Normal and aberrant tissue distribution of Loma salmonae (Microspora) within rainbow trout, Oncorhynchus mykiss (Walbaum), following experimental infection at water temperatures within and outside of the xenoma-expression temperature boundaries

Temperatures above 20 degreesC or below 9 degreesC interrupt the life cycle of the gill intracellular microsporidian parasite Loma salmonae (Microspor a) prior to sporogony, inhibiting the production of xenomas. This study int ended to characterize this life-cycle failure. Juvenile rainbow trout, Onco rhynchus mykiss (Walbaum), were experimentally infected with L. salmonae sp ores, and the effect of water temperature on the progress of infection, as determined by polymerase chain reaction, was compared for fish held at wate r temperatures of 5, 15 and 21 degreesC. At 15 degreesC, parasite DNA was f irst detected in the heart (3 days post-exposure [PE]), and then in the gil ls and spleen (2 weeks PE). Branchial xenomas developed by week 4 PE. In co ntrast, at 5 degreesC, the arrival of the parasite in the heart was delayed until 7 days PE. However, even though parasite DNA was detected in the gil ls at 7 days PE, xenomas failed to form in the gill, and by week 4 PE, para site DNA was no longer detected. In fish held at 21 degreesC, parasite DNA was detected in the heart, gills and spleen by 3 days post-infection, and s imilar results were observed at 7 days PE. Xenomas also failed to form in t hese fish and parasite DNA was no longer detected by week 2 PE. Within the range of temperatures tested in this study, spore germination and delivery of their DNA into the hose through the intestinal wall was not blocked by t emperature. At 5 or 21 degreesC, migration to the heart and gills occurred, but at aberrant periods of time. The normal life cycle of L. salmonae may depend on the completion of relatively lengthy, but yet unknown, stages of development within the heart, prior to reaching the gill. This development may be adversely affected by temperature, and explain the temperature limit s of this parasite.