Cutaneous dyskeratosis in free-ranging desert tortoises, Gopherus agassizii, in the Colorado Desert of Southern California

High mortality rates and a shell disease originally described as shell necrosis were observed in the population of desert tortoises (Gopherus agassizii) in the Colorado Desert on the Chuckwalla Bench Area of Critical Environmental Concern, Riverside County, California, USA. In a retrospective review of photographic slides of desert tortoises from the Chuckwalla Bench, the disease was evident in 1979 when tortoises on a permanent study site were first photographed. Lesions were present in both sexes and all size classes of tortoises in all years in which tortoises were photographed. In those tortoises where sequential photographs were taken, the most severe lesions were seen in 1988. Although the disease was present on the carapace, plastron, and thickened forelimb scutes, the plastron was more severely affected than other areas of the integument. The affected portions of the shell were gray-white and sometimes orange and had a roughened flaky appearance. The lesion commenced at seams between scutes and spread toward the middle of each scute in an irregular pattern. Shell biopsies of nine affected tortoises were evaluated by light mi croscopy. No inflammatory infiltrates were present in the lesions, and although bacterial organisms were identified in tissue sections, they were superficially located and were considered to be secondary invaders. Special staining indicated a loss of the normal integrity of the horny material covering affected scutes. For the most part, the epithelial cells that formed a pseudostratified layer under affected portions of each scute remained intact. Although the location and histologie appearance of the lesion were compatible with a dyskeratosis and were suggestive of either a deficiency disease or toxicosis, the exact cause of the disease could not be determined.