Transmission factors for microsporidial gill disease caused by Loma salmonae

Loma salmonae causes microsporidial gill disease (MGD) in farmed Pacific salmonids, Oncorhynchus spp., resulting in respiratory distress, secondary infections and often mortality. The infection occurs in the gills and to a lesser extent in other vascularized tissues with a final development stage of a spore-laden xenoma within the endothelial and pillar cells of the gill. The overall research goal was to identify important host, parasite and environmental factors associated with the transmission of L. salmonae in rainbow trout (RBT). Environmental factors. Water temperature is considered to be an important environmental variable in the transmission of many fish diseases. Using a cohabitation challenge model, RBT held at 19°C had the least number of days to the development of branchial xenomas compared to fish held at either 11° and 15°C and the latter two temperatures groups showed similar xenoma onset rate. A subsequent trial revealed that water temperature affects xenoma clearance and recovery time, so that as the water temperature increased, the time required for the dissolution of all branchial xenomas decreased. Host factors. Investigations under the domain of host factors centered on the effects of various feeding rates, the dependency of fish size at the time of exposure and further studies investigating the efficacy of monensin therapy. Although feeding rate did not alter the onset or resulting intensities of branchial xenomas, fish size was found to be a significant factor. Small RBT (17 to 23 g) had significantly faster rate of xenoma development and increased xenoma intensity with the median onset time approximately 1 week sooner compared to the 2 larger size groups. Another host factor considered was the potential use of monensin therapy to treat MGD by investigating the minimum dose and treatment time required for therapeutic success. Pathogen factors . The third domain of the disease triad contains factors that directly involve the pathogen. A novel challenge model using only effluent water revealed that L. salmonae was transmitted to naive RBT without the need for physical contact with infectious fish. (Abstract shortened by UMI.).