Sulfadimethoxine
**Sulfadimethoxine** is a long-acting, broad-spectrum sulfonamide antimicrobial agent used widely in veterinary medicine. **Key Clinical Highlights:** * **Primary Uses:** FDA-approved for respiratory, genitourinary, enteric, and soft tissue infections in dogs, cats, horses, and cattle. It is also heavily utilized (though off-label in some species) for the treatment of **coccidiosis**. * **Spectrum of Activity:** Effective against many gram-positive bacteria (e.g., *Streptococcus*, *Staphylococcus*, *Nocardia*) and some gram-negative bacteria. It also has notable activity against protozoa like *Coccidia* and *Toxoplasma*. * **Limitations:** Bacteriostatic when used alone. Efficacy is significantly reduced in the presence of pus, necrotic tissue, or extensive cellular debris. Bacterial resistance is increasingly common. * **Species Specifics:** Highly protein-bound and well-distributed. In dogs, it relies heavily on renal excretion rather than hepatic metabolism, and extensive renal tubular reabsorption gives it a long half-life.
Mechanism: Sulfonamides are bacteriostatic agents that disrupt bacterial folic acid synthesis. * **Mechanism:** Sulfadimethoxine acts as a structural analog of **para-aminobenzoic acid (PABA)**. * **Pathway:** It competitively inhibits the bacterial enzyme **dihydropteroate synthase** โ blocks the incorporation of PABA into **dihydropteroic acid** โ prevents the biosynthesis of **tetrahydrofolic acid** (active folic acid). * **Result:** Without tetrahydrofolic acid, susceptible bacteria and protozoa cannot synthesize purines, pyrimidines, and ultimately DNA/RNA, halting replication. * *Note:* Mammalian cells are unaffected because they utilize preformed dietary folic acid rather than synthesizing it de novo.
Dosing by species
- Coccidiosis (Camelids - E. alpacae, E. lamae, E. punoensis, E. peruviana) ยท 15 mg/kg PO twice daily for 5 days ยท PO ยท q12h ยท 5 days ยท Monitor for signs of polioencephalomalacia.
- Coccidiosis (Camelids - E. macusaniensis) ยท 110 mg/kg PO q24h ยท PO ยท q24h ยท 10-15 days ยท Administer directly to animal, not in water supply.
- Susceptible infections ยท 25 mg/kg PO, IV, or IM once daily ยท PO, IV, IM ยท q24h
- Susceptible infections ยท 100 mg/kg PO, IV or IM once daily ยท PO, IV, IM ยท q24h
- Susceptible infections ยท 55 mg/kg PO, or IV, or SC initially, then 27.5 mg/kg once daily thereafter ยท PO, IV, SC ยท q24h
- Coccidiosis ยท 50 mg/kg once daily for 10-14 days ยท PO ยท q24h ยท 10-14 days ยท Will eliminate oocyst excretion in most cats
- Susceptible infections ยท 25 mg/kg PO, SC or IM once daily ยท PO, SC, IM ยท q24h
- Coccidiosis ยท 25 mg/kg PO once daily for 14 days ยท PO ยท q24h ยท 14 days
- Susceptible infections ยท 110 mg/kg PO or IV once daily ยท PO, IV ยท q24h
Routes of administration
Contraindications
- Hypersensitivity to sulfonamides, thiazides, or sulfonylurea agents
- Severe renal or hepatic impairment
- Doberman Pinschers (highly susceptible to poly-systemic immune complex disease)
- Dehydrated patients (increased risk of crystalluria)
Adverse effects
- Crystalluria, hematuria, and renal tubule obstruction (especially with acidic/concentrated urine)
- Keratoconjunctivitis sicca (KCS or 'dry eye') in dogs
- Hypersensitivity reactions (rashes, dermatitis, fever)
- Bone marrow depression
- Nonseptic polyarthritis (especially in Doberman Pinschers)
- Focal retinitis
- Vomiting and diarrhea (due to altered gut flora)
- Muscle weakness, blindness, ataxia, and collapse (if injected IV too rapidly)
- Tissue irritation and necrosis (if injected IM or SC)
Drug interactions
- Antacids ยท May decrease the oral bioavailability of sulfonamides if administered concurrently.
Monitoring
- Clinical efficacy and resolution of infection/coccidiosis
- Hydration status and urine output
- Schirmer Tear Test (STT) in dogs (monitor for KCS, especially with prolonged use)
- Complete Blood Count (CBC) if used long-term (monitor for bone marrow depression)
- Signs of hypersensitivity (fever, joint pain, rash)
Overdose
Acute toxicity secondary to overdoses occurs only rarely in veterinary species. In addition to standard adverse effects (crystalluria, GI upset, hypersensitivity), **CNS stimulation** and **myelin degeneration** have been noted after very high dosages.
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.