Acinetobacter lwoffii, an emerging pathogen for fish in Schizothorax genus in China

Acinetobacter lwoffii, a serious human pathogen, has been identified as a cause of nosocomial infections such as bacteremia, pneumonia and meningitis. There are only a few studies reporting A. lwoffii as a pathogen of fish. During 2016 and 2017, six bacterial strains, isolated from diseased fish of the Schizothorax genus, were identified as A. lwoffii by morphology, biochemical tests, 16S rDNA and gyrB gene sequencing analysis. One of these isolates was selected for experimental infection of Sclizothorax prenanti, Schizothorax davidi and Schizothorax wangchiachii, to confirm its pathogenicity. Experimentally infected fish showed similar symptoms to those observed in fish after natural outbreaks. Susceptibility of the isolates to 14 antibiotics was tested using a disc diffusion method; all isolates were resistant to cephalothin, aminoglycosides and β-lactams, and sensitive only to some fluoroquinolones and tetracyclines. Histological examination revealed that A. lwoffii infection could cause pathological lesions in multiple organs and tissues, especially in liver, kidney, spleen and heart. These lesions included extensive haemorrhage, vacuolar degeneration, necrosis and inflammatory cell infiltration. To our knowledge, this is the first report on A. lwoffii as a virulent pathogen for fish of the Schizothorax genus.