Amphotericin B is one of the oldest and most potent antifungal agents available in veterinary medicine. A polyene macrolide antibiotic derived from Streptomyces nodosus, it remains a critical tool for treating severe, life-threatening systemic fungal infections in birds, particularly aspergillosis. Despite its significant toxicity profile, amphotericin B is often indispensable when other antifungals have failed or when rapid fungicidal action is required. This is a prescription medication that must be administered under close veterinary supervision.
What Is Amphotericin B Used For?
- Aspergillosis — severe or disseminated aspergillosis in birds, often used in combination with azole antifungals
- Systemic candidiasis — invasive yeast infections unresponsive to other treatments
- Other systemic mycoses — cryptococcosis, zygomycosis, and other deep fungal infections
- Nebulization therapy — topical delivery directly to the avian respiratory tract and air sacs
In avian medicine, amphotericin B is most commonly used as a nebulized treatment to deliver the drug directly to infected air sacs and lungs, reducing systemic toxicity while maximizing local drug concentrations. Intratracheal and intravenous routes are also used in severe cases.
How Does Amphotericin B Work?
Amphotericin B binds to ergosterol in the fungal cell membrane, creating pores that cause leakage of essential intracellular ions (potassium, sodium, hydrogen) and small molecules. This disruption of membrane integrity leads to fungal cell death. The drug is fungicidal at therapeutic concentrations, meaning it kills fungi rather than merely inhibiting their growth.
The selectivity of amphotericin B for fungal cells over host cells is based on its preferential binding to ergosterol (found in fungal membranes) over cholesterol (found in animal cell membranes). However, this selectivity is incomplete, which accounts for the drug’s toxicity to host tissues, particularly the kidneys.
Dosage
Dosing must be determined and monitored by an experienced avian veterinarian.
| Route | Typical Dose | Frequency | Notes |
|---|---|---|---|
| Nebulization | 1 mg/mL in sterile water | Every 12 hours, 15—30 min sessions | Preferred route in avian medicine |
| Intravenous | 1—1.5 mg/kg | Every 48—72 hours | Dilute in 5% dextrose, infuse slowly |
| Intratracheal | 1 mg/kg diluted | Every 12—24 hours | Direct instillation for tracheal lesions |
Lipid-encapsulated formulations (AmBisome, Abelcet) are significantly less nephrotoxic than conventional amphotericin B deoxycholate (Fungizone) but are considerably more expensive. When intravenous therapy is necessary, lipid formulations are preferred in avian patients to minimize kidney damage.
Nebulization is the most commonly used route in birds because it delivers drug directly to the respiratory system — the primary site of aspergillosis infection — while minimizing systemic absorption and toxicity.
Side Effects
Common side effects include:
- Decreased appetite during treatment
- Mild lethargy
- Local irritation of airways during nebulization (transient coughing or respiratory effort)
Serious side effects requiring immediate veterinary attention:
- Nephrotoxicity — kidney damage is the most significant and dose-limiting toxicity; signs include decreased urine output, elevated uric acid (birds excrete uric acid rather than urea), lethargy, and dehydration
- Hypokalemia — potassium wasting through damaged renal tubules, causing weakness or cardiac arrhythmias
- Anemia — suppression of red blood cell production with prolonged use
- Phlebitis — vein inflammation at IV injection sites
- Anaphylaxis — rare but possible acute allergic reactions during IV infusion
In birds, early signs of toxicity may include polyuria (increased watery component of droppings), fluffed feathers, and progressive lethargy. Elevated uric acid on blood work is a key indicator of renal damage.
Monitoring
- Baseline renal values (uric acid, phosphorus) and complete blood count before starting therapy
- Renal values every 2 to 3 days during intravenous therapy
- Electrolytes (potassium, sodium) during intravenous therapy
- Hydration status assessment at each treatment visit
- Pre-hydration with IV or subcutaneous fluids before each IV dose to reduce nephrotoxicity
Avian-Specific Considerations
The unique avian respiratory anatomy makes nebulization therapy particularly effective:
- Air sac system — birds have nine air sacs that extend throughout the body cavity and even into some bones; nebulized particles can reach these structures directly
- Unidirectional airflow — unlike mammalian tidal breathing, avian respiration moves air in one direction through the lungs, potentially improving nebulized drug distribution
- Pneumatic bones — hollow bones connected to the air sac system can harbor fungal infection; nebulized drug may penetrate these sites
- Nebulizer particle size — particles of 0.5 to 3 micrometers are ideal for reaching the lower respiratory tract and air sacs in birds
- Treatment environment — birds should be nebulized in a small, enclosed chamber to maximize drug exposure while minimizing handler exposure
Combination Therapy
Amphotericin B is frequently used in combination with other antifungals for aspergillosis:
- Nebulized amphotericin B + oral voriconazole — the most common current combination for avian aspergillosis
- Nebulized amphotericin B + oral itraconazole — an alternative when voriconazole is not available or tolerated
- Terbinafine — may be added as a third agent in refractory cases
Surgical debridement of accessible fungal granulomas combined with medical therapy often yields the best outcomes for localized aspergillosis.
Storage
Conventional amphotericin B (Fungizone) should be stored in the refrigerator (2—8 degrees C / 36—46 degrees F) and protected from light. Reconstituted solutions should be used within 24 hours if refrigerated. Lipid formulations have specific storage requirements — follow manufacturer guidelines. Nebulization solutions should be prepared fresh for each treatment session.
Always consult your avian veterinarian before starting, stopping, or changing any medication. Amphotericin B therapy requires close medical supervision and regular monitoring. This information is for educational purposes and does not replace professional veterinary advice.