Investigation of farmed Nile tilapia health through histopathology

Highlights

• Ranking of lesions in tilapia maintained under normal conditions of culture

• Hyperplasia of the secondary lamellae and lamellar fusion related to trichodinid infestation

• Parasites and bacteria mixed infections in cultured fish

• First report of epitheliocystis in cultured tilapia in the Southern Brazil

Abstract

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.

Introduction

Aquaculture continues to be the fastest growing animal production sector, currently accounting for half the world's supply of fish for human consumption (FAO, 2016). Nile tilapia (Oreochromis niloticus) is the second most widely cultured fish species in the world and the first in Brazil (Vicente et al., 2014) being widely marketed (Fitzsimmons et al., 2011). According to the topic, the search for well-being in farmed fish has been highlighted, resulting in legislation in most European countries (Galhardo and Oliveira, 2006). However, scientific information on well-being in farmed fish is still scarce (Ashley, 2007, Chandroo et al., 2004, Huntingford and Kadri, 2014, Saraiva et al., 2015, Tavares-Dias and Martins, 2017).

Various methods of assessing the health status of aquatic organisms may reflect on fish welfare and lead to significant improvement in productivity. Among them, the parasitological and histopathological diagnoses are presented as tools for analysis and prevention of diseases (Resende, 2009, Sampaio et al., 2013). Another commonly used way to determine fish welfare is the relative condition factor (Kn), a parameter measured by the relationship between the observed weight and the expected weight for a given length (Guidelli et al., 2009). It is expected that, under normal conditions, Kn will be equal to 1, but it is known that it can be influenced by numerous factors such as nutrition, contamination and parasites (Yamada et al., 2008). In turn, histopathological analyzes are highlighted, as they are characterized as confirmatory and definitive diagnosis, in the face of adverse situations (Mcgavin and Zachary, 2013, Santos et al., 2017).

Histopathological changes result from a variety of biochemical and physiological changes in the organism (Hinton and Laurén, 1990) being able to integrate the effects of abiotic and biotic factors on organ function and fish health (Handy et al., 2002, Zimmerli et al., 2007; Van Dyk et al., 2009). Tissue alterations in fish may be due to stressing agents of the environment (Brum et al., 2014) or induced by pathogens (Guerra-Santos et al., 2012, Santos et al., 2017), compromising the growth and survival of animals. Thus, histopathological examination is a good indicator of fish health status, and the relevance of each lesion depends on how it affects organ function and the fish's ability to survive (Bernet et al., 1999).

This tool is widely used in fish, most of which are for experimental purposes (Figueiredo-Fernandes et al., 2007, Santos et al., 2012, Alim and Matter, 2015) and also as biomarkers (Liebel et al., 2013, Lins et al., 2010), but rarely applied in farmed fish (Coz-Rakovac et al., 2005, Raskovic et al., 2013, Saraiva et al., 2015).

In Brazil, studies of this nature were carried out by Jerônimo et al. (2014) and Santos et al. (2017), exclusively related to tissue changes by parasites, so that little is known about tissue changes and adaptations of Nile tilapia under farming conditions. This study aimed to evaluate the health status of Nile tilapia by histopathological examination, providing a ranking of the degree of lesions in fish kept under normal culture conditions.