Induced resistance in rainbow trout, Oncorhynchus mykiss (Walbaum), to gill disease associated with the microsporidian gill parasite Loma salmonae

Branchial xenomas were detected by week 5 and disappeared by week 10 after naive juvenile rainbow trout, held at 14.5 degrees C, were fed or intubated with Loma salmonae-infected gill tissue preparations. Upon re-challenge wi th L. salmonae, these fish were protected from disease and branchial xenoma s did not develop. Branchial xenomas were never detected in naive fish held at 10 degrees C and exposed to L. salmonae. When these fish were re-challe nged with L. salmonae at 14.5 degrees C, they were also protected from the disease. Branchial xenomas also developed after naive fish, held at 14.5 de grees C, were injected intraperitoneally with a semipurified preparation of fresh spores, but generally did not develop after intraperitoneal injectio n with a preparation of spores subjected to freezing and thawing before use . Fish that had received fresh spores intraperitoneally were completely res istant to disease when re-challenged via oral delivery of spores, whereas t hose that had received frozen spores were incompletely, but significantly, protected from disease compared with naive fish. We conclude that infection with L. salmonae induces strong protection towards the disease upon re-exp osure to spores, and that the protection does not depend on the completion of the parasite's life cycle, thus establishing the basis for further resea rch on vaccine development for this disease.