Diseases Caused by Amoebae in Fish: An Overview

Abstract

Parasitic and amphizoic amoebae are ubiquitous and can affect a huge variety of hosts, from invertebrates to humans, and fish are not an exception. Most of the relationships between amoebae and fish are based on four different types: ectocommensals, ectoparasites, endocommensals and endoparasites, although the lines between them are not always clear. As ectocommensals, they are located specially on the gills and particularly the amphizoic Neoparamoeba perurans is the most relevant species, being a real pathogenic parasite in farmed salmon. It causes amoebic gill disease, which causes a progressive hyperplasia of epithelial cells in the gill filaments and lamellae. Nodular gill disease is its analogue in freshwater fish but the causative agent is still not clear, although several amoebae have been identified associated to the lesions. Other species have been described in different fish species, affecting not only gills but also other organs, even internal ones. In some cases, species of the genera Naegleria or Acanthamoeba, which also contain pathogenic species affecting humans, are usually described affecting freshwater fish species. As endocommensals, Entamoebae species have been described in the digestive tract of freshwater and marine fish species, but Endolimax nana can reach other organs and cause systemic infections in farmed Solea senegalensis. Other systemic infections caused by amoebae are usually described in wild fish, although in most cases these are isolated cases without clinical signs or significance.

Keywords: AGD; Endolimax nana; NGD; Neoparamoeba perurans; systemic amoebiasis.

Figure 1

Gills affected by amoebic gill disease (AGD) and nodular gill disease (NGD): (A), gill filaments of turbot affected by amoebic gill disease. Note the typical white multifocal lesions. (B–D), Paraffin-embedded histological sections: (B), extensive hyperplasia of epithelial gill cells in Atlantic salmon, with synechiae and lacunae between filaments; (C), detail of amoebae within lacunae between gill filaments in AGD in Atlantic salmon; (D), flattened amoebae attached to the surface of proliferative gill lesions of NGD in rainbow trout; (E), transmission electron micrographs of gill with amoebae attached to the epithelium in NGD in rainbow trout.

Figure 2

Senegalese soles clinically infected by Endolimax piscium: (A), conspicuous lump from the abdominal cavity, and noticeable at the skin surface (arrow); (B), nodule in muscle with an abscess-like aspect with a soft and liquefied consistency. (C–E), paraffin-embedded histological sections: (C), skeletal muscle showing the typical granulomatous inflammatory reaction to E. piscium; (D), E. piscium cells within the intestinal epithelium (arrow); (E), in situ hybridisation of histological sections where parasite cells can be recognised stained in purple.