Nodular gill disease (NGD) is an emerging condition associated with amoeba trophozoites in freshwater salmonid farms. However, unambiguous identification of the pathogens still must be achieved. This study aimed to identify the amoeba species involved in periodic NGD outbreaks in two rainbow trout (Oncorhynchus mykiss) farms in Northeastern Italy. During four episodes (February-April 2023), 88 fish were euthanized, and their gills were evaluated by macroscopic, microscopic and histopathological examination. The macroscopic and microscopic severity of the lesions and the degree of amoebae infestation were scored and statistically evaluated. One gill arch from each animal was put on non-nutrient agar (NNA) Petri dishes for amoeba isolation, cultivation and subsequent identification with SSU rDNA sequencing. Histopathology confirmed moderate to severe lesions consistent with NGD and mild to moderate amoeba infestation. The presence of amoebae was significantly correlated with lesion severity. Light microscopy of cultured amoebae strains and SSU rDNA analysis revealed the presence of a previously characterized amoeba Naegleria sp. strain GERK and several new strains: two strains from Hartmannelidae, three vannelid amoebae from the genus Ripella and cercozoan amoeba Rosculus. Despite the uncertainty in NGD etiopathogenesis and amoebae pathogenic role, identifying known and new amoebae leans towards a possible multi-aetiological origin.

Aquatic Diagnostic Services Atlantic Veterinary College University of Prince Edward Island Charlottetown Prince Edward Island Canada

Department of Animal Medicine Production and Health University of Padova Legnaro Padua Italy

Department of Comparative Biomedicine and Food Science University of Padova Legnaro Padua Italy

Department of Veterinary Medical Sciences Alma Mater Studiorum University of Bologna Ozzano dell'Emilia Bologna Italy

Institute of Parasitology Biology Centre CAS Ceske Budejovice Czech Republic

Servizio Tecnico Commerciale Aquafeed Gruppo Veronesi Verona Veneto Region Italy

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RefSeq
HM363631, HM363629, AY145442