Transmission of infectious salmon anaemia (ISA) through natural secretions and excretions from infected smolts of Atlantic salmon Salmo salar during their presymptomatic phase

Abstract

Short-term (48 h) exposure of healthy Atlantic salmon Salmo salar L. smolts to infectious salmon anemia (1SA)-inoculated cohort smolts showed that the disease was transmitted with near 100% mortality from Day 7 post-inoculation and onwards. This is more than a week before the inoculated fish show any clinical signs and long before the typical petechial bleedings occur. A bloodborne transmission of the disease is therefore unlikely. Skin mucus, faeces, urine and blood, isolated from ISA-inoculated smolt, transmitted the disease to healthy cohort smolt with 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 smolt. After i.p. injection, skin mucus had somewhat lower infectivity than blood homogenates. Furthermore, in some experiments application of skin mucus to the gills was as efficient as i.p. inlection for transmission of the &sease When introduced into the stomach none of the inocula caused ISA. Coprophagy thus seems to be ineffective in the transrn~ssion of ISA under laboratory c o n d ~t ~o n s .