values) was determined using data from all 20 horses after 30 minutes of sample storage. Results: There was a significant effect of storage time on R, K, and α but not MA. Reference intervals were: R, 3.65–6.4 minutes; K, 1.8–5.45 minutes; α, 33.4–66.2°; MA, 41.2–64.1 mm; lysis at 30 minutes post‐MA (LY30), <2.75%; and lysis at 60 minutes post‐MA (LY60), 1.55–9.5%. Conclusions: TEG can be performed on equine citrated blood samples using recombinant human TF to activate clot formation. TEG parameters were significantly affected by storage time, suggesting an incomplete inhibition of coagulation in citrated blood.     

Pharmacokinetics and adverse effects of oral meloxicam tablets in healthy adult horses

The objective of this study was to assess the pharmacokinetic profile and determine whether any adverse effects would occur in seven healthy adult horses following oral meloxicam tablet administration once daily for 14 days at a dose of 0.6 mg/kg·bwt. Horses were evaluated for health using physical examination, complete blood count, serum chemistry, urinalysis, and gastroscopy at the beginning and end of the study. Blood was collected for the quantification of meloxicam concentrations with liquid chromatography and mass spectrometry. The mean terminal half‐life was 4.99 ± 1.11 h. There was no significant difference between the mean C_max, 1.58 ± 0.71 ng/mL at T_max 3.48 ± 3.30 h on day 1, 2.07 ± 0.94 ng/mL at T_max 1.24 ± 1.24 h on day 7, and 1.81 ± 0.76 ng/mL at 1.93 ± 1.30 h on day 14 (P = 0.30). There was a statistically significant difference between the T_max on the sample days (P = 0.04). No statistically significant increase in gastric ulcer score or laboratory analytes was noted. Oral meloxicam tablets were absorbed in adult horses, and adverse effects were not statistically significant in this study. Further studies should evaluate the adverse effects and efficacy of meloxicam tablets in a larger population of horses before routine use can be recommended.     

Effect of feeding on the pharmacokinetics of oral minocycline in healthy adult horses

Minocycline is commonly used to treat bacterial and rickettsial infections in adult horses but limited information exists regarding the impact of feeding on its oral bioavailability. This study's objective was to compare the pharmacokinetics of minocycline after administration of a single oral dose in horses with feed withheld and with feed provided at the time of drug administration. Six healthy adult horses were administered intravenous (2.2 mg/kg) and oral minocycline (4 mg/kg) with access to hay at the time of oral drug administration (fed) and with access to hay delayed for 2 hr after oral drug administration (fasted), with a 7‐day washout between treatments. Plasma concentration versus time data was analyzed based on noncompartmental pharmacokinetics. Mean ± SD bioavailability (fasted: 38.6% ± 4.6; fed: 15.7% ± 2.3) and C_max (fasted: 1.343 ± 0.418 μg/ml; fed: 0.281 ± 0.157 μg/ml) were greater in fasted horses compared to fed horses (p < .05 both). Median (range) T_max (hr) in fasted horses was 2.0 (1.5–3.5) and in fed horses was 5.0 (1.0–8.0) and was not significantly different between groups. Overnight fasting and delaying feeding hay 2 hr after oral minocycline administration improve drug bioavailability and thus plasma concentrations.     

Pharmacokinetics and bioequivalence of 2 meloxicam oral dosage formulations in healthy adult horses

Meloxicam, a non-steroidal anti-inflammatory drug, is approved for use in horses in several countries, but an equine formulation is not available in North America. However, meloxicam is being used in an extra-label manner in horses in Canada. The purpose of this study, therefore, was to assess the bioequivalence of an approved oral meloxicam suspension (Metacam 15 mg/mL for horses; Boehringer Ingelheim Vetmedica GmBH, Ingelheim, Germany) from the European Union with human meloxicam tablets (Meloxicam 15 mg tablets; TEVA Canada, Toronto, Ontario) compounded with molasses to improve palatability and administration. The geometric mean ratios (GMR test/reference) and the 90% confidence intervals of the pivotal pharmacokinetic parameters (area under the curve and maximum concentration) were within the defined limits of 80% to 125% generally accepted for products to be considered bioequivalent. Therefore, use of human meloxicam tablets compounded with molasses would be expected to produce a similar clinical response in horses as the approved oral product from the European Union.     

