Bacterial gill disease of salmonids; relationship between the severity of gill lesions and bacterial recovery

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Abstract
Thirteen naturally occurring outbreaks of bacterial gill disease (BGD) affecting commercially reared salmonids in Ontario (mainly rainbow trout, Oncorhynchus mykiss) were examined histologically and bacteriologically. Bacteria recovered from healthy and diseased gills were enumerated, and the severity of the lesions from each outbreak was graded based on the histopathological changes. A positive correlation was found between the presence of gill lesions and the number of bacteria recovered from diseased gills. Clinically healthy rainbow trout had 4.95 .times. 103 colony forming units (CFU) per g of gill tissue and were histologically normal. The mean of the total yellow pigmented bacteria (YPB) recovered from healthy gills was 0.73 .times. 103 CFU g-1 with the YPB accounting for roughly 15% of the total recovered. During BGD, a significant increase was observed in the total aerobic, total YPB, and percent YPB from diseased gills. Mild outbreaks of BGD displayed limited lamellar epithelial and chloride cell hypertrophy and had, on average, 1.50 .times. 104 CFU g-1 gill tissue. Moderate outbreaks of BGD, associated with 9.57 .times. 104 CFU g-1, displayed increased evidence of lamellar epithelial and chloride cell degeneration, necrosis, and exfoliation with moderate epithelial hyperplasia. Severe BGD had 3.91 .times. 106 CFU g-1 gill tissue and displayed extensive lamellar epithelial hyperplasia and fusion. The mean of the total YPB recovered from gills from 13 outbreaks was 12.71 .times. 105 CFU g-1; the YPB accounted for approximately 35% of the total bacteria recovered, and are considered to be the most significant isolates for most outbreaks in Ontario hatcheries. Statistical analysis on the severity of lesions and bacterial recovery indicated a strong association between the severity of lesions and the presence of filamentous bacteria.

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