Pathogenesis of the Acute Ulceration Response (AUR) in Fish

Abstract

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, which named as 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 microscopically change in the fins occurred within 15 min with the swelling of the outermost layers of the epidermis and epidermal erosion. After 30-min stress, epidermis at the distal edges of fins developed ulceration. The middle and basal epidermis developed severe spongiosis, and the dermis and hypodermis became edematous. The epidermal ulceration appeared on all fins of the stressed fish was significantly greater than from the unstressed fish. A time-course study of the acute stress response showed a significant correlation between confinement period and severity of AUR. In separate experiment, the size of the acclimation space, temperature during acclimation and confinement, and exogenous adrenergic modulators influenced the risk of developing AUR. The acute confinement can also cause AUR in a wide array of fish species, including guppy (Poecilia reticulata), angelfish (Pterophyllum scalare) and channel catfish (Ictalurus punctatus), but not in rainbow trout (Oncorhynchus mykiss). As AUR might be expected to predispose fish to secondary microbial infections, water mold infection was exhibited in AUR hybrid striped bass after exposed to low concentration of Saprolegnia zoospores and caused high mortality (87.5%). However, bacterial numbers in AUR lesions remained low throughout the recovery period in a healthy environment. In conclusion, AUR might play a critical role in skin ulcer epidemics of many fish species after an acute stress. The physical environmental manipulations might be used to reduce the risk of fish developing AUR. Furthermore, the environmental pathogen load plays a critical role that lead to devastating disease losses.