Multinucleated cells, often simply called giant cells, are found in a variety of situations in teleost fish. They are not uncommon in granulomatous inflammatory responses, such as bacterial kidney disease, proliferative kidney disease, or as a response to vaccine, and are a result of fusion of macrophages or epithelioid cells. They are also found in virus infections, so-called syncytial giant cells, in which a number of non-macrophage cells, such as hepatocytes, fuse together. But giant cells are also found in normal fish as osteoclasts. Although...