Interaction of water temperature and challenge model on xenoma development rates for Loma salmonae (Microspora) in rainbow trout, Oncorhynchus mykiss (Walbaum).

This study evaluated the regulatory effects of water temperature on the development of branchial xenomas caused by Loma salmonae using a high-dose per os-challenge model compared with a low-dose cohabitation-challenge model. Approximately 275 juvenile rainbow trout (RBT), Oncorhynchus mykiss, were randomly distributed to six tanks with two tanks each maintained at 11, 15 and 19 degrees C. Fish in one tank from each temperature setting were exposed per os to macerated L. salmonae-infected gill material and fish in the other tank from each temperature setting were exposed to L. salmonae using the cohabitation-challenge model. Fish were monitored for the development of branchial xenomas beginning at day 21 post-exposure. Survival analyses were used to evaluate the effect of water temperature and challenge model on the number of days until the first visible branchial xenoma was detected. The survivor curves for the per os-challenge model revealed that there was at least one significant difference, whereas the cohabitation challenge did not reveal any significant differences amongst the temperature settings. The proportional hazards model revealed a significant interaction between the challenge model used and water temperature. This indicated that the effect of water temperature was different depending on challenge model. Additionally, from the mean xenoma intensities, on average, the per os-challenged fish showed higher xenoma intensity compared with the cohabitation-challenged fish. Overall, the impact of water temperature on disease pathogenesis was greater when the RBT were per os challenged compared with using the cohabitation model.