Vaccine-associated granulomatous inflammation and melanin accumulation in Atlantic salmon, Salmo salar L., white muscle.

The purpose of this study was to investigate the nature of variably sized pigmented foci encountered in fillets of farmed Atlantic salmon, Salmo salar L. The material was sampled on the fillet production line and on salmon farms from fish with an average size of 3 kg from various producers. The fish had been routinely vaccinated by injection. Gross pathology, histology, immunohistochemistry using antisera against major histocompatibility complex (MHC) class II beta chain and transmission electron microscopy (TEM) were used to characterize the changes. Macroscopically, melanized foci were seen penetrating from the peritoneum deep into the abdominal wall, sometimes right through to the skin, and also embedded in the caudal musculature. Histological investigation revealed muscle degeneration and necrosis, fibrosis and granulomatous inflammation containing varying numbers of melano-macrophages. Vacuoles, either empty or containing heterogeneous material, were frequently seen. The presence of abundant MHC class II+ cells indicated an active inflammatory condition. TEM showed large extracellular vacuoles and leucocytes containing homogeneous material of lipid-like appearance. The results showed that the melanized foci in Atlantic salmon fillet resulted from an inflammatory condition probably induced by vaccination. The described condition is not known in wild salmon and in farmed salmon where injection vaccination is not applied.

Granulomatous uveitis associated with vaccination in the atlantic salmon.

This study addressed histologic and immunopathologic changes in ocular tissues and investigated the distribution of major histocompatibility class II (MHC class II)-positive cells in Atlantic salmon (Salmo salar) suffering from severe postvaccination disease. Twenty-nine fish with generalized inflammation, probably a result of vaccination, were investigated. One individual that had escaped vaccination was included in the study. Material was investigated by cultivation methods for fungi and bacteria. Histology using conventional staining procedures and immunohistochemistry with antisera against MHC class II beta chain were performed. No growth was observed from the cultivation investigations. Histology revealed occlusion of the lumen in the larger choroid vessels and in the choriocapillaris, inflammatory infiltrations and loss of structure in the choroid rete, and, in some cases, aggregations of multinucleated giant cells (MGC) and Splendore-Hoeppli material. Immunohistochemistry demonstrated massive MHC class II+ cellular infiltrations in the uveal tract. Such infiltrations were also seen in the ventral ciliary cleft, a condition that is associated with glaucoma. Immunoreactive cells included dendritelike cells, epithelioid cells, and MGCs. The endothelia of smaller vessels were frequently MHC class II+, and immunoreactive infiltrations were seen in the optic nerve in several individuals. No pathologic changes were detected in the unvaccinated individual. In conclusion, generalized inflammatory reactions in fish may lead to severe ocular inflammation, occlusion of uveal vessels, and perivascular changes with MHC class II+ upregulation in cells in the uveal tract and optic nerve.