The acute ulceration response (AUR): A potentially widespread and serious cause of skin infection in fish

In previous studies, we found that rapidly confined hybrid striped bass (Morone saxatilis male×Morone 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 degeneration, erosion, and ulceration on the body skin and fins, as well as corneal ulceration, in stressed hybrid striped bass. In the present study, we have shown that acute confinement stress can also cause AUR in a taxonomically wide array of fish species, including guppy (Poecilia reticulata), freshwater angelfish (Pterophyllum scalare) and channel catfish (Ictalurus punctatus) after a 2-h stress. However, we could not induce AUR in rainbow trout (Oncorhynchus mykiss). The AUR lesions were similar to those seen previously in hybrid striped bass. As AUR might be expected to predispose fish to secondary microbial infections, we examined the skin of hybrid striped bass for bacterial infection after experimental induction of AUR. These experiments showed that even fish with severe skin damage could rapidly heal their wounds without obvious consequences within several days. Bacterial numbers in AUR lesions remained low, ∼104 cfu/g of fin tissue, throughout the recovery period. However, if hybrid striped bass with AUR were exposed to even low doses (1 zoospore/ml) of the water mold Saprolegnia, a relatively weak, opportunistic pathogen, the fish developed severe saprolegniosis as soon as 48 h after challenge, with 87.5% infected and 87.5% dead within 4 days post-challenge. In contrast, none of the control fish (no AUR, but exposed to Saprolegnia) developed saprolegniosis. These data provide strong evidence that AUR might play a critical role in skin ulcer epidemics of many fish species that are preceded by an acute stress. Furthermore, our data suggest that environmental pathogen load plays a critical role in determining if AUR lesions will heal spontaneously or instead will lead to devastating disease losses.