Sulfadiazine/Trimethoprim and Sulfamethoxazole/Trimethoprim
Potentiated sulfonamides (often referred to as TMS, SMZ-TMP, or co-trimoxazole) are broad-spectrum, bactericidal antimicrobial combinations used extensively in veterinary medicine. **Key Clinical Pearls:** - **Excellent Tissue Penetration:** Highly lipid-soluble, allowing therapeutic concentrations to be reached in difficult-to-penetrate tissues including the **prostate**, **blood-brain barrier (CNS)**, and **eye**. - **Broad Spectrum:** Effective against many Gram-positive and Gram-negative organisms, including many strains of methicillin-resistant *Staphylococcus* (MRSA/MRSP), *Nocardia*, and certain protozoa (*Toxoplasma*, *Coccidia*, *Pneumocystis*). - **Inactivation by Pus:** The drug's efficacy is significantly inhibited by purulent debris and necrotic tissue (which are rich in PABA and thymidine). Abscesses must be drained for the drug to be effective. - **Breed Sensitivity:** Doberman Pinschers have a known breed-specific susceptibility to sulfonamide-induced poly-systemic immune complex disease (hypersensitivity). - **Formulations:** While veterinary-approved trimethoprim/sulfadiazine products exist, many practitioners utilize human-approved trimethoprim/sulfamethoxazole (Bactrimยฎ, Septraยฎ) off-label with similar efficacy.
Mechanism: Potentiated sulfas exhibit **synergistic, bactericidal** activity by sequentially blocking the bacterial folic acid synthesis pathway. Mammalian cells are largely unaffected because they utilize preformed dietary folate rather than synthesizing it. - **Sulfonamides (Bacteriostatic alone):** Act as structural analogues of para-aminobenzoic acid (PABA). They competitively inhibit the bacterial enzyme **dihydropteroate synthase** โ blocks the conversion of PABA to dihydrofolic acid (DFA). - **Trimethoprim (Bactericidal alone):** Reversibly inhibits the bacterial enzyme **dihydrofolate reductase** โ blocks the conversion of DFA to tetrahydrofolic acid (THFA). - **Synergy:** The sequential blockade completely depletes THFA, an essential cofactor for bacterial DNA and RNA synthesis, leading to rapid bacterial cell death. > *Pharmacologic Note:* The optimal *in vitro* ratio for most susceptible bacteria is 1:20 (trimethoprim:sulfa), but synergistic activity occurs across a wide range (1:1 to 1:40). The serum concentration of the trimethoprim component is generally considered the primary driver of efficacy.
Dosing by species
- UTI ยท 30 mg/kg PO q24h ยท PO ยท q24h ยท 7-14 days
- UTI, soft tissue infections ยท 15 mg/kg PO q12h ยท PO ยท q12h ยท 7-14 days
- Susceptible infections ยท 30 mg/kg q12h ยท PO ยท q12h ยท If treating Nocardia, double dose
- Toxoplasmosis ยท 15 mg/kg PO q12h ยท PO ยท q12h ยท 28 days
- Bacterial UTI ยท 30 mg/kg q12h PO ยท PO ยท q12h
- Susceptible infections ยท 30 mg/kg PO twice daily ยท PO ยท q12h
- Coccidiosis ยท 30 mg/kg PO once daily ยท PO ยท q24h ยท 14 days
- Susceptible infections ยท 44 mg/kg once daily IM or IV using 48% suspension ยท IM/IV ยท q24h
- Susceptible infections ยท 25 mg/kg, IV or IM q24h ยท IV/IM ยท q24h
- Susceptible infections (Calves) ยท 48 mg/kg IV or IM q24h ยท IV/IM ยท q24h
Routes of administration
Contraindications
- Hypersensitivity to sulfonamides, thiazides, or sulfonylurea agents
- Severe renal or hepatic impairment
- Doberman pinschers (highly susceptible to immune complex disease)
- Marked blood dyscrasias
- Animals intended for food (in the USA/certain jurisdictions)
Adverse effects
- Dogs: Keratoconjunctivitis sicca (KCS/dry eye - potentially irreversible)
- Dogs: Hypersensitivity reactions (Type 1 anaphylaxis or Type 3 serum sickness, polyarthritis, urticaria, facial swelling)
- Dogs: Acute neutrophilic hepatitis with icterus, idiosyncratic hepatic necrosis
- Dogs: Vomiting, anorexia, diarrhea
- Dogs: Hemolytic anemia, agranulocytosis
- Dogs: Hypothyroidism (with extended therapy)
- Dogs: Crystalluria, hematuria, polyuria, polydipsia
- Cats: Anorexia, crystalluria, hematuria, leukopenias, anemias
- Horses: Transient pruritus (after IV injection), diarrhea, hypersensitivity, hematologic effects
- Injection site reactions: Swelling, pain, tissue damage (IM, SC, or extravasation)
Drug interactions
- Amantadine ยท May cause toxic delirium (reported in humans)
- Antacids ยท May decrease the bioavailability of sulfonamides if administered concurrently
- Cyclosporine ยท May increase the risk of nephrotoxicity
- Digoxin ยท May increase digoxin levels
- Diuretics, Thiazide ยท May increase risk for thrombocytopenia
- Hypoglycemic agents, oral ยท May potentiate hypoglycemic effects
- Methotrexate ยท May displace from plasma proteins and increase risk for toxic effects; can also interfere with MTX assays
- Phenytoin ยท May increase half-life of phenytoin
- Tricyclic antidepressants ยท May decrease efficacy of the antidepressant
- Warfarin ยท May prolong INR/PT and increase bleeding risk
Monitoring
- Clinical efficacy
- Adverse effects (GI, hypersensitivity)
- Complete blood counts (CBC) periodically with chronic therapy
- Schirmer Tear Test (STT) in dogs to monitor tear production (e.g., at 5 days, then every 2-3 weeks)
- Thyroid function tests (baseline and ongoing) for dogs on long-term treatment
Overdose
Manifestations of acute overdosage include GI distress (nausea, vomiting, diarrhea), CNS toxicity (depression, headache, confusion), facial swelling, bone marrow depression, and elevated serum aminotransferases. **Treatment:** - **Oral Overdose:** Empty the stomach following usual protocols and initiate symptomatic/supportive therapy. - **Fluid Therapy:** Maintain hydration. Acidification of urine may increase renal elimination of trimethoprim but increases the risk of sulfonamide crystalluria (especially with sulfadiazine). - **Monitoring:** Monitor complete blood counts and liver parameters. - **Bone Marrow Suppression:** If severe and associated with chronic overdose, may be treated with folinic acid (leucovorin). - *Note:* Peritoneal dialysis is not effective in removing TMP or sulfas from circulation.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.