Fig. 4

Parvicapsula pseudobranchicola infection in the pseudobranchs of Atlantic salmon, in histological sections stained using in situ hybridization (ISH) or hematoxylin-eosin (HE). a-c At 35 days post-sea-transfer (PST) most parasite stages occurred inside the primary filament vessels (a, ISH, arrows). The parasite in c (HE) is in an adjacent section to those in b (ISH). d, e At 49 days PST, most parasite cells occur in the lamellar lacunae (d; ISH), but the tissues appear normal (e, HE). f. 89 days PST, when all parasites were extravascular, but small (ISH). g Detail showing parasite cells (ISH); h Accompanying tissue destruction; pseudobranch cell depletion and high cellularity due to numerous parasite cells (HE)