Loma salmonae-associated xenoma onset and clearance in rainbow trout, Oncorhynchus mykiss (Walbaum): comparisons of per os and cohabitation exposure using survival analysis

Survival analysis techniques were used to compare experimental exposure methods of Loma salmonae (Microspora) in rainbow trout (RBT) by measuring xenoma onset and clearance time. Twenty-eight naive RBT were exposed per os (fed L. salmonae spores) and 28 RBT were exposed by cohabitation with 28 L. salmonae-infected RBT. Exposed fish were examined once every week (7 days) post exposure (PE). For xenoma onset, the median survival time, the time to first appearance of branchial xenomas, was 6 weeks PE for both per os and cohabitation exposure. For xenoma clearance, the median survival time, the time to total clearance of branchial xenomas, was 10 weeks for per os exposure and 12 weeks for cohabitation exposure. There was a significant difference between the survival curves of per os fish compared with cohabited fish for both xenoma onset and xenoma clearance. The incidence rate of xenoma development was greater for per os exposure (0.1745 cases per fish-week) compared with cohabitation exposure (0.1342 cases per fish-week). Similarly, the rate of xenoma clearance was greater for per os exposure (0.1028 cases per fish-week) compared with cohabitation exposure (0.0885 cases per fish-week). Differences between exposure methods were attributed to differences in the frequency and duration of exposure to spores. Survival analysis was useful for examining the onset and clearance of L. salmonae and may have applications for understanding disease dynamics in aquaculture..