The biology of infection by loma salmonae (microsporidia)

• Main Author: Shaw, Ross Winning
• Format: Text
• Language: English
• Published: The University of British Columbia 1999
• Subjects: Arctic; Pacific; Loma; Atlantic salmon
• Online Access: https://dx.doi.org/10.14288/1.0099447; https://doi.library.ubc.ca/10.14288/1.0099447

Loma salmonae is an important gill pathogen of Pacific salmon Oncorhynchus spp. in the Pacific Northwest. Epizootics of the parasite have caused considerable economic loss in Pacific salmon farming in British Columbia. A study was undertaken on the basic biology of L. salmonae since very little information exists to aid fish-farmers and scientists. Loma salmonae transmission was examined by challenging fish by per os (oral), injection, and cohabitation with infected fish. Loma salmonae is transmissible by experimental (e.g. intraperitoneal, intramuscular injection) and natural (e.g. peros, cohabitation of naive and infected fish) exposure routes. Autoinfection of hosts is possible as demonstrated by intravascular injections of spores producing infection. The route of infection in salmon was examined using histological techniques. Sporoplasms from extruded spores were detected in epithelial cells of the alimentary canal. The parasite may move through these cells and enter the blood stream eventually reaching the gills. Purified spores were held under various laboratory conditions to examine viability. Spores were still infective after 95 d at 4 °C in fresh or sea water. Spores were killed when frozen or exposed to 100 or 200 ppm iodophor, although some spores survived the latter treatment. Host specificity of L. salmonae was examined by per os exposure of salmonids and nonsalmonds. All Oncorhynchus spp. tested were susceptible, but Atlantic salmon and Arctic char were resistant. All nonsalmonids tested were resistant. A Northern Stream strain of Chinook showed higher susceptibility to the parasite when compared to a Southern Coastal or hybrid strains. Macrophage phagocytic ability was investigated as a possible mechanism to explain differences between strains and susceptible or resistant salmonids. No differences were found between strains but Atlantic salmon macrophages have a higher phagocytic response to L. salmonae spores than Chinook. The possibility of nonsalmonid reservoirs for L. salmonae was examined by collecting different life stages of salmonids and nonsalmonids near Vancouver Island. Loma salmonae was present throughout the life cycle of wild salmonids and a new Loma sp. is described from Cymatogaster aggregata using morphology, transmission studies, and rDNA sequencing.