UBC Theses and Dissertations
Vertical transmission of pathogens and of immunity in salmonids Brown, Laura Louise
Bacterial kidney disease (BKD) in salmonids is caused by Renibacterium salmoninarum (Rs) which can be transmitted vertically through salmonid eggs. This study investigated various phenomena in salmonid eggs relating to the control of this salmonid pathogen. First, it was shown that exposure of coho salmon at the egg stage to Rs (and certain of its antigens) induces at least partial immunosuppression. Coho salmon eggs were injected with various Rs antigens. The eggs were fertilized and the progeny were challenged with live, virulent Rs. Fish that had been injected, as eggs, with a 57 kDa protein of Rs (p57) demonstrated a significantly higher cumulative l5ercent mirtality (50%) than the saline-injected controls (14%). A decreased proportion of phagocytic cells isolated from fish exposed, as eggs, to 100 ng p57, showed respiratory burst activity (0.4%) compared with the proportion of cells isolated from the saline-injected group (16.5 %) that showed respiratory burst activity. Second, a polymerase chain reaction (PCR) protocol to detect DNA from Rs within individual salmonid eggs was developed, the intent being to devise a detection system that would permit fish culturists to avoid the pathogen. Using two primers that were designed to amplify a 501 base-pair fragment of the gene encoding p57, the reaction was shown to be sufficiently sensitive to detect DNA from as few as two Rs cells per egg. The results indicate that the PCR is a more sensitive and specific method of screening broodstock salmonids and their eggs for Rs than the immunodiagnostic assays now routinely used. Third, it was shown that protective immunoglobulins can be passively transferred into salmonid eggs as a means of immunizing juvenile salmonids against vertically transmitted pathogens. Resistance to Vibrio anguillarum (Va) was used as the criterion of passive transfer because it is known that resistance to vibriosis is antibody-mediated. Antibodies raised against Va in rabbits and in chinook salmon were injected into female coho salmon before spawning. Based on immunoassays of egg contents, rabbit antibodies (but not salmonid antibodies) appeared to be incorporated into the eggs. Contents of eggs from all injected broodstock (including saline-injected controls) were injected into naïve coho salmon fry, which were then challenged with live Va. Egg material from broodstock injected with rabbit antiserum conferred protection against Va; in contrast, egg material from broodstock injected with chinook antibodies or saline did not. Samples of fry from the injected broodstock were also challenged with Va following absorption of their yolk sacs. Such fry from salmon injected with rabbit or chinook antibodies were not protected. Overall, the results suggest that passive vertical transfer of immunity occurs within salmonids with mammalian but not salmonid antibodies; further, the protection is not maintained for long after the yolk sac is absorbed. The significance of these results is discussed.
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