Risk factors linked to the onset of Nodular Gill Disease (NGD) in rainbow trout farms in North-eastern Italy

Nodular gill disease (NGD) is an emerging disease caused by amoebae affecting freshwater salmonids. It can cause high mortality and severe economic losses, in particular in rainbow trout (Oncorhynchus mykiss) farms. In Italy, most of the production of this species is found in the North-eastern regions. Therefore, an area in this part of the Country was selected to perform an epidemiological study with the aim of identifying the risk factors related to the onset of the disease. The survey was carried out between 2017 and 2019 and involved all the rainbow trout farms located in the Autonomous Province of Trento (North-eastern Italy). We administered a questionnaire to the 50 farmers, requesting information on the health status of the farms, their structural and management features, the applied biosecurity measures and the proximity and hydrological connection to other farms. Data were stored in a dedicated Access database, validated and analysed through statistical software STATA 12.1 and SAS 9.4. Univariate statistics were used to summarize the data characteristics. Afterwards, almost 70 variables were analysed through bivariate analysis (Chi-square or Fisher exact test for categorical variables, Wilcoxon/Mann-Whitney nonparametric test for two independent samples when quantitative variables were considered). A significant association (p-value<0.05) with NGD was detected for 7 variables. A logistic regression analysis was then performed with stepwise selection on variables that resulted significantly associated with NGD. The data analysis revealed that 42% of the farms experienced at least one episode of NGD in the previous 5 years. Furthermore, the reported cases increased during the study period, particularly in winter. The identified risk factors for the onset of NGD were the presence of other diseases in the farm in recent years (OR=17.496; 95% IC=2.746; 111.454), such as viral haemorrhagic septicaemia (VHS), infectious haematopoietic necrosis (IHN) and enteric redmouth disease (ERM), and having farms in the 5 km upstream (OR=24.679; 95% IC=2.885; 211.116). The results show that the disease is widespread among rainbow trout farms in the selected study area and underline the relevance of the exposure via water to amoebae and lack of biosecurity measures for the onset of the disease.