A Morphological Study of the Epitheliocystis Agent in Farmed Atlantic Salmon

During the last few years there have been several reports of epitheliocystis on farmed Atlantic salmon Salmo salar along the Norwegian coast, and heavy mortalities have been associated with the disease in some cases. Epitheliocystis has previously been described from juvenile steelhead Oncorhynchus mykiss, juvenile lake trout Salvelinus namaycush, and several different nonsalmonid species. These studies of epitheliocystis suggest that the agent belongs to the order Rickettsiales or Chlamydiales. The epitheliocystis agent found in Atlantic salmon was an intracellular, gram-negative bacteria with three distinctive developmental stages: reticulate bodies (RB), intermediate bodies (IB), and elementary bodies (EB). All three stages were present within membrane-bound vacuoles in gill epithelial cells. The bodies were surrounded by two trilaminar membranes and contained ribosomes and nucleic acids. The nucleic acids in the IB and EB were condensed to a nucleoid, while the nucleic acids in the RB were found as strands of chromatin. The RB could be as long as 10 μm and could branch. The EB were coccoid with a diameter of about 0.5 μm. Based on the morphology (i.e., normal anatomy of the agent, formation and structure of altered [abnormal] forms of the agent, and features of interaction with host cells), it is suggested that the epitheliocystis agent from Atlantic salmon belongs to the order Chlamydiales.