Influence of feeding ratio and size on susceptibility to microsporidial gill disease caused by Loma salmonae in rainbow trout, Oncorhynchus mykiss (Walbaum)

Abstract Two trials were designed to quantify the effect of feeding ratio and fish size on the cohabitation transmission of Loma salmonae , the causative agent of microsporidial gill disease (MGD) in salmonids, Oncorhynchus spp. To evaluate the effect of feeding ratio on disease onset, groups of 45 rainbow trout, O. mykiss (Walbaum) (RBT), were fed daily at 1% (low), 2% (medium) or 4% (high) of the fish biomass in the tank. There were three tanks at each feeding level: two tanks were exposed to the pathogen and one was a control. For the second objective, 300 RBT were separated into seven tanks so that the weight classifications were small (17–23 g), medium (32–38 g) and large (57–63 g). Each size class was done in duplicate with one control tank containing medium‐sized fish. Separately for each trial, on day 0 post‐exposure (PE) five highly infectious RBT were added to each tank (not including the control tanks) to begin the cohabitation exposure period. Beginning on day 21 PE and continuing biweekly until days 70 and 77 PE for the feeding and fish size trials, respectively, each fish was evaluated for visible branchial xenomas to determine disease onset time. Using survival analysis, the survival curves for the low, medium and high feedings were not significantly different from each other. However, there were significant differences amongst the small, medium and large weight classes in the size trial. The median numbers of days to the development of branchial xenomas was 31, 38 and 42 for small, medium and large size fish, respectively. On any given day, a medium or large sized fish had a hazard ratio for developing branchial xenomas of 0.66 and 0.63, respectively, compared with a small fish. In addition to host species and host strain differences, fish size is now considered a host risk factor for the development of MGD.