🐎 Vet’s 2025 Guide to Myofibrillar Myopathy in Horses

By Dr Duncan Houston BVSc

1. What Is MFM?

Myofibrillar Myopathy (MFM) is a muscle disorder characterised by defective alignment and deterioration of muscle fibres, with abnormal accumulation of structural proteins like desmin and disrupted Z‑discs—hallmarks visible under specialised stains in muscle biopsy samples :contentReference[oaicite:3]{index=3}. It’s a relatively newly identified myopathy, distinct from glycogen storage disorders such as PSSM.

2. Who Is Affected?

MFM primarily affects Warmbloods and Arabian-type horses. Warmbloods typically show pain and refusal to collect, while Arabians may tie up unusually during or after exercise :contentReference[oaicite:4]{index=4}. Most cases are first recognized in horses aged 6–10 when athletic demands increase :contentReference[oaicite:5]{index=5}.

3. Clinical Signs of MFM

• Poor performance, exercise intolerance, declining stamina
• Muscle stiffness, shifting hind‑leg lameness, reluctance to move forward or collect
• Muscle pain or discomfort during or after work
• Arabians may show intermittent cramping, tying‑up episodes, and elevated muscle enzymes (CK/AST); Warmbloods usually have normal enzymes :contentReference[oaicite:6]{index=6}.

4. How It Differs from Other Myopathies

MFM can resemble PSSM or RER, but differs in diagnostics and management. Unlike PSSM1, there is no known specific genetic mutation; unlike RER, enzyme levels may not rise, especially in Warmbloods :contentReference[oaicite:7]{index=7}. Definitive diagnosis requires biopsy and detection of desmin-positive aggregates :contentReference[oaicite:8]{index=8}.

5. Diagnosing MFM

1. Thorough clinical and lameness exam ruling out orthopaedic, dental, saddle-fit, or neurologic issues.
2. Bloodwork – CK/AST may be elevated in Arabians, typically normal in Warmbloods :contentReference[oaicite:9]{index=9}.
3. Muscle biopsy (gluteal or semimembranosus), stained for desmin/Z-disc proteins – gold standard to confirm MFM :contentReference[oaicite:10]{index=10}.
4. Exclude PSSM (genetic test) and other exertional myopathies :contentReference[oaicite:11]{index=11}.

6. Underlying Causes & Genetics

MFM appears to have a genetic component, potentially polygenic, and heritable forms have been identified in Warmblood family lines :contentReference[oaicite:12]{index=12}. However, no single mutation has been identified. Environmental and training stress may influence disease progression.

7. Management & Treatment Approach

7.1 Diet Strategy

• High‑quality protein (12–14%) to support muscle repair, including essential amino acids like cysteine and branched‑chain amino acids :contentReference[oaicite:13]{index=13}.
• Forage 1.5–2% body weight/day, with grass or grass-legume hay; limit high‑NSC feeds unless needed for condition :contentReference[oaicite:14]{index=14}.
• Supplement antioxidants (vitamin E, CoQ10) and specialized products like MFM Pellets for cellular protection :contentReference[oaicite:15]{index=15}.

7.2 Exercise & Conditioning

• Daily turnout with room to move; long slow warmups to increase structural protein turnover :contentReference[oaicite:16]{index=16}.
• Warmbloods: schedule 3–4 days on, 1–2 days off work to allow muscle recovery :contentReference[oaicite:17]{index=17}.
• Arabians: consistent exercise helps prevent tying-up.

7.3 Medical & Supportive Therapies

• NSAIDs/analgesics during flare-ups of muscle pain.
• Physiotherapy, massage, stretching, and core-strengthening.
• Monitor muscle enzyme levels for Arabians.
• Review fitness and tack to minimise strain.

8. Prognosis & Long-Term Outlook

MFM is not curable, but many horses maintain good performance with proper management. Warmbloods may continue competition with careful scheduling and rest; Arabians need muscle enzyme monitoring. Some may need early retirement from high-level athleticism :contentReference[oaicite:18]{index=18}.

9. Breeding & Herd Considerations

Because heritability is possible, breeding horses with confirmed MFM should be approached with caution. No carrier tests exist—owners should aim to selectively reduce incidence by not breeding affected individuals.

10. Ask A Vet Support 🩺

At Ask A Vet, we support MFM cases through:

• Video/photo gait and posture assessment
• Diet review & tailoring, including nutritionist referrals
• Exercise schedule design & warm‑up protocols
• Coordination of diagnostic testing—enzymes, biopsy planning
• Management of flare‑ups and performance check‑ins

Download the Ask A Vet app for expert muscle-care guidance and long-term wellness support in 2025 and beyond! ❤️

11. Quick Reference Table

12. Final Thoughts

Myofibrillar Myopathy is a challenging but manageable muscular condition. With early recognition, dietary optimisation, consistent turnout, and professional diagnostics, many affected horses enjoy robust athletic lives. Partner with Ask A Vet for ongoing nutrition coaching, exercise programming, and veterinary support to keep your horse strong and comfortable in 2025 and beyond. 🐴💪