Chronic idiopathic myopathy in Icelandic horses: A case series

Exertional myopathies are recognised as a cause of poor performance in equines. In Icelandic horses presenting reduced performance and/or multi-limb lameness, no specific myopathy has been identified.

To characterise the clinical presentation and histopathological findings in muscle biopsy samples from Icelandic horses demonstrating poor performance.

Case series.

Phenotypic characteristics, clinical examination and diagnoses of 17 Icelandic horses were studied. All horses had a resting serum creatine kinase (CK) and aspartate transaminase activities measured, and CK activities 4-6 and 24 h post-exercise were measured in some horses. The semimembranosus muscle was biopsied in all horses and used to categorise horses into two groups: those with histopathological evidence of chronic idiopathic myopathy and those with normal biopsy findings.

Twelve horses displayed non-specific histological features of muscle degeneration and regeneration consistent with a myopathy. The affected horses had significantly increased CK activities after exercise (median 1637 U/L, IQR 861-2480 U/L) compared with the group with histologically normal muscle (median 234 U/L, IQR 211-457 U/L, p = 0.02). Phenotypic traits, such as tachypnoea, fatigue, stumbling and reluctance to tölt or canter, were present in both groups.

The small sample size and absence of a control group with normal performance.

This case series suggests the presence of a novel myopathy in Icelandic horses. In horses of this breed, exhibiting decreased performance, multi-limb lameness and/or increased serum CK activity at rest or post-exercise, an underlying myopathy should be considered.