Pathogenesis of the acute ulceration response (AUR) in fish

We demonstrated that rapidly confined hybrid striped bass (Morone saxatilis male x M. chrysops female) developed a syndrome characterized by the immediate and dramatic loss of their skin. We have named this phenomenon the Acute Ulceration Response (AUR). AUR is characterized by the rapid onset of severe epidermal erosion, ulceration and degeneration on the body skin and fins, as well as corneal ulceration, in stressed hybrid striped bass. Grossly, the distal edges of the fins became obviously ragged and blanched. The earliest microscopic change in the fins occurred within 15 min, with swelling of the outermost layers of the epidermis and epidermal erosion. After 30-min stress, the epidermis at the distal edges of the fins became ulcerated. Both apoptotic and necrotic epithelial cells were observed at 30 min confinement stress. The middle and basal epidermis developed severe spongiosis, and the dermis and hypodermis became edematous. Epidermal ulceration appeared on all fins of stressed fish and was significantly greater compared to fins from unstressed fish. A time-course study of the response to acute confinement stress showed a significant correlation between confinement period and severity of AUR.; In separate experiments, the size of the acclimation space, temperature during acclimation and confinement, and exogenous adrenergic modulators were shown to influence the risk of developing AUR. Acute confinement can also cause AUR in a wide array of fish species, including guppy (Poecilia reticulata), freshwater angelfish (Pterophyllum scalare) and channel catfish (Ictalurus punctatus), but not in rainbow trout (Oncorhynchus mykiss). AUR might be expected to predispose fish to secondary microbial infections, and this was supported by a study in which high mortality (87.5%) from severe water mold (Saprolegnia ) infection developed in hybrid striped bass having AUR that were subsequently exposed to a low concentration of Saprolegnia zoospores. However, when fish with AUR were placed into a healthy environment (no pathogens added), microbial load (as reflected by bacterial concentration) in AUR lesions remained low throughout the recovery period. In conclusion, AUR might play a critical role in skin ulcer epidemics of many fish species after an acute stress. Furthermore, manipulation of the physical environment might reduce the risk of fish developing AUR and environmental pathogen load seems to play a critical role in determining whether AUR lesions will lead to devastating disease losses.