The interbranchial lymphoid tissue (ILT) was recently described in the gills of salmonids. This study
examined changes in the ILT during a parasitic infection in marine environment, using amoebic gill
disease (AGD) as a model. Atlantic salmon (Salmo salar) experimentally infected with Neoparamoeba
perurans were sampled at 0, 3, 7, 14 and 28 days post challenge. Transversal sections of three areas of the
gills (dorsal, medial and ventral) were histologically assessed for morphological and cellular changes.
AGD induced morphological changes and a cellular response in the ILT of affected fish. These changes
included a significant increase in the ILT surface area in fish 28 days after AGD challenge, compared to
control fish at the same time point. The length of the ILT increased significantly 28 days post exposure in
the dorsal area of the gill arch in the fish affected by AGD. The lymphocyte density of the ILT increased
after AGD challenge, peaking at 7 days post exposure; however, by 28 days post exposure, a reduction of
lymphocyte density to values close to pre-infection levels was observed. PCNA immunostaining revealed
that epithelial hyperplasia was the most likely factor contributing to the ILT enlargement in the affected
fish.