Pathology of bacterial gill disease: sequential development of lesions during natural outbreaks of disease

The sequential pattern of bacterial colonization and the sequential development of branchial lesions during typical outbreaks of bacterial gill disease (BGD) was studied among 4 groups of commercially reared rainbow trout fingerlings, Oncorhynchus mykiss. During a 5-month monitoring regime, before the onset of natural outbreaks of BGD, gill morphology of examined fish remained unaltered. Bacterial colonization of the gills was immediately preceeded by several gill changes which were widely present in fish from all groups under study. These changes, which were detectable at the ultrastructural level only, included cytoplasmic blistering and degeneration of the microridges of the superficial filament epithelium, in addition to slight topographical irregularity of the filament tips suggestive of mild hyperplasia. Bacterial colonization began at these altered filament tips before spreading to more proximal regions of the filament and adjacent lamellar surfaces. Explosive increases in proportional morbidity and mortality coincided with the development of the following gill lesions: extensive bacterial colonization of lamellar surfaces, lamellar epithelial hydropic degeneration and necrosis, and lamellar oedema. In contrast, gill lesions such as lamellar fusion, epithelial hyperplasia and various metaplastic responses were detected either as subacute (3-5 days) or chronic (7-14 days) changes. The role of epithelial necrosis as an important event during BGD is discussed as a possible mechanism leading to development of sub-acute and chronic gill lesions..