Nodular gill disease causing proliferative branchitis and mortality in Chinook salmon (Oncorhynchus tshawytscha)

A period of escalating mortality occurred among Chinook salmon (1-5 g) held in freshwater raceways at a commercial culture facility. The mortality rate peaked at 1.5% of the population per day, water temperature was 9-10 degrees C, and water quality had recently deteriorated due to an influx of suspended solids. The affected fish did not respond to several chloramine-T bath treatments (10 ppm for 1 hour).

The dead fish did not exhibit flared opercula typical of infection with bacterial gill disease (BGD) nor did the remaining fish show obvious signs of respiratory distress. Clinical lesions were limited to the gills of the fish, and were consistent with a proliferative branchitis, including substantial hyperplasia and fusion of lamellae.

Numerous angular amoebic organisms (approximately 10 x 8 microm), with a central, large vesicular nucleus and faintly acidophilic cytoplasm, visible on H&E-stained sections, were located on the hyperplastic epithelia of the lamellae, filaments, and remaining lining of the branchial cavity. Morphologically, the amoebae were similar to representatives of the genus Cochliopodia spp. described from North American salmonids, that