Presence of infectious salmon anaemia virus (ISAV) in tissues of Atlantic salmon, Salmo salar L., collected during three separate outbreaks of the disease

Abstract

. Infectious salmon anaemia (ISA) is a viral disease of farmed Atlantic salmon, Salmo salar L., in Norway. The enveloped virus particles (100nm) believed to be the causative agent of the disease have been observed budding from endothelial cells in heart blood vessels. However, it is not known if the virus propagates in endothelial cells in all tissues/organs, if other target cells exist or if material collected from different salmon farms with natural outbreaks of ISA contain the same virus particles. Salmon smolts from three hatcheries with no history of disease were taken into the laboratory and experimentally challenged with ISA collected from Atlantic salmon during natural outbreaks of the disease in three different fish farms outside Bergen. Norway. Tissues for TEM studies were Collected from: (1) organs that showed clinical signs of ISA (i.e. used in the diagnosis of the disease); (2) tissues believed to be important in transmission of the virus (integument, kidney, urinary bladder, gut and somatic muscle); and (3) hormone‐producing tissues (pituitary gland, saccus vasculosus, thymus, thyroid, ultimobranehial gland, gonad, head kidney, heart and ventral aorta). The same virus as that believed to be the causative agent of ISA was found in all tissues examined from the challenged fish, i.e. a multiorgan infection with the same virus present in salmon from all three fish farms. The virus particles are about 100 nm in diameter, consisting of a slightly pleomorphic unit membrane envelope within which are a number of granules about 10–12nm in diameter. The granules seemed to be arranged in two concentric circles (spheres). The virus was seen budding from the surface of endothelial cells in blood vessels/sinus only. However, the virus was found intracellularly in both endothelial cells and in leucocytes.

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

“…The passage of the causative agent through 0.22 pm filters and our electron microscope findings of numerous virus particles in tissues as well as in the inhabitated water mass are in full agreement with the viral origin of ISA as previously indicated by others (Dannevig & Falk 1994, 1995, Nylund et al 1995b. The morphology of the virus particles is closely similar to that observed in the above-mentioned works.…”

Infectious Salmon Anemia Virus RNA in Fish Cell Cultures and in Tissue Sections of Atlantic Salmon Experimentally Infected with Infectious Salmon Anemia Virus

“…Electron microscopy has been used to show that the virus buds from endothelial cells in heart blood vessels and leucocytes. 15,26 The ISH technique will give a better spatial localization of the virus in the tissues/cells than electron microscopy. The optimal specific hybridization signal in tissue depends partly on the optimum permeabilization of the tissues/cells that harbor the viral genetic material.…”

Multiplication and haemadsorbing activity of infectious salmon anaemia virus in the established Atlantic salmon cell line.

“…The disease is characterized by severe anaemia in the terminal stages and mortality is usually high (Thorud & Djupvik, 1988), but asymptomatic ISA virus (ISAV) infections have been demonstrated in other salmonids (Nylund et al, 1994). Enveloped 100 nm virus particles have been detected in tissue from ISA-diseased Atlantic salmon by transmission electron microscopy (TEM) studies (Nylund et al, 1995). ISAV has not previously been isolated in any of the commercially available continuous fish cell lines, but the establishment of a long-term cell line from Atlantic salmon Author for correspondence : Ann-Inger Sommer.…”

“…AS cells, both non-infected and ISAV-infected, were fixed 11 days after inoculation using 1 % glutaraldehyde and 4 % formaldehyde in phosphate buffer, 220 mOSm, and embedded in Epon\Araldite. Thin sections of pelleted cells were stained for 1n5 h with 2 % aqueous uranyl acetate solution (Nylund et al, 1995), and stained for 12 min with Reynold's lead citrate before examination in a JEOL JEM 1010 transmission electron microscope. Numerous virus particles somewhat variable in size and form were seen.…”