Rifampin
Rifampin is a highly lipophilic, semi-synthetic antimicrobial agent belonging to the rifamycin class. It is a cornerstone therapy in veterinary medicine for treating deep-seated, intracellular infections, most notably **_Rhodococcus equi_** in foals. Because of its exceptional ability to penetrate caseous material, bone, abscesses, and macrophages, it is highly effective against intracellular pathogens like *Mycobacterium* spp., *Staphylococcus aureus*, and *Lawsonia intracellularis*. It also possesses some antifungal and antiviral activity at high concentrations. > **Clinical Pearl:** Rifampin must **never** be used as a monotherapy. Bacteria can develop resistance rapidly via a single-step mutation. It is always prescribed in combination with other antimicrobials (e.g., macrolides like erythromycin or clarithromycin in horses, or cephalosporins/sulfonamides in dogs).
Mechanism: Rifampin can be bactericidal or bacteriostatic depending on the concentration and the susceptibility of the target organism. * **Target:** It binds strongly to the beta subunit of bacterial **DNA-dependent RNA polymerase**. * **Pathway:** Binding inhibits the enzyme โ suppresses the initiation of chain formation for RNA synthesis โ halts bacterial protein production โ cell death. * **Selectivity:** Rifampin is highly selective for bacterial enzymes and does not inhibit mammalian RNA polymerase, providing a wide margin of safety for the host.
Dosing by species
- Susceptible infections (e.g., mycobacteriosis, Rhodococcus equi, chlamydiosis, bartonellosis) ยท 10-15 mg/kg ยท PO ยท q24h ยท Duration depends on the specific infection being treated ยท Must be used in combination with other antimicrobial drugs to prevent the emergence of resistant organisms.
- Treatment of Rhodococcus equi (C. equi) infections in foals ยท 5 mg/kg PO two times daily ยท PO ยท q12h ยท With erythromycin 15-25 mg/kg, PO q12-24h. Clarithromycin may be superior due to erythromycin side effects.
- Treatment of Rhodococcus equi (C. equi) infections in foals ยท 5 mg/kg PO two times daily or 10 mg/kg PO once daily ยท PO ยท q12h or q24h ยท 4-9 weeks ยท With erythromycin 25 mg/kg, PO q6-8h.
- Treatment of proliferative enteropathy caused by Lawsonia intracellularis in foals ยท 10 mg/kg PO once daily ยท PO ยท q24h ยท minimum of 21 days ยท In combination with erythromycin estolate (25 mg/kg PO q6-8h)
- Treatment of mycobacteriosis ยท 45 mg/kg PO once daily ยท PO ยท q24h ยท In combination with ethambutol (30 mg/kg PO once daily) and one of the following: clofazimine (6 mg/kg PO once daily) or isoniazid (30 mg/kg PO once daily).
- Combination therapy of atypical Mycobacteria infections; treatment of resistant Staph endocarditis ยท 10-20 mg/kg PO q8-12h ยท PO ยท q8-12h ยท In combination with amoxicillin/clavulanate or trimethoprim/sulfa
Routes of administration
Contraindications
- Hypersensitivity to rifampin or other rifamycins
- Use with extreme caution in patients with preexisting hepatic dysfunction
- Pregnancy (teratogenic at high doses)
- Pre-existing liver disease or hepatic dysfunction
Adverse effects
- Red-orange discoloration of bodily fluids (urine, tears, sweat, saliva)
- Gastrointestinal distress (anorexia, vomiting, diarrhea)
- Elevated liver enzymes and potential hepatotoxicity
- Rashes and erythema (noted in cats and humans)
- In horses (especially with macrolides): mild diarrhea to severe enterocolitis, hyperthermia, and acute respiratory distress
- Intravenous use in horses (rare): CNS depression, sweating, hemolysis, anorexia
- Elevated serum hepatic enzymes
- Clinical hepatitis
- Orange-red discoloration of urine, saliva, tears, and feces
- Gastrointestinal upset (anorexia, vomiting, diarrhea)
Drug interactions
- Fluoroquinolones ยท In vitro antagonism has been reported; concurrent use should be avoided.
- Barbiturates ยท Decreased serum levels and shortened half-life due to hepatic microsomal enzyme induction by rifampin. ยท major
- Benzodiazepines (e.g., diazepam) ยท Decreased serum levels and shortened half-life due to hepatic enzyme induction.
- Chloramphenicol ยท Decreased serum levels and shortened half-life due to hepatic enzyme induction.
- Corticosteroids ยท Decreased serum levels and shortened half-life due to hepatic enzyme induction.
- Dapsone ยท Decreased serum levels and shortened half-life due to hepatic enzyme induction.
- Ketoconazole ยท Decreased serum levels and shortened half-life due to hepatic enzyme induction.
- Propranolol ยท Decreased serum levels and shortened half-life due to hepatic enzyme induction.
- Quinidine ยท Decreased serum levels and shortened half-life due to hepatic enzyme induction.
- Warfarin ยท Decreased serum levels and shortened half-life due to hepatic enzyme induction.
- Theophylline ยท Increased rate of metabolism of theophylline due to hepatic enzyme induction. ยท major
- Itraconazole ยท Increased rate of metabolism of itraconazole due to hepatic enzyme induction, leading to subtherapeutic antifungal levels. ยท major
Monitoring
- Clinical efficacy and resolution of infection
- Liver function tests (especially with long-term therapy)
- Chest radiographs and plasma fibrinogen levels (prognostic indicators for C. equi infections in foals after 1 week of therapy)
- Baseline and periodic serum hepatic enzyme levels (ALT, AST, ALP, Bilirubin)
- Clinical signs of hepatitis (icterus, anorexia, vomiting, lethargy)
Overdose
Clinical signs of oral rifampin overdose are generally extensions of its adverse effects, including **gastrointestinal distress**, **orange-red coloring of fluids**, and **skin erythema** (noted particularly in cats). Massive overdoses carry a significant risk of **hepatotoxicity**. Dogs may exhibit central nervous system depression. **Treatment:** * Empty the gut following standard decontamination protocols for massive oral overdosage. * Monitor liver enzymes closely. * Initiate supportive treatment as necessary.
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.