Regardless of the cause of the inflammatory response, granulomatous inflammation and granulation tissue can resemble neoplasms, and the suffix of the term granuloma adds to the potential confusion. A common cause of granulomas in fish is mycobacteria (Nigrelli and Vogel, 1963; Beckwith and Malsberger, 1980), but similar lesions are caused by other pathogens (Majeed et al., 1981; McVicar and McLay, 1985) or are idiopathic (Munkittrick et al., 1985). In some cases, granulomatous exudate can occur in multiple sites, displace normal tissue, and cause a distention of the body (Fig. 3.3). Identification of the infiltrating cells as macrophages is difficult in routinely stained sections, and these lesions could be mistaken for neoplasia, especially when the cause of the lesion is not apparent. Granulation tissue and granulomas have been the cause of erroneous reports of neoplasms in experimental studies (Beckwith and Malsberger, 1980).

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