TRANSMISSION OF INFECTIOUS SALMON ANEMIA (ISA) THROUGH NATURAL SECRETIONS AND EXCRETIONS FROM INFECTED SMELTS OF ATLANTIC SALMON SALMO-SALAR DURING THEIR PRESYMPTOMATIC PHASE

Short-term (48 h) exposure of healthy Atlantic salmon Salmo salar L. s melts to infectious salmon anemia (ISA)-inoculated cohort smelts showe d that the disease was transmitted with near 100% mortality from Day 7 post-inoculation and onwards. This is more than a week before the ino culated fish show any clinical signs and long before the typical petec hial bleedings occur. A bloodborne transmission of the disease is ther efore unlikely. Skin mucus, faeces, urine and blood, isolated from ISA -inoculated smelt, transmitted the disease to healthy cohort smelt wit h variable efficiency depending on how the inoculum was administered. All the sources were infectious and transmitted the disease with high efficiency when injected intraperitoneally (i.p.) into cohort smelt. A fter i.p. injection, skin mucus had somewhat lower infectivity than bl ood homogenates. Furthermore, in some experiments application of skin mucus to the gills was as efficient as i.p. injection for transmission of the disease. When introduced into the stomach none of the inocula caused ISA. Coprophagy thus seems to be ineffective in the transmissio n of TSA under laboratory conditions. Skin mucus from non-inoculated c ohabitants exposed to ISA-inoculated smelts for 2 d transmitted the di sease with close to 100% efficiency to healthy cohort smelts when inje cted i.p. This indicates that the infectious agent is waterborne and a bsorbed by the skin mucus rather than being secreted with the skin muc us. Since healthy smelts have an intact skin barrier, proximity to ino culation directly to the vascular bed seems unlikely. An ultrastructur al study of 10 different organs, all in close proximity to the secreti ons/excretions, revealed that at early stages of the disease, the viru s was exclusively found in the pillar cells and endocardial cells. Thi s indicates that the gills are the most Likely port of entry of the vi rus. It also supports a causal relation between the observed virus and the disease.