The presence of metacercariae of Centrocestus formosanus was detected on the gills of goldfish (Carassius auratus L.) imported from Singapore and obtained from two pet shops. This is the first report of the parasite in Croatia. The histopathological changes observed lead to the conclusion that the infection could be one of the reasons for the death of goldfish.

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.

The impact of the long-term historical contamination of the Mrežnica River with textile industry wastewaters on metal/metalloid bioaccumulation in the muscles of the young northern pike (Esox lucius) (0+ to 3+) was evaluated, as well as the association of bioaccumulated metal/metalloid concentrations in the muscle with fish physiology. Increased levels of several elements (As, Bi, Cs, Co, Cu, Zn), bioaccumulated in fish muscle, were occasionally found in front of the former factory, but the obtained metal/metalloid concentrations in muscle were in general either comparable or even lower than in the fish from moderately contaminated freshwaters. Calculated target hazard quotients indicated that the current risk for humans, arising from consumption of the northern pike meat originating from a historically contaminated section of the Mrežnica River, was negligible. The influence of biological factors, especially seasonal physiological changes, on metal/metalloid bioaccumulation was confirmed, indicating the importance of the involvement of these parameters in the freshwater pollution assessment, but also in the estimation of the human health hazard. The increased bioaccumulation of several metals/metalloids in fish muscle at a historically contaminated site pointed to the need for continuous supervision of fish health and biodiversity in freshwaters impacted by currently suspended long-term contamination sources.

Reply to comment on "Structural changes in the rat placenta during the last third of gestation discovered by stereology"

Populations of northern pike (Esox lucius) and pumpkinseed (Lepomis gibbosus) from the Mrežnica River were found to be infected with the tapeworm Triaenophorus nodulosus. In both species, the mean intensity of infection was low, suggesting a well-balanced host–parasite relationship. This study investigates pathological changes caused by adult T. nodulosus in its definitive host, the northern pike, and the associated intestinal immune response. The infection had no detectable adverse effects on either the northern pike population or the host’s body condition index. Histological examination revealed lesions both at the site of tapeworm attachment and in areas adjacent to the free strobila, involving the lamina propria and submucosa. A moderate, multifocal, ulcerative, and necrotizing enteritis was observed, accompanied by an increased number of mast cells (MCs), which were identified as the predominant immune cells involved in the E. lucius–T. nodulosus interaction. MCs, mostly degranulated, were found in the lamina propria and superficial submucosa at the attachment site. Immunofluorescence revealed a subpopulation of piscidin 1-positive MCs in the same layers, with a higher number at the attachment site compared to unaffected intestinal areas. This represents the first evidence of piscidin 1 involvement in intestinal host defence against cestode infections in teleosts.