Few infectious agents specifically target the gastrointestinal tract of fish. The gastrointestinal tract often becomes secondarily involved as a systemic extension from the primary organ infected. An endocrine disorder resulting in a nodular mass or masses of nonneoplastic thyroid epithelial cell proliferation in the oral and branchial cavity of fish. Goiter has been reported in numerous species of wild and captive freshwater, brackish and marine fishes. Thyroid tissue in fish lacks a discrete fibrous capsule, is capable of ectopic growth to nonpharyngeal sites, and is frequently predisposed to hyperplastic proliferation. A variety of tumors have been reported from the oral and branchial cavity of fishes and include spontaneous neoplasia in addition to those caused by exposure to environmental agents and infectious agents. Affected fish often present with emaciation, lethargy, hyperpigmentation and a history of increasing mortality. The bacteria is most likely shed into the water through the feces of infected fish and acquired by ingestion of water or contaminated feed.