Investigation of farmed Nile tilapia health through histopathology

Confinement is a stressful practice for fish, which can culminate in tissue damage and production losses. Histopathological examination is a technique capable of forthcoming the normal and pathological conditions in fish. It reflects the animal's health and welfare, being able to apply measures to mitigate the stress and diseases in fish farming. This study evaluated the health status of cultured Nile tilapia using histopathological examination as a tool. Fragments of gills, liver, heart and spleen of 60 tilapias from fish farms in southern Brazil were collected and fixed. All crops were free of sanitary problems and fish were clinically healthy. This study showed a positive correlation between Trichodina species, protozoans and hyperplasic epithelium in the secondary lamellae and lamellar fusion, as well as the role of parasites in the inflammatory response in tilapia gills. Nonetheless, alterations such as hyperplasia of interlamellar epithelium were persistent even in non-parasitized animals, suggesting being an adaptation to the confinement environment. The presence of melanomacrophage centers in the liver and spleen were characteristic of the chronic stress that the cultured fish are exposed to. Histological findings such as epitheliocystis in the gills and bacterial colonies in the liver, spleen and heart, showed that even with absence of mortality, bacteremia were present in healthy farmed fish, as well as the parasitic diseases culminating in mixed infections. This study presented unprecedented results on tissue changes in farmed tilapia, serving as an assistance for what can be considered normal in the histopathological examination in farmed fish. Fish herein used did not present any production loss and did not have the relative condition factor (Kn) influenced by the changes, indicating a good health status. Thus, this study reports the first occurrence of epitheliocystis in tilapia gills grown in southern Brazil.