Effects of long-term oral administration of levothyroxine sodium on glucose dynamics in healthy adult horses

OBJECTIVE: To determine the effects of long-term oral administration of levothyroxine sodium (L-T(4)) on glucose dynamics in adult euthyroid horses. ANIMALS: 6 healthy adult mares. PROCEDURES: Horses received L-T(4) (48 mg/d) orally for 48 weeks. Frequently sampled IV glucose tolerance test procedures were performed on 3 occasions (24-hour intervals) before and at 16, 32, and 48 weeks during the treatment period. Data were assessed via minimal model analysis. The repeatability of measurements was evaluated. RESULTS: During treatment, body weight decreased significantly from the pretreatment value; mean +/- SD weight was 49 +/- 14 kg, 43 +/- 7 kg, and 25 +/- 18 kg less than the pretreatment value at weeks 16, 32, and 48, respectively. Compared with pretreatment findings, 1.8-, 2.4-, and 1.9-fold increases in mean insulin sensitivity (SI) were detected at weeks 16, 32, and 48, respectively; SI was negatively correlated with body weight (r = -0.42; P < 0.001). During treatment, glucose effectiveness increased and the acute insulin response to glucose decreased. Overall mean within-horse coefficients of variation were 5% and 29% for plasma glucose and serum insulin concentrations, respectively, and 33%, 26%, and 23% for SI, glucose effectiveness, and the acute insulin response to glucose, respectively. CONCLUSIONS AND CLINICAL RELEVANCE: Long-term administration of L-T(4) was associated with weight loss and increased SI in adult euthyroid horses, although other factors may have confounded results. Levothyroxine sodium may be useful for the treatment of obesity and insulin resistance in horses, but further studies are required.     

Evaluation of a chromogenic assay to measure the factor Xa inhibitory activity of unfractionated heparin in canine plasma

BACKGROUND: Unfractionated heparin (UFH) has a complex pharmacologic profile that necessitates patient monitoring to prevent inadequate anticoagulation or overdosage and hemorrhage. Factor Xa inhibitory assays (to measure anti-Xa activity) are used to adjust UFH dosage and define safe and effective regimens for specific thrombotic disorders in humans. OBJECTIVE: In this study, the accuracy, linearity, and clinical utility of a chromogenic assay were assessed for monitoring UFH anti-Xa activity in canine plasma samples. METHODS: A commercial assay (Rotachrom Heparin, Diagnostica Stago, Parsippany, NJ, USA) was used to measure anti-Xa activity in canine plasma samples spiked with different concentrations of UFH. Background absorbance and assay linearity were compared for canine and human plasmas. Percentage recovery of UFH anti-Xa activity and intra- and interassay imprecisions were investigated by multiple measurements of canine plasma to which known amounts of UFH were added. The spiked plasma samples also were used to determine the heparin sensitivity of an activated partial thromboplastin time (aPTT) test. RESULTS: Canine plasma samples were assayed at a higher dilution than were human plasma samples (3:8 versus 4:8) to eliminate higher background anti-Xa activity in canine plasma. Using this modification, the recovery of anti-Xa activity in canine plasma was linear (R2 > .9) at concentrations of 0 - 0.75 U/mL UFH. Intra- and interassay imprecisions for plasma samples containing 0.5 U/mL UFH were <10%, whereas samples containing 0.25 U/mL UFH had imprecisions of 13% and 24%, respectively. The anti-Xa activity range of 0.5 - 0.75 U/mL caused prolongation of aPTTs to 1.5 - 2.5 times the assay mean. CONCLUSION: Plasma anti-Xa activity of dogs treated with UFH can be accurately monitored using this commercially available chromogenic assay.     

Measurement of intraocular pressure in healthy anesthetized horses during hoisting

Objective

To measure intraocular pressure (IOP) in horses during hoisting after induction of anesthesia.

Safety and efficacy of subcutaneous alpha-tocopherol in healthy adult horses

Vitamin E is essential for neuromuscular function. The primary treatment, oral supplementation with natural (‘RRR’) α-tocopherol, is not effective in all horses. The objectives of this pilot study were to evaluate the safety and efficacy of a subcutaneously administered RRR-α-tocopherol preparation. Horses were randomly assigned in a cross-over design to initially receive RRR-α-tocopherol (5000 IU/450 kg of 600 IU/mL) subcutaneously (n = 3) or orally (n = 3) or were untreated sentinels (n = 2). Tissue reactions following injection in Phase I of the study necessitated adjustment of the preparation with reduction of the RRR-α-tocopherol concentration to 500 IU/mL in Phase 2. Following an 8-week washout period, horses received the reciprocal treatment route with the new preparation (5000 IU/450 kg of 500 IU/mL). Serum, CSF and muscle α-tocopherol concentrations were determined by high-performance liquid chromatography over a 14-day period during each phase. Serum and CSF α-tocopherol concentrations increased significantly postinjection only when the 500 IU/mL product was administered (P<0.0001). There was no significant difference in the muscle concentration of α-tocopherol following either treatment. All eight horses had marked tissue reaction to subcutaneous injection, regardless of product concentration. Whilst we have demonstrated that this route may be a useful alternative to oral supplementation, the marked tissue reaction makes use of such products limited at this time to only the most refractory of cases.     

Evaluation of intramuscular anesthetic protocols in healthy domestic horses

ObjectiveTo assess anesthetic induction, recovery quality and cardiopulmonary variables after intramuscular (IM) injection of three drug combinations for immobilization of horses.Study designRandomized, blinded, three-way crossover prospective design.AnimalsA total of eight healthy adult horses weighing 470–575 kg.MethodsHorses were administered three treatments IM separated by ≥1 week. Combinations were tiletamine–zolazepam (1.2 mg kg⁻¹), ketamine (1 mg kg⁻¹) and detomidine (0.04 mg kg⁻¹) (treatment TKD); ketamine (3 mg kg⁻¹) and detomidine (0.04 mg kg⁻¹) (treatment KD); and tiletamine–zolazepam (2.4 mg kg⁻¹) and detomidine (0.04 mg kg⁻¹) (treatment TD). Parametric data were analyzed using mixed model linear regression. Nonparametric data were compared using Skillings–Mack test. A p value <0.05 was considered statistically significant.ResultsAll horses in treatment TD became recumbent. In treatments KD and TKD, one horse remained standing. PaO₂ 15 minutes after recumbency was significantly lower in treatments TD (p < 0.0005) and TKD (p = 0.001) than in treatment KD. Times to first movement (25 ± 15 minutes) and sternal recumbency (55 ± 11 minutes) in treatment KD were faster than in treatments TD (57 ± 17 and 76 ± 19 minutes; p < 0.0005, p = 0.001) and TKD (45 ± 18 and 73 ± 31 minutes; p = 0.005, p = 0.021). There were no differences in induction quality, muscle relaxation score, number of attempts to stand or recovery quality.Conclusions and clinical relevanceIn domestic horses, IM injections of tiletamine–zolazepam–detomidine resulted in more reliable recumbency with a longer duration when compared with ketamine–detomidine and tiletamine–zolazepam–ketamine–detomidine. Recoveries were comparable among protocols.     

Evaluation of plasma concentration after intravenous and intramuscular penicillin administration over 24 hr in healthy adult horses

Penicillin is administered intravenously (IV) or intramuscularly (IM) to horses for the prevention and treatment of infections, and both routes have disadvantages. To minimize these shortcomings, a 24‐hr hybrid administration protocol (HPP) was developed. Our objective was to determine penicillin plasma concentrations in horses administered via HPP. Venous blood was collected from seven healthy horses administered IV potassium penicillin G at 0 and 6 hr and IM procaine penicillin G at 12 hr. Blood was collected at 2‐hr intervals from 0 to 20 hr and at 24 hr. Plasma penicillin concentrations were measured using liquid chromatography and mass spectrometry. Penicillin susceptibility from equine isolates was examined to determine pharmacodynamic targets. The MIC₉₀ of penicillin for 264 isolates of Streptococcus sp. was ≤0.06 μg/ml. For the 24‐hr dosing interval, the mean plasma penicillin concentration was >0.07 μg/ml. Five horses (72%) exceeded 0.06 μg/ml for 98% of the dosing interval, and two horses exceeded this value for 52%–65% of the dosing interval. The HPP achieved mean plasma penicillin concentrations in healthy adult horses above 0.07 μg/ml for a 24‐hr dosing interval. However, individual variations in plasma concentrations were apparent and deserve future clinical study.