The Horse

OCT 2018

The Horse:Your Guide To Equine Health Care provides monthly equine health care information to horse owners, breeders, veterinarians, barn/farm managers, trainer/riding instructors, and others involved in the hands-on care of the horse.

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LETTERS E-mail us at, or write us at The Horse, 3101 Beaumont Centre Circle, Suite 100, Lexington, KY 40513. Letters may be edited for space limitations and must include the author's name and contact information. 12 October 2018 The Horse | EQUISUL - SDT ® (Sulfadiazine/Trimethoprim) Oral Suspension For use in horses only. NADA 141-360 CAUTION Federal law (USA) restricts this drug to use by or on the order of a licensed veterinarian. DESCRIPTION EQUISUL-SDT is a broad-spectrum antimicrobial from the potentiated sulfonamide class of chemotherapeutic agents. These two drugs block different sequential steps in the biosynthesis of nucleic acids. Sulfadiazine inhibits bacterial synthesis of dihydrofolic acid by competing with para-aminobenzoic acid. Trimethoprim blocks the production of tetrahydrofolic acid from dihydrofolic acid by reversibly inhibiting dihydrofolate reductase. The effect of the dual action is to reduce the minimum inhibitory concentration of each agent (synergism) and to convert a bacteriostatic action to a bactericidal action. Sulfadiazine is the non-proprietary name for 4-amino-N-2-pyrimidinylbenzenesulfonamide. Trimethoprim is the non-proprietary name for 5-[(3,4,5¬trimethoxyphenyl) methyl]-2,4-pyrimidinediamine. Figure 1. Structure of sulfadiazine Figure 2. Structure of trimethoprim Each mL of EQUISUL-SDT contains 400 mg combined active ingredients (333 mg sulfadiazine and 67 mg trimethoprim) in an aqueous suspension. INDICATION EQUISUL-SDT is indicated for the treatment of lower respiratory tract infections in horses caused by susceptible strains of Streptococcus equi subsp. zooepidemicus. DOSAGE AND ADMINISTRATION Shake well before use. Administer EQUISUL-SDT orally at the dosage of 24 mg combined active ingredients per kilogram body weight (10.9 mg/lb) twice daily for 10 days. EQUISUL-SDT can be administered by volume at 2.7 mL per 45.4 kg (2.7 mL/100 lb) body weight. CONTRAINDICATIONS EQUISUL-SDT is contraindicated in horses with a known allergy to sulfadiazine, sulfonamide class antimicrobials, or trimethoprim. WARNING Do not use in horses intended for human consumption. HUMAN WARNINGS Not for use in humans. For use in animals only. Keep this and all drugs out of the reach of children. Consult a physician in the case of accidental human exposure. Antimicrobial drugs, including sulfonamides, can cause mild to severe allergic reactions in some individuals. Avoid direct contact of the product with the skin, eyes, mouth, and clothing. Persons with a known sensitivity to sulfonamides or trimethoprim should avoid exposure to this product. If an allergic reaction occurs (e.g., skin rash, PRECAUTIONS Prescribing antibacterial drugs in the absence of a proven or strongly animals and may increase the risk of development of drug-resistant animal pathogens. The administration of antimicrobials, including sulfadiazine and trimetho- prim, to horses under conditions of stress may be associated with acute diarrhea that can be fatal. If acute diarrhea or persistent changes in fecal consistency are observed, additional doses of EQUISUL-SDT should not be administered and appropriate therapy should be initiated. The safe use of EQUISUL-SDT has not been evaluated in breeding, preg- nant, or lactating horses. Potentiated sulfonamides should only be used in to the fetus. Use of potentiated sulfonamides during pregnancy has been associated with an increased risk of congenital abnormalities that may placenta, are excreted in milk, and may cause hyperbilirubinemia-induced neurotoxicity in nursing neonates. Decreased hematopoetic activity and blood dyscrasias have been asso- ciated with the use of elevated doses and/or prolonged administration of potentiated sulfonamides. EQUISUL-SDT should be discontinued if prolonged clotting times, or decreased platelet, white blood cell or red blood cell counts are observed. Sulfonamides should be used with caution in horses with impaired hepatic function. Although rare, sulfonamide use has been associated with fulminant hepatic necrosis in humans. Neurologic abnormalities have been reported in several species following administration of potentiated sulfonamides. In horses, potentiated sulfonamides have been associated with gait alterations and behavior changes that resolved after discontinuation of the drug. The safe use of EQUISUL-SDT has not been evaluated in horses less than 1 year of age. ADVERSE REACTIONS breeds, ranging from 1 to 25 years of age, which had been treated with either EQUISUL-SDT (n = 182) or with a saline control (n = 88) are summarized in Table 1. At least one episode of loose stool of varying severity was observed in 69 of 182 (38%) of the EQUISUL-SDT-treated horses, and 29 of 88 (33%) saline control horses. Of those animals experiencing loose stool, 2 of 182 (1.1%) of the EQUISUL-SDT-treated horses and 0 of 88 (0%) placebo-treated horses were removed from the Both cases of diarrhea in this study were self-limiting and resolved without treatment within 5–10 days after discontinuation of EQUISUL-SDT. Table 1. Number of Horses with Adverse Reactions During the Field Study with EQUISUL-SDT Adverse Reactions Equisul-SDT (n=182) Saline control (n=88) Loose stool (including diarrhea) 69 (38%) 29 (33%) Colic 3 (1.6%) 2 (2.2%) Diarrhea 2 (1.1%) 0 (0%) To report suspected adverse events, for technical assistance or to obtain a copy of the MSDS, contact Aurora Pharmaceutical LLC at 888-215-1256 or For additional information about adverse drug experience reporting for animal drugs, contact FDA at 1-888-FDA-VETS or online at http://www.fda. gov/AnimalVeterinary/SafetyHealth. CLINICAL PHARMACOLOGY Following oral administration, EQUISUL-SDT is rapidly absorbed and widely distributed throughout body tissues. Sulfadiazine levels are usually highest in the kidney, while the tissue concentration in other tissues is only slightly lower than plasma concentrations. Concentrations of trimethoprim are usually higher in the lungs, kidney, and liver than in the blood. Sulfadiazine and trimethoprim are both eliminated primarily by concentrations of both sulfadiazine and trimethoprim are several-fold higher than blood concentrations. 1 Sulfadiazine and trimethoprim are 20% and 35% bound to plasma protein, respectively. Administration of sulfadiazine and trimethoprim with food has no apparent effect on the absorption of sulfadiazine but the absorption of trimethoprim is decreased. Based on a study in fed horses, trimethoprim concentrations following repeat oral administration of 24 mg/kg EQUISUL-SDT to 6 horses reached peak concentration in 0.5 to 12.0 hours. The median plasma elimination half-life was 3 hours, with a range of 2.31 to 4.96 hours. Peak sulfadiazine concentrations were reached within 1.0 to 12.0 hours in the same study. The median plasma elimination half-life for sulfadiazine was approximately 7.80 hours, with a range of 6.78 to 10.39 hours. Only minor accumulation of both drugs was observed following repeat oral administration of EQUISUL-SDT and both drugs reached steady state by day 3. Sulfadiazine and trimethoprim key steady state parameters associated with administration in 6 fed horses over a period of 7 days are found in Table 2. Table 2. Median (Range) of sulfadiazine and trimethoprim pharmacokinetics parameters following repeat dosing of 24 mg/kg bid EQUISUL-SDT for 7 days to six horses in fed condition Drug Sulfadiazine Trimethoprim Tmax (hr) 4.75 (1.00–12.00) 8.50 (0.50–12.00) Cmax (µg/mL) 17.63 (10.10–31.15) 0.78 (0.60–1.14) AUC 0–12 (last dose) (hr*µg/mL) 159.35 (73.90–282.54) 5.47 (3.31–10.91) T 1/2 (hr) 7.80 (6.78–10.39) 3.00 (2.31–4.96) MICROBIOLOGY EQUISUL-SDT is the combination of the sulfonamide sulfadiazine and trimethoprim. These two drugs block sequential steps in nucleic acids biosynthesis. Sulfadiazine inhibits bacterial synthesis of dihydrofolic acid by competing with para-aminobenzoic acid. Trimethoprim blocks the production of tetrahydrofolic acid from dihydrofolic acid by reversibly inhibiting dihydrofolate reductase. The two drugs act synergistically, reducing the minimum inhibitory concentration of each, while enhancing the bacteriostatic action of each separately to a bactericidal action when combined. EQUISUL-SDT administered as a combined sulfadiazine-trimethoprim dose of 24 mg/kg body weight twice daily for 7 days provided concen- trations of sulfadiazine and trimethoprim with T>MIC90 (%T) values of 100% and 98% respectively. The minimum inhibitory concentration (MIC) values for EQUISUL-SDT against indicated pathogens isolated from lower respiratory tract infections in horses enrolled in a 2010–2011 effective- accordance with the Clinical and Laboratory Standards Institute (CLSI) Approved Standard M31-A3 using a broth microdilution system and 3% lysed horse blood. Table 3. Trimethoprim/sulfadiazine minimum inhibitory concentration (MIC) values a of isolates recovered from horses with lower respiratory infection caused by Streptococcus equi subsp. zooepidemicus treated with EQUISUL-SDT in the U.S. (2010–2011) Treatment Outcome Success Failure Number of Isolates 65 c 46 Time of Sample Collection Pre-Treatment Pre-Treatment MIC 50 b (µg/mL) 0.25/4.75 0.25/4.75 MIC 90 b (µg/mL) 0.25/4.75 0.25/4.75 MIC Range (µg/mL) 0.12/2.4 to 0.5/9.5 0.12/2.4 to 0.5/9.5 a The correlation between in vitro susceptibility data and clinical effectiveness is unknown. b The lowest MIC to encompass 50% and 90% of the most susceptible isolates, respectively. c One isolate of S. equi subsp. zooepidemicus was not tested. EFFECTIVENESS effectiveness of EQUISUL-SDT administered at 24 mg/kg body weight, orally, twice daily for 10 days for the treatment of lower respiratory tract infections in horses caused by Streptococcus equi subsp. zooepidemicus. In this study, a total of 182 horses were treated with EQUISUL-SDT, and 88 horses were treated with saline. One hundred seventy-three horses (112 EQUISUL-SDT and 61 saline) were included in the statistical analysis. Therapeutic success was characterized by absence of fever and no improvement or resolution of clinical signs of lower respiratory tract infection by Day 17. The observed success rates are 58.9% (66/112) and 14.8% (9/61) for the EQUISUL-SDT and saline-treated groups, respectively. Table 4 summarizes the statistical analysis results on the overall success rate. Table 4. Overall Clinical Effectiveness Results Equisul-SDT Saline P-value* Least Square Means 61% 13.1% 0.0123 * P-value and estimated success rates are based on back-transformed mean estimates from the statistical analysis. ANIMAL SAFETY In a target animal safety study, EQUISUL-SDT was administered orally to 32 healthy adult horses at 0 (0X), 24 (1X), 72 (3X), or 120 (5X) mg/kg twice daily for 30 days. Loose stool was the most common abnormal observation. Observations of loose stool (pellets with liquid or unformed/ cowpile stool) occurred more often in horses treated with EQUISUL-SDT with the incidence of loose stool increasing in a dose related manner. All incidents of loose stool were self-limiting and resolved without treatment. higher mean serum creatinine concentrations, and those in the 3X and albumin concentrations. Statistically higher mean neutrophil counts and mean serum gamma glutamyl transferase (GGT) activity were seen in the 1X and 5X groups. Individual animal creatinine, GGT, and albumin concentrations remained within the reference range. Individual animal elevations in absolute neutrophil counts ranged up to 7.09 x 10 3 /mcL (reference range: 1.96-5.31 x 10 3 /mcL). Based upon blood concentrations obtained during the study, it was noted that the sulfadiazine and trimethoprim plasma concentrations did not increase in proportion to dose. For sulfadiazine, a 3X and 5X dose resulted in an average exposure of 2.0X and 2.6X the concentrations observed following a 1X dose. For trimethoprim, the corresponding values were 2.5X and 3.5X as compared to the 1X dose. Furthermore, marked intersubject variability, particularly with sulfadiazine, resulted in substantial overlap of individual subject blood levels across the three dosing groups. STORAGE CONDITIONS Store at 59°– 86° F (15°– 30° C). Brief periods up to 104° F (40° C) are permitted. Protect from freezing. HOW SUPPLIED EQUISUL-SDT is available in the following package sizes: 135 mL 280 mL 560 mL 900 mL [footnote] 1 Kahn CM, Line S, eds. The Merck Veterinary Manual. 10th Ed. Merck & Co. 2010. DEALING WITH PYTHIOSIS I am a lifetime horse person and an avid reader of The Horse. I learn something practical with every edition. I write because my horse Camilla has pythiosis (a rare skin infection that causes ulcerative tumorlike masses). She was diagnosed in August 2017. I have a wonderful vet, Dr. Wendy Cusick, who had us start chemotherapy, then immunotherapy, within three weeks of the original outbreak. We have used the immuno- therapy to clear up her condi- tion three times. The booster shot only lasted two months. She has lived in the barn in a 24-by-24-foot area for the last year. Camilla is an athletic 16.2-hand Hanoverian who excelled in the show ring. Now she is wasting away in the barn. Each relapse has a minor "water incident" associated with it. Camilla will be com- pletely healed, then, if turned out when humidity is above 75% or there is a bit of dew on the ground, she breaks out. What are people doing to manage this? Are there boots to keep the heel and pastern dry without irritating the scarred area? Are people bandaging? Are they using salves? What do they do for turnout? Where do they ride? I'm afraid to take Camilla out of the barn. She has no quality of life. With the recent annual hurricanes coming up from the Gulf of Mexico, pythiosis is almost becoming common here. I sure would like to see an article on how to manage the chronic pythiosis horse. Mary Farr Florida Editor's note: It's been a few years since we've focused on equine pythiosis (TheHorse. com/149887); thanks for the prompt for an update! EPIZOOTIC CLARIFICATION The prompt for contacting you has to do with a commen- tary in the August issue. Spe- cifically, I am referring to lines 8-9, second paragraph in the published version, that states in parenthesis "(outbreaks that could potentially affect humans)." This is incorrect (as a definition of epizootic). African horse sickness virus is not known to affect humans in the course of natural occur- rences of the disease. The only historically recorded instances where the causal virus has been implicated in infecting humans have been seen in those working with certain se- rotypes of the virus in facilities producing commercial African horse sickness vaccine. Peter Timoney, MVB, MS, PhD, FRCVS Lexington, Kentucky Editor's note: We inadver- tently provided the definition for zoonotic instead of epizo- otic (relating to or denoting a disease that is temporarily prevalent and widespread in an animal population) and regret the error. Thank you for bring- ing this to our attention.

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