Young rapidly growing domestic animals of several species are subject to angular limb deformities. In sheep the condition has been named 'bentleg' or 'bowie'. It most commonly affects the forelimbs of young rapidly growing ram lambs and may be unilateral or bilateral and either valgus or varus. Most reports of angular limb deformity in sheep describe an asymmetric lesion in the fast-growing physis of the distal radius, and to a lesser extent in the distal metacarpals or metatarsals (Maxie, 2016).
This report describes a case of angular limb deformity affecting nearly 60 percent of a cohort of young merino rams grazed on oats and improved pasture on the Central Tablelands of NSW.
The owner of a self-replacing fine wool merino flock run between Bathurst and Lithgow reported distal limb abnormalities in a high proportion of the young home-bred rams. The rams, a mix of spring 2013- and autumn 2014-drop lambs, appeared to be normal when classed by the owner in March 2014. Affected rams were first noticed by the owner at foot trimming in early August and had a high prevalence of abnormalities when examined in September 2014. The rams had run on a paddock of grazing oats (Avena sativa) with some other grasses and clover, then grazed a perennial ryegrass (Lolium perenne) paddock for 2-3 weeks. They also had intermittent access to a homemade mineral mix (ingredients were dolomite, bentonite, rock phosphate, zinc and copper supplements, urea at 10 percent and salt). The owner had not seen this problem previously (or subsequently).
Seventy-three well-grown 8-12-week-old, fine-wool merino rams were examined on 18 September 2014. Forty-three (59 percent) displayed moderate to marked valgus and occasionally varus deviations of distal front limbs. The deviations most often involved the distal metacarpus and P1 and the physes of these bones were, in some cases, palpably enlarged. Rams from both the spring 2013 and autumn 2014 cohorts were affected and a range of sires contributed affected progeny.
Three affected rams were examined and blood tested on 23 October 2014 and two were necropsied.
On 25 November 2014, 45 of the rams were inspected. Most had moderate to marked fetlock valgus deviations of both front feet.
Both distal forelimbs (skinned, distal radius/ulna to hoof), and one rib from two euthanased rams were submitted for examination.
Case 1 (ram 7686). The left lateral, right lateral and right medial claw hooves were overgrown. There was a moderate medial deviation (varus) of the left lateral claw. On longitudinal section, the distal metacarpal physes and metaphyses of both front legs were mildly and irregularly thickened. Metacarpal bones also showed some loss of definition of distal physeal-metaphyseal junctions. The proximal metacarpus was grossly normal. The rib and costochondral junction were grossly normal.
Case 3 (ram 7616). There was mild lateral deviation (valgus) of all claws, which were mildly to moderately overgrown. The left carpometacarpal joint was moderately enlarged, with enlargement of carpal bones and moderate thickening of the joint capsule. On longitudinal section, the distal metacarpal physes and metaphyses of both front legs were mildly and irregularly thickened. Metacarpal bones also showed was some loss of definition of the distal physeal-metaphyseal junctions. The proximal metacarpi were grossly normal bilaterally. The rib and costochondral junction were grossly normal.
Distal ends of metacarpal bones were cut longitudinally using a bandsaw, decalcified and processed for histological examination. The areas examined included epiphyseal articular cartilage, the epiphysis, physis and metaphysis. Various physeal lesions were present in some physes. These included focal areas of necrotic physeal cartilage, sometimes associated with disorderly tongues of proliferative, hypertrophic chondrocytes, which extended into the metaphysis immediately beneath the cystic area. Focal horizontal clefts were also present within the proliferative or hypertrophic zones of the physeal cartilage. On the metaphyseal side of such clefts the physeal cartilage was narrow and irregular with loss of normal endochondral ossification structure. Associated with this loss of normal endochondral ossification, adjacent metaphyseal bone was condensed across the chondro-osseous junction.
On the 18 September and the 23 October three young rams were blood tested. On all occasions, calcium and magnesium were within the normal range. Phosphorus levels were in the high normal range, except for one sample (collected on the 23 October) in which the blood phosphorus was slightly above normal (2.66 mmol/L, normal 1.13-2.58 mmol/L). Vitamin D assays were not available at the time of examination.
The affected rams were culled as being unsuitable for breeding. The owner has not grazed young rams on oats since this incident and has not seen the problem in subsequent years. He considers that the problem coincided with a season in which the ram lambs grew exceptionally well.
Both necropsied rams have angular limb deformities associated with lesions of the distal metacarpal physis (growth plate). The lesions reflect defective endochondral ossification. The multifocal clefts and necrosis present within physeal cartilage are not consistent with rickets. The lesions have some similarities to osteochondrosis, however are not specific for osteochondrosis. Osteochondrosis-like lesions have been reported previously in angular limb deformities in sheep. Osteochondrosis is characterised by focal failure of endochondral ossification and may affect the physeal cartilage and/or the cartilage underlying epiphyseal articular cartilage. The disease has a variety of different clinical manifestations in pigs, horses, dogs, cattle and sheep. The cause of angular limb deformity in sheep is unknown, but there is a strong association with high-energy rations and rapid growth (Maxie, 2016).