Experimental intra-ovum infection of salmonid eggs with Renibacterium salmoninarum and vertical transmission of the pathogen with such eggs despite their treatment with erythromycin

Studies with eggs of steelhead trout Salmo gairdneri (hchardson) and coho salmon Oncorhynchus kisutch (Walbaum) showed that the eggs can be infected internally with Renibacterium salmonlnarum by immersing them prior to feruzation and water-hardening in coelomic fluid containing h ~g h numbers of R. salmoninarum cells. These results support our hypothesis that, in vivo, salmonid eggs acquire their intra-ovum R. salmoninarum infections from heavily infected coelomic fluid that surrounds them in 'culprit' fish following their release to the body cavity. Whether eggs are also infected early in oogenesis is stdl not known. Intra-ovum infection rates obtained were low (3.5 to 5.0 %) and did not increase if eggs were fertilized while still immersed in the infected coelomic fluid. These and other data discussed in the paper lead to the conclusion that the male salmonid plays a relatively unimportant role in the vertical transmission of R. salmoninarum. Water-hardening of experimentally infected coho salmon eggs in erythromycin did not appear to affect the intra-ovum infection rate, and fry hatching from the treated eggs carried the pathogen.