The salmonid pathogen Renibactenum salmoninarum is now known to be transmitted from parent to progeny within the salmonid egg.Two groups of experiments were conducted to evaluate the possible efficacy of the antibiotic erythromycin in preventing this mode of R. salmoninarum transmission.The first evaluated the widely practised disinfection technique of waterhardening eggs in aqueous erythromycin.Eggs treated in this manner rapidly accumulated the antibiotic, but only in the perivitehne fluid, not in the yolk where the intra-ovum pathogen is thought to occur.Further, the erythromycin was leached from the egg too rapidly to permit any bactericidal activity.The second group of experiments investigated erythromycin levels attained In eggs when the broodfish were injected with the antibiotic prior to spawning.Fish injected with erythromycin (20 mg antibiotic [kg fish]-') 30 to 56 d prior to spawning ylelded eggs containing levels of the antibiotic that exceeded the minimum lethal concentration determined for R. salmoninarum.Moreover, the drug was not leachable and it occurred in the yolk where it would contact the intra-ovum pathogen.We failed in an attempt to infect antibiotic-containing eggs from erythromycin-injected fish using R. salmoninarum and an in vitro infechon method always previously successful with antibiotic-free eggs.This encouraging result Indicates that further studies to evaluate the broodstock injection procedure are clearly warranted.
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Evelyn et al. (1986) studied this question.