| Summary: | 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.
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