A prospective longitudinal study of "Candidatus arthromitus"-associated rainbow trout gastroenteritis in the UK

Rainbow trout gastroenteritis (RTGE) is an emerging disease of farmed rainbow trout (Oncorhynchus mykiss) reported in Croatia, France, Italy, Spain and the UK. The impact of RTGE at the site-level varies, and daily mortalities of 0.5-1% are common. Gross lesions include severe enteritis with congestion and oedema, and the segmented filamentous bacterium "Candidatus arthromitus" has been suggested as a possible aetiological agent. This paper presents findings from a prospective longitudinal epidemiology study that took place from June to November 2006 in 12 RTGE-positive UK sites, examining the impact and presentation of the disease, spread pattern, risk factors for disease presence and severity and the efficacy of current control strategies. RTGE was present in a total of 164 production units across the 12 sites, representing 39% of all stocked units. Fish losses due to RTGE totalled 61.4 tonnes. The plotting of epidemic curves revealed a propagating epidemic pattern in 51% affected units, suggesting RTGE is infectious. This was supported by risk analysis of fish transfers and site layouts, which revealed an increased risk of becoming RTGE positive after contiguity or fish transfer to and from a previously affected unit. Conditional multivariate logistic regression analysis identified eight variables significantly associated with the presence of RTGE. Seven variables were risk factors, including higher stocking densities, mortalities due to predation or handling, higher water temperature, higher daily feed input and contiguity or fish movement to and from a previous case. One variable was protective: the absence of an aeration system in a unit. General linear modelling identified a significant association of mean feed input per fish during an outbreak with RTGE cumulative mortality. Finally, great variability was observed with regards to the control strategies targeted to RTGE in these sites, suggesting an absence of a common strategy. In feed NaCl treatments apparently presented a palliative effect if given to affected fish during the clinical outbreaks, but they did not prevent recurrence.