THE MERCK VETERINARY MANUAL
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Physitis in Horses

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Physitis involves swelling around the growth plates of certain long bones in young horses. It can be a component of osteochondrosis. Suggested causes include malnutrition, conformational defects, faulty hoof growth, excessive exercise, obesity, and toxicosis. The condition is seen frequently in well-grown, fast-growing, heavy-topped foals during the summer when the ground is dry and hard, and on stud farms where the calcium:phosphorus ratio in the diet is imbalanced. This suggests that it is a result of overload of the physeal area due to excessive loading or weakened bone and/or cartilage or a combination of these factors. Rapid growth may increase periosteal tension at the physeal attachment causing increased flaring.

Physitis most commonly involves the distal extremities of the radius, tibia, third metacarpal or metatarsal bone, and the proximal aspect of the first phalanx. It is characterized by flaring at the level of the growth plate, giving a typical “boxy” appearance to the affected joints. Radiographs aid the clinical assessment. Microscopically, the physeal cartilage appears crushed and thinned, and new bone is formed.

Treatment consists of reducing food intake to reduce body weight or at least growth rate; confining exercise to a yard or a large, well-ventilated loose box with a soft surface (eg, peat moss, deep straw, shavings, or sand); ensuring that the feet are carefully and frequently trimmed; and correcting the diet if necessary. The calcium:phosphorus ratio should be adjusted to 1.6:1, and protein content limited to <10% of dry matter. In general terms, bran should not be fed, and dicalcium phosphate or bone flour (10–30 g daily) should be added to the diet. Vitamin D supplements (PO or parenteral) are indicated, but the dosage must be monitored closely to avoid hypervitaminosis D.

As a preventive measure, the older foal or yearling that is fat or heavy-topped should be watched carefully for clinical signs, especially when the ground is hard and dry. When these conditions prevail, feed rations and exercise should be restricted.

Last full review/revision March 2012 by Stephen B. Adams, DVM, MS, DACVS; Joerg A. Auer, DrMedVet, Dr h c, MS, DACVS, DECVS; James K. Belknap, DVM, PhD, DACVS; Jane C. Boswell, MA, VetMB, CertVA, CertES (Orth), DECVS, MRCVS; Peter Clegg, MA, Vet MB, PhD, CertEO, DECVS, MRCVS; Andrew L. Crawford, BVetMed, CertES (Orth), MRCVS; Jean-Marie Denoix, DVM, PhD, Agregé; Marcus J. Head, BVetMed, MRCVS; C. Wayne McIlwraith, BVSc, PhD, DSc, FRCVS, DACVS, DACVSMR; James Schumacher, DVM, MS, DACVS, MRCVS; John Schumacher, DVM, MS, DACVIM; Roger K. W. Smith, MA, VetMB, PhD, DEO, DECVS, MRCVS; Chris Whitton, BVSc, FACVSc, PhD

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