Sulfadiazine

Sulfadiazine is a sulfonamide antimicrobial used alone in some historical regimens and more often in potentiated combinations. Trimethoprim broadens antibacterial use through sequential folate-pathway blockade, while pyrimethamine combinations have selected antiprotozoal applications. Its practical veterinary role therefore depends on whether the drug is administered alone or with a partner.

Group

Sulfonamide antibacterial

Mechanism of Action

Sulfadiazine competes with para-aminobenzoic acid at microbial dihydropteroate synthase, reducing formation of folate required for nucleic-acid synthesis. The resulting effect is usually bacteriostatic. Trimethoprim or pyrimethamine inhibits a subsequent folate-pathway step, producing sequential blockade in susceptible organisms. The partner drug and organism determine the activity of the combination.

Dose Rates

Dogs

Use / stage	Dose	Route	Schedule	Instructions
Susceptible bacterial infection — Loading	100 mg/kg	IV, PO	Once
Susceptible bacterial infection — Maintenance	50 mg/kg	IV, PO	Every 12 h

Cats

Use / stage	Dose	Route	Schedule	Instructions
Susceptible bacterial infection — Loading	100 mg/kg	IV, PO	Once
Susceptible bacterial infection — Maintenance	50 mg/kg	IV, PO	Every 12 h

Indications

Treatment of selected sulfadiazine-susceptible bacterial infections in veterinary patients.
Treatment of selected susceptible respiratory, urinary or soft-tissue infections with a trimethoprim–sulfadiazine combination.
Treatment of equine protozoal myeloencephalitis with an appropriate sulfadiazine–pyrimethamine regimen.
Combination treatment of selected toxoplasmosis or other susceptible protozoal infections under a disease-specific specialist plan.

Contraindications

Sulfonamide hypersensitivity
Doberman pinschers may have greater susceptibility to reactions.

Interactions

Warfarin, methenamine, dapsone, etodolac, methotrexate and pyrimethamine may interact; cyclosporine exposure may fall.
IV trimethoprim-sulfonamide combinations with equine detomidine can provoke arrhythmias; this warning is not a general oral interaction.

Adverse effects

Hypersensitivity, cytopenias, hepatopathy, keratoconjunctivitis sicca, arthropathy and thyroid suppression with prolonged therapy.

Preparations

Preparation	Form	Strength	Route
Sulfadiazine 500 mg	Tablet	500 mg / tablet	PO

Monitoring and Administration

Assess clinical and microbiological response; monitor CBC, hepatic signs, tear production and thyroid interpretation during prolonged treatment. A combination dose must not be treated as a single sulfonamide component dose.

Potentiated trimethoprim-sulfadiazine protocols depend on the combination ratio and combined-total dose; they cannot be entered as the same milligram amount of sulfadiazine alone.