ABSTRACT
In May and June 2015, moderate and severe lesions were observed in Litopenaeus vannamei reared in clear seawater while, at the same time, lesions in shrimp reared in biofloc were considerably fewer. The signs of disease included anorexia, lethargy, melanization, expanded chromatophores, luminescence and necrotic areas in the uropods, suggesting a possible vibriosis. However, lesions observed in shrimp reared in biofloc disappeared after a certain time and without mortality in tanks, whereas mortality and severe signs continued to be observed in shrimp reared in clear seawater. To treat the possible vibriosis, oxytetracycline was administered only in clear seawater tanks, but the results were not successful. Bacterial cultures from hepatopancreas tissues of shrimp from both rearing systems confirmed a vibriosis outbreak only in the clear seawater system. Subsequently, Vibrio harveyi, Vibrio rotiferianus, Photobacterium sp. and Photobacterium damselae were identified from bacterial culture previously isolated for both rearing systems by molecular methods. Shewanella sp. was isolated and identified only in biofloc. To understand the possible pathogenicity and resistance mechanisms of the Vibiro strains for both rearing systems, pathogenicity (toxR) and oxytetracycline resistance-related genes (tet(B), tet(D), tet(G)) were determined. Although these genes were expressed for both rearing systems, biofloc proved to have the ability to control the development of the disease, in comparison to clear water, where the vibriosis was evident regardless of the administration of oxytetracycline as a treatment.
