27 Incidence rate of lesions in the distal forelimb of growing, exercising sheep as a model for horses

Abstract

The use of circular exercise is prominent in the equine industry, but this practice may result in joint damage which can be indicated by macroscopic characteristics such as cartilage lesions. Compromised integrity of the joint often results in pain that presents as lameness and decreased performance. Cartilage lesions can also be indicative of chronic disabling conditions such as osteoarthritis. To evaluate the effects of circular exercise on joint health, 42 sheep (4 mo old, 40 ± 1 kg) were used as a model for young horses. Animals were striated by sex and weight, then randomly assigned to the following treatments: straight line slow, straight line fast, small circle slow, small circle fast, large circle slow, large circle fast, and non-exercised control. Straight line exercise was conducted on a treadmill, and circular exercise was conducted via mechanical walker in a clockwise direction on a 12-m diameter circle (small) or an 18-m diameter circle (large). Exercise occurred 4 d/wk for 12 wks starting at 390 m/d (slow: 5.0 min/d at 1.3 m/s; fast: 4 min/d at 1.3 m/s and 0.7 min/d at 2.0 m/s). Exercise increased weekly until animals reached 2,340 m/d (slow: 30 min/d at 1.3 m/s; fast: 24 min/d at 1.3 m/s and 3.9 min/d at 2.0 m/s). Sheep were housed in group pens (34 m2, bedded with straw) and fed a standard balanced diet with free choice grass hay. At the end of the study,animals weighed an average of 54 ± 1 kg and were humanely euthanized via captive bolt. All carpal and metacarpophalangeal joint capsules were opened and analyzed for number of lesions and lesion surface area using ImageJ software (NIH). Data were analyzed by Proc Freq and Proc Mixed in SAS 9.4. Only one sheep (straight line fast group) displayed an acute lameness for 5 d. At the completion of the study, sheep had reached 70% of their mature BW, increasing their size by 35% during the 3 mo, averaging 0.17 kg/d. Within the 42 sheep, a total of 322 lesions were observed with 99% in the carpal joint and 1% in the metacarpophalangeal joint. Average lesion surface area in the carpal joint was 22 ± 1 mm2 versus 1.6 ± 0.3 mm2 in the metacarpophalangeal joint (P < 0.05). Within the carpal joint, 52% of lesions were found in the carpometacarpal joint, 36% in the middle carpal joint, and 12% in the radiocarpal joint. No differences were found among treatment groups or limbs. These data suggest exercise was not the sole contributor to lesion development, emphasizing that growth rate, diet, and genetic components must also be considered when evaluating the cause of cartilage lesions.