Fish gills, owing to their status as a multifunctional organ, have always fascinated researchers. In spite of the intense work done on the morphologic examination of gills, the organ is relatively under-used in health evaluation in fish. The existing literature on this topic is reviewed here. Our review summarises important diagnostic guidelines for the examination of gill structure and describes the morphological lesions that develop under the influence of different biological and physicochemical factors. The picture that should emerge is that of an organ that is extremely sensitive to all types of handling and unfavourable changes in the external and internal environments. We conclude that studying the morphology of the fish gills provides an opportunity to assess fish health status as well as information on possible health hazards coming from their environment.

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Massive deaths were reported on a chinchilla (Chinchilla lanigera) farm, which over 10 months led to the elimination of the herd. After three months of feeding, longitudinal precipitates inside the pellets were noted. The first symptoms were observed two weeks after the introduction of feed, and included lethargy, decreased mobility, and reduced appetite, as well as increased reactions to external stimuli. Over time, single deaths, hair loss and gnawing, white discolorations on teeth, and polyuria were reported. Haemorrhages of the reproductive tract and mass abortions were observed. Necropsy revealed the presence of transudate with pH = 10, severe hyperaemia of the intestinal mesentery, and extensive regressive lesions in the gastric and intestinal mucosa. Both grossly and microscopically, lesions were noted in the liver, kidneys, adrenal glands, and lungs. The urinary bladder was overfilled. No lesions were observed in the spleen or mesenteric lymph nodes. Histopathology of skin specimens revealed atrophy of the hair follicles. Diagnosis was sodium carbonate intoxication. Owing to the specific features of this species, it was impossible to introduce emetic-based treatment in order to eliminate the toxic agent from the body. No therapeutic measures were undertaken because of the late diagnosis of the toxic agent and late removal of feed, as well as extensive lesions on the gastrointestinal mucosa.

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