Emergence of a new multidrug-resistant and highly virulent serotype of Streptococcus agalactiae in fish farms from Brazil

Abstract Streptococcus agalactiae is one of the major etiological agents of streptococcosis in cultured and wild fish. Although there are ten serotypes described for this pathogen, the serotypes Ib, Ia and III are the most common fish diseases worldwide. In Brazil, only serotypes Ib and Ia have been implicated in fish disease so far, with Ia being rare. At the end of 2016, a disease outbreak resulted in high mortality rates in six tilapia farms in northeastern Brazil. Gram-positive cocci were isolated from the brain and kidney of 28 of 29 collected moribund animals, as confirmed by species-specific PCR for S . agalactiae . The isolates were resistant to all aminoglycosides tested, including sulphazotrim (sulfamethoxazole with trimethoprim), tetracycline, ampicillin and all fluoroquinolones and were therefore characterized as multidrug-resistant. All of these isolates were characterized molecularly as serotype III. The biofilm formation ability of one of these isolates was higher than that of serotype Ib, which is the most frequent serotype in the country, suggesting an adaptation of the pathogen for maintenance in the aquatic environment. An experimental challenge performed with water temperatures of 22 °C and 31 °C showed mortality rates of 10% and 80%, respectively. Our findings indicate that sanitary barriers in fish culture have weak points and that field and laboratory monitoring should be intensified to prevent and control the transmission of these new strain variants to other regions. Furthermore, the emergence of multidrug-resistant strains in aquatic environments, especially fish pathogens, is alarming and may have unpredictable consequences.