Exploring Aeromonas veronii in Migratory Mute Swans (Cygnus olor): A Debut Report and Genetic Characterization

<i>Aeromonas veronii</i> (<i>A. veronii</i>) is a ubiquitous bacterium in terrestrial and aquatic environments. It has a significant impact on animal and human health, with it becoming an emerging crucial pathogen worldwide. However, there have been no reports of mute swan infections. In the present study, after an observation of pathological changes, one bacterial strain isolated from a dead migratory mute swan was identified as <i>A. veronii</i> HNZZ-1/2022 based on its morphology, matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS), and sequence analysis of the <i>16S rRNA</i> and <i>gyrB</i> genes. To explore its pathogenicity, virulence gene detection and a gosling infection experiment were subsequently carried out, respectively. Six virulence genes for cytotonic enterotoxins (<i>alt</i>), lateral elastase (<i>ela</i>), lipase (<i>lip</i>), cytotoxic enterotoxin (<i>act</i>), aerolysin (<i>aerA</i>), and polar flagellin (<i>fla</i>) were present in the template DNA of <i>A. veronii</i> HNZZ-1/2022. Experimentally infected goslings exhibited hemorrhages of various different degrees in multiple organs. The half-maximal lethal dose (LD₅₀) value of <i>A. veronii</i> strain HNZZ-1/2022 was estimated to be 3.48 × 10⁸ colony forming units (CFUs) per mL for goslings. An antimicrobial susceptibility test showed that the <i>A. veronii</i> HNZZ-1/2022 strain was resistant to meropenem, ampicillin, and enrofloxacin. To date, this is the first report of <i>A. veronii</i> in migratory mute swans, thus expanding the currently known host spectrum. These results suggest that the migratory mute swan is a new host for <i>A. veronii</i> and demonstrate the need for extensive surveillance and research of <i>A. veronii</i> to minimize its transmission between animals, the environment, and humans.