In mariculture facilities, bacterial infections pose significant production challenges, with potentially catastrophic impacts on fish species. Bacterial co-infections are a widespread phenomenon in the natural marine environment, although their impact on aquatic organisms remains poorly investigated. This study aimed to detail the pathological findings associated with a natural bacterial co-infection caused by three different pathogens, namely Photobacterium damselae subsp. piscicida, Tenacibaculum maritimum and Vibrio sp., as the cause of mass mortality in European sea bass. The fish had been reared in open-net cages in Sicily and later transferred for experimental research purposes to a user establishment after immunization with an inactivated vaccine. Macroscopic, cytological and histopathological examinations were performed on 109 animals, and bacterial species were identified by the 16S rRNA gene. Overall, ulcerative skin lesions, necrotizing myositis and tail rot with occasional tail loss were associated with tenacibaculosis and vibriosis, while P. damselae subsp. piscicida mainly caused granulomatous inflammation in the spleen and head kidney. Finally, an injection site reaction due to the oil-adjuvanted vaccine administered intraperitoneally was observed in the abdominal fat. Understanding the impact of bacterial pathogens is essential to manage the health and welfare of farmed fish, and the importance of a good health monitoring program cannot be overstated to avoid outbreaks and the possible emergence of new pathogens due to the intensification of the production systems, antibiotic resistance and climate changes. The study would also highlight the importance of the quarantine period when animals supplied for research come from aquaculture farms and how the main goal in the near future should be to better define the procedures to provide completely pathogen-free animals.

Bacterial diseases are widespread in pet birds, posing a severe threat in nestlings and birds with weakened immune systems, often resulting in high mortality during the first days after hatching. This study aimed to describe the pathological findings of a natural bacterial co-infection with Klebsiella pneumoniae and Pseudomonas aeruginosa in nestling European goldfinches (Carduelis carduelis), as a cause of mortality after hatching. Eight nestlings kept in a breeding facility showed an inability to move, anorexia, and respiratory distress, and were found dead between 1 and 4 days of life. Gross pathological findings included diffuse pneumonia with edema and multifocal hemorrhages and occasionally hepatic necrosis. On histopathology, the main findings in all examined birds included severe, subacute bronchopneumonia and severe necrotizing hepatitis. Gram-negative bacilli were observed in parabronchial walls, within pulmonary arteries and surrounding hepatocellular necrotic foci. Lungs, livers and kidneys were sampled for bacteriological examination, resulting in two Gram-negative bacterial isolates. Four housekeeping genes (i.e., 16S rRNA; rpoB; khe; ecfX) were amplified and sequenced for bacterial identification at species level. Although K. pneumoniae and P. aeruginosa are common Gram-negative pathogens and are often co-isolated in human bacterial pneumonia, co-infection with these bacteria has not been documented in nestling goldfinches to date. Pathogen identification is essential for formulating a correct etiological diagnosis and further selecting the most appropriate therapeutic strategy.