Pachydermatoperiostosis, a rare form of hypertrophic osteoarthropathy, is of unknown etiology and previously thought limited to humans. The only periosteal reaction previously reported in prosimians is related to renal disease. Notation of hypertrophic osteoarthritis in three prosimians led to recognition that this was the first non-human documentation of the disease. Three related red ruffed lemurs (Varecia variegatus rubra) had diaphyseal periosteal reaction classic for hypertrophic osteoarthropathy. Workup was negative for known underlying causes and for the secondary hyperparathyroidism which produces bone alterations in black Eulemur macao, black and white Varecia variegatatus varigatus and ringtail Lemur catta lemurs. Recognition of facial coarsening allows identification of the primary form of hypertrophic osteoarthropathy, categorized in humans as primary hypertrophic osteoarthropathy. This is the first recognition of the phenomenon in the order primates, exclusive of humans, and represents a new model for this rare disease
