Vet Guide to Equisul‑SDT® (Sulfadiazine/Trimethoprim) in 2025 🐾

Hello, I’m **Dr Duncan Houston, BVSc**—founder of Ask A Vet. In this detailed guide, learn how **Equisul‑SDT®**, an FDA-approved oral suspension of sulfadiazine and trimethoprim, offers safe, effective treatment for lower respiratory infections in horses caused by Streptococcus zooepidemicus and other susceptible pathogens. Discover proper dosing, side-effect management, contraindications, and veterinarian-led monitoring strategies. 💊🐴

📘 What Is Equisul‑SDT®?

Equisul‑SDT® is a **potentiated sulfonamide**—combining sulfadiazine and trimethoprim to synergistically block sequential steps in folic acid production, converting bacteriostatic to bactericidal action :contentReference[oaicite:3]{index=3}.

✅ Indications & Mechanism

• Indicated for: FDA-approved oral treatment of lower respiratory infections in horses caused by susceptible organisms, primarily Streptococcus equi subsp. zooepidemicus :contentReference[oaicite:4]{index=4}.
• Mechanism: Sulfadiazine blocks dihydropteroate synthase; trimethoprim inhibits dihydrofolate reductase—together suppress DNA synthesis and bacterial growth :contentReference[oaicite:5]{index=5}.

📐 Dosing & Administration

• Standard dose: 24 mg combined actives per kg bodyweight (≈10.9 mg/lb) **twice daily for 10 days**, equivalent to ~2.7 mL per 100 lb :contentReference[oaicite:6]{index=6}.
• How to give it: Shake well, draw dose by syringe through cap, and administer orally over tongue :contentReference[oaicite:7]{index=7}.
• Onset & levels: Both drugs reach steady-state by Day 3; trimethoprim is detectable within 0.5–12 h, and sulfadiazine within 1–12 h post-administration :contentReference[oaicite:8]{index=8}.

⚠️ Safety, Contraindications & Monitoring

• Contraindicated: Known allergy to sulfonamides or trimethoprim, and use in animals meant for human consumption—withdrawal not established :contentReference[oaicite:9]{index=9}.
• Precautions: Avoid extra-label use in pregnant, lactating, or <1-year-old horses—potential for folate deficiency, congenital defects, or hematopoietic side effects :contentReference[oaicite:10]{index=10}.
• Adverse effects: - Loose stool seen in 38% of treated horses; colic in ~1.6%; serious diarrhea rare (~1.1%) and self-limiting :contentReference[oaicite:11]{index=11}. - Potential hepatic or hematologic side effects—monitor for anemia, thrombocytopenia, clotting changes, elevated liver enzymes and creatinine :contentReference[oaicite:12]{index=12}.
• Monitoring Plan: - Evaluate fecal consistency throughout therapy—discontinue if colic or persistent diarrhea occurs. - Perform baseline bloodwork (CBC, liver & kidney) before use, then repeat periodically during prolonged treatments (>10 days).
• Drug interactions: Avoid concurrent use of other nephrotoxins; no specific interactions identified, but monitor organ function.

📈 Efficacy & Evidence

• Field study: 182 horses treated vs. 88 controls showed 59% success in treated group vs. 15% control by Day 7–10 (P=0.0123) :contentReference[oaicite:13]{index=13}.
• Pharmacokinetic data confirm >98% of dosing interval maintains plasma concentrations above MIC₉₀ :contentReference[oaicite:14]{index=14}.
• Safety demonstrated at 1×, 3×, and 5× dosage in healthy horses for 30 days—while mild loose stool was noted, no severe toxicity occurred :contentReference[oaicite:15]{index=15}.

🏡 Real‑World Example

A 500 kg performance horse with confirmed S. zooepidemicus pneumonia received Equisul‑SDT at 24 mg/kg BID. By Day 5, fever resolved, cough reduced, and feed intake improved. Mild soft stool noted on Day 3; resolved after Day 8 without intervention. Follow-up bloodwork remained normal.

❓ Frequently Asked Questions

Can I give human Bactrim instead?

No—Equisul‑SDT is FDA-approved for horses and ensures correct dosing, flavor, and regulatory compliance.

What if my horse develops diarrhea?

Pause treatment and call your vet. Diarrhea can be serious—supportive care may be needed.

Can treatment duration be extended?

Only under veterinary decision, with regular monitoring for hematologic or hepatic side effects.

📌 Final Takeaways

• Equisul‑SDT® is the only FDA-approved oral sulfadiazine/trimethoprim for equine respiratory disease.
• Dosing: 24 mg/kg BID for 10 days (~2.7 mL per 100 lb). Shake before use, administer by syringe.
• Common side-effects include mild diarrhea; serious reactions are rare but require discontinuation.
• Contraindicated in allergy, pregnant or food-intended horses; safety untested in foals.
• Veterinarian monitoring of stool, bloodwork, and clinical response is essential for safe use. 🐴❤️

Considering Equisul‑SDT® for your horse’s respiratory infection? Download the Ask A Vet app for dosage schedules, side-effect alerts, bloodwork tracking, and 24/7 veterinary support—ensuring swift, safe recovery in 2025. 🐾