Epidemiological and pathogenetic issues of nodular gill disease, an emerging pathology in Italian farmed rainbow trout (Oncorhynchus mykiss)

Introduction: In the latest years the nodular gill disease (NGD) is one of the most serious health problem in rainbow trout cultured in Northern Italy. This disease caused by amoebae has been reported to affect salmonids farmed in freshwater environment of Northern Europe and North America. Relatively little is known about epidemiology and pathogenesis of NGD. Methodology: To clarify the possible risk factors related to the occurrence of NGD, a study area (province of Trento - Northern Italy) was identified where an epidemiological investigation at farm level was planned. Data required were collected during onsite visits by a veterinarian. A standardized questionnaire, implemented and tested during the first steps of the project, is being used for data collection. Information on disease status, species stocked, farm activities undertaken, risk factors related to NGD introduction and spread are included in the questionnaire together with data on management practices and biosecurity measures applied at farm level. Anamnestic and clinical information related to the health status of each farm are reported in the questionnaire. To assess distances from case farms identified as hazards and upstream-downstream proximity to other farms, data on the location of farms, stored in a regional web GIS, will be available. A database will be implemented in order to validate, store and analyze the data collected during the project. Data collection started in November 2017 and is still in progress. To understand the pathogenesis of NGD three tanks, respectively from three trout farms located in the province of Trento, were monitored every fifteen days from September 2016 to March 2017. Ten fish from each tank (five samples at the inlet and five at the outlet) were taken during each monitoring phase and subjected to necropsy and to microscopic and histological examination of the gills. Expected results: The epidemiological investigation provides information about the real diffusion of NGD in the province of Trento and identify the factors that may favor the development of disease. The study of pathogenesis could reveal the timing and mode of NGD development.