"Rash" skin disease of rainbow trout [Oncorhynchus mykiss]

“Rash”, a subchronic, debilitating and non-fatal inflammatory skin disease has been found in cultured rainbow trout (Oncorhynchus mykiss) in several areas of Japan over the past five years. This condition was observed from small fish (11cm in body length) to mature-size animals, however, most of the affected fish were market size (in Japan, 20cm/120g). Morbidity sometimes reaches as high as 48% and causes a significant economic impact, since those fish loose commercial value. A self-limited clinical course and healing of the lesion could be observed after 6 to 8 weeks. “Rash” signs included the presence of bright red, non-raised, ulcerated or not, defined to petechial scattered lesions on the ventral and/or lateral surfaces of the fish. Histopathological features included a subchronic focal to non-focal, non-suppurative dermatitis with various degrees of ulceration and an extensive mononuclear inflammatory infiltration. “Rash” etiology still remains unknown.