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.

Amoeba species colonizing the gills of rainbow trout (Oncorhynchus mykiss) in Swiss aquaculture

“…This study identified seven amoeba strains from rainbow trout gills with NGD in Swiss freshwater aquaculture. Amoeba are free‐ranging organisms in water and soil, with the potential to causing disease and death in humans and animals (Padrós & Constenla, 2021). In saltwater salmonids, Neoparamoeba perurans causes AGD and is accompanied by multiple other amoeba species that are far less abundant on the gills than N. perurans (Boerlage et al, 2020; Cano et al, 2019; Crosbie et al, 2012; Dyková & Lom, 2004; English & Lima, 2020; English, Swords, et al, 2019; English, Tyml, et al, 2019; Herrero et al, 2018; Kim et al, 2017; Lima et al, 2017; Marcos‐López & Rodger, 2020; Nowak et al, 2014; Padrós & Constenla, 2021; Taylor et al, 2009).…”

“…The analogous disease in freshwater fish is NGD. However, the causative agent of NGD remains unclear (Padrós & Constenla, 2021) and a multi‐amoeba and multifactorial aetiology has also been suggested (Dykova & Tyml, 2016). Several amoeba species have been described in NGD‐affected fish farmed in Europe.…”

“…Amoeba are highly variable eukaryotic organisms distributed in soil and aquatic habitats, which can parasite the gills of rainbow trout ( Oncorhynchus mykiss ) and cause disease (Daoust & Ferguson, 1985; Dyková et al, 2010; Dykova & Tyml, 2016; Tubbs et al, 2010). Several amoeba species have been found in salmonids: Neoparamoeba perurans causing amoebic gill disease (AGD; Boerlage et al, 2020; English, Swords, et al, 2019; English, Tyml, et al, 2019; Marcos‐López & Rodger, 2020; Padrós & Constenla, 2021), alongside likely non‐pathogenic strains, Vexillifera sp., Pseudoparamoeba sp., Nolandella sp. and Vannella sp.…”

Identification of Vermamoeba vermiformis and Tetramitus sp. Isolated from the Gills of Oreochromis sp. (Red Hybrid Tilapia)

“…The fish gills are easily accessible to surrounding pathogens (Pratte et al 2018). Irritation of the gills caused by infection with pathogens can cause respiratory failure and osmoregulatory complications in fish (Padrós and Constenla 2021). The anatomy of fish gills includes the arch of the gills, primary lamellae, and secondary lamellae, which are perpendicularly arranged with primary lamellae.…”

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“…Other abundant pathogenic genera, such as Paramoeba and Hartmanella, including harmful amoeba species, were detected in the sediment and water column. These species have been associated with NGD in freshwater fish and with Amoebic Gill Disease (AGD) in other species, such as Atlantic salmon (Salmo salar), rainbow trout (Oncorhynchus mykiss), brown trout (Salmo trutta) or turbot (Scophthalmus maximus) (150). Moreover, although Nolandella, the most abundant amoeba genus in our study, does not seem to be pathogenic, it has been reported to colonize the gills of farmed Atlantic salmon with AGD (151).…”