Avian Gastric Yeast (Macrorhabdus ornithogaster) 2025 🐦 | Vet Guide by Dr Duncan Houston BVSc

Avian gastric yeast, historically called “megabacteria,” is a fungal infection—***Macrorhabdus ornithogaster***—that targets the digestive system of many bird species. Left undiagnosed, it causes weight loss, malnutrition, and often death. In this thorough 2025 guide, we explore causes, signs, diagnostics, treatments, supportive care, and proactive prevention for your birds and flock.

1. 🧬 What Is Avian Gastric Yeast?

This rod-shaped yeast invades the junction between the proventriculus (glandular stomach) and gizzard. Once thought to be a bacterium, we now know it’s a fungus impacting species from budgerigars and cockatiels to chickens and parrots :contentReference[oaicite:3]{index=3}.

2. 🐦 Affected Birds & Risk Factors

• Common in budgies, cockatiels, finches, chickens—but can infect many species :contentReference[oaicite:4]{index=4}.
• Often seen in birds >2 years or those under stress from overcrowding, poor diet, breeding, or illness :contentReference[oaicite:5]{index=5}.

3. 🚨 Clinical Signs: Watch for Two Forms

Acute

• Sudden refusal to eat, possible bloody regurgitation, rapid decline, death within days :contentReference[oaicite:6]{index=6}.

Chronic

• Persistent weight loss despite appetite
• Regurgitation, undigested seeds in droppings, diarrhea :contentReference[oaicite:7]{index=7}
• Lethargy, fluffed feathers, dull appearance :contentReference[oaicite:8]{index=8}
• Proventricular dilation on radiographs in some birds :contentReference[oaicite:9]{index=9}

4. 🔬 Diagnosis: Confirm the Infection

• Fresh fecal/crop wash cytology: rod-shaped yeast under microscope :contentReference[oaicite:10]{index=10}.
• PCR testing: more accurate, especially with intermittent shedding :contentReference[oaicite:11]{index=11}.
• X‑rays: may show proventricular enlargement :contentReference[oaicite:12]{index=12}.
• Histopathology: definitive but usually post-mortem :contentReference[oaicite:13]{index=13}.

5. 💊 Treatment Strategies

All treatment protocols require avian veterinarian oversight:

• Amphotericin B: 100 mg/kg by mouth twice daily for 4 weeks—most effective, but stressful :contentReference[oaicite:14]{index=14}.
• Fluconazole: alternative if amphotericin not tolerated :contentReference[oaicite:15]{index=15}.
• Nystatin: 300,000–600,000 IU/kg twice daily for ~10–14 days; less effective :contentReference[oaicite:16]{index=16}.
• Sodium benzoate: experimental via drinking water with mixed efficacy :contentReference[oaicite:17]{index=17}.
• Acidifiers: apple cider vinegar or vitamin C can help by lowering stomach pH :contentReference[oaicite:18]{index=18}.

6. 🛡️ Supportive & Environmental Care

• Ensure clean, digestible diet and abundant water.
• Minimize stress: improve hygiene, enrich environment, isolate infected birds.
• Weigh birds regularly to monitor intake and response :contentReference[oaicite:19]{index=19}.
• Treat entire flock due to high contagion; disinfect cages and utensils often :contentReference[oaicite:20]{index=20}.

7. 📉 Prognosis & Monitoring

• Mortality ranges between 10–80%, based on infection severity and treatment success :contentReference[oaicite:21]{index=21}.
• Post-treatment fecal exams or PCR at 14 and 30 days are essential to confirm clearance :contentReference[oaicite:22]{index=22}.
• Relapse can occur, especially if environmental stressors persist :contentReference[oaicite:23]{index=23}.

8. ✅ Summary Table

9. 🧭 Final Thoughts

Avian gastric yeast is a serious but treatable condition if caught early. The best outcomes come from thorough diagnostics, strong antifungal protocols, supportive care, and strict environmental control. Regular monitoring and proactive flock management are key to recovery and prevention.

— Dr Duncan Houston BVSc

👉 For treatment schedules, medication reminders, or flock screening tools, visit AskAVet.com or download the Ask A Vet app. Our avian team is ready to support your bird’s health journey! 📱🐦