Bone spavin in Icelandic horses : aspects of predisposition, pathogenesis and prognosis

Bone spavin is an osteoarthrosis (OA) of the centrodistal tarsal joint (CD), the tarsometatarsal joint (TMT) and occasionally the proximal intertarsal joint (PIT). In order to detect and describe the earliest changes compatible with OA, specimens from the CD of young Icelandic horses were examined by high detail radiography (0-6 year, n = 111) and histology (0-4 year, n = 82). Histological chondronecrosis was seen in 33% of the joints, located both medially and laterally. Radiographic sclerosis of the subchondral bone was recorded in 60% of the specimens, most often medially. The medial subchondral bone sclerosis was not correlated with the chondronecrosis, but laterally the bone sclerosis was considered to be secondary to the cartilage lesions.In a field survey, 614 Icelandic horses in the age range of 6-12 year (mean age 7.9 year), and in use for riding were examined radiographically and clinically for OA in the distal tarsal joints, to estimate the prevalence and clinical relevance of the disease in the riding horse population, to evaluate the effect of potential (environmental and intrinsic) risk factors and to estimate the heritability ofthe disease. Radiographic signs ofOA in the distal tarsal joints (RS) were found in 30.3% of the horses and hindlimb lameness after flexion test ofthe tarsus was found in 32.4%. There was a significant correlation between the two diagnostic methods and 16.4% ofthe horses had both ofRS and lameness after flexion test. The survival culling rate ofthe horses in the five following years was significantly affected by RS and horses with the combination of RS and a positive flexion test had the poorest prognosis.The prevalence of RS was strongly correlated to age and tarsal angle (increased as the tarsal angle decreased). The birthplace was also significantly associated with RS, and considered to be an indirect genetic effect. The prevalence oflameness after flexion test was not influenced by age but a significant effect of sire was established. The prevalence was higher for horses that were broken to saddle late (6 year or older) and for horses that had not participated in a stud show. The heritability of age-at-onset of RS, reflecting the predisposition for OA, was estimated to be 0.33 and a similar figure was found for the heritability oflameness after flexion test.It was concluded that bone spavin is a common disease in Icelandic horses affecting their durability, although often subclinically manifested. The high prevalence of histological findings in the young horses (1-4 year) and radiographical findings in the 6-12 year old horses demonstrated a progressive nature of the disease although the progression may be slow. The initiation of the disease was unrelated to the use of the horses for riding and workload was not found to effect the development ofthe disease negatively. The mediumhigh heritability together with the association to the tarsal angle and the radiographic pattern of uneven distribution of load in the CD joint, strongly indicate that poor tarsal conformation or architecture of the distal tarsal joints is the main etiological factor ofthe disease