Resistance to reinfection in chinook salmon Oncorhynchus tshawytscha to Loma salmonae (Microsporidia)

Chinook salmon Oncorhynchus tshawytscha were experimentally infected per os with Loma salmonae and held in flow-through seawater tanks at 12 to 14 degrees C. The fish exhibited 100% infection when first examined at 7 wk post initial exposure (p.e.), and. by 20 wk p.e. they had completely recovered from gill infections. The recovered fish were then re-exposed the following week. All of these fish showed strong protection to new L. salmonae infections, while naive fish exposed to the same inoculum developed the infection. Most of the re-exposed fish exhibited a few free spores or spares within phagocytes in the kidney interstitium at 20 to 29 wk p.e., but xenomas were not detected in either the gills or visceral organs. The kidney is the primary site of reticulo-endothelial activity, and thus these spores were likely deposited in the kidney by entrapment by fixed macrophages. It is possible that these spores provide immunologic stimuli to reinforce the resistance to new L. salmonae infections.