Impact of a water temperature shift on xenoma clearance and recovery time during a Loma salmonae (Microsporidia) infection in rainbow trout Oncorhynchus mykiss.

Previous studies have modelled the relationship between water temperature and the rate of sporulation as defined by xenoma formation during microsporidial gill disease (MGD) in salmon caused by Loma salmonae. Although offering insight into the epidemiology of MGD, a key unexplored area is the role of temperature in the rate of xenoma dissolution including spore release into the environment, and this is crucial to our ability to model horizontal transmission of MGD within confined net-pen populations of farmed salmon. Results from a previous trial suggested that xenoma dissolution may be dramatically hastened as water temperature declines, thus introducing a critical anomaly into any predictive exercise. The data generated herein was evaluated using the statistics of survival analysis to re-establish the baseline relationship of xenoma formation and dissolution relative to water temperature and to compare these results with those of previous studies. We infected 30 individuals of Oncorhynchus mykiss (Walbaum) with macerated xenoma-laden gill material, and afterwards allocated them to tanks with water temperatures of 11, 15, or 19 degrees C and monitored them through a disease cycle. Xenoma onset and clearance times were similar to previous findings with both events being accelerated at higher water temperatures, thereby suggesting a similar temperature response in the current strain to those used in previous studies. Another group of 45 fish was infected with L. salmonae and held at 15 degrees C until xenomas formed, and were subsequently shifted to 11, 15, or 19 degrees C. The median xenoma dissolution time in these tanks was 49, 35 and 28 d, respectively, similar to rates observed when water temperature remained constant. Thus we rejected the hypothesis that a sudden change in water temperature triggers rapid or anomalous xenoma dissolution.