Localization of the initial developmental stages of Loma salmonae in rainbow trout (Oncorhynchus mykiss)

The intracellular microsporidian parasite Lorna sahnanae affects salmonids of the genus Oncorhynchus and is a significant cause of economic losses in pen-reared Chinook salmon (O. tshawytscha) in British Columbia. Lorna salmo nae infection is easily recognized by the xenomas that form in the gills, b ut early stages of infection are difficult to detect in histologic sections . In situ hybridization (ISH), using an L. sabnonae-specific digoxigenin-la beled single-stranded DNA probe, was used to detect the parasite during the early stages of infection. Loma-salmonae was detected in the gut mucosal e pithelium as early as 24 hours postexposure (PE), and it localized in the l amina propria of the intestine within 24 hours of infection. After the para site was detected in the lamina propria, dividing merogonic stages in infec ted cells in the heart were detected by ISH as early as 2 days PE, providin g the first evidence of parasitaemia and hematogenous distribution of thus parasite in infected blood cells. The parasites inside the infected cells a ppeared to be undergoing merogony as they passed through the heart, indicat ing that proliferation may start at the site of infection, before the paras ite arrives to the gills for their final developmental phase. This is the f irst time that L. salmonae passage through the intestinal wall and migratio n to the heart has been visualized; however, the identity of the cells harb oring the parasite has yet to be determined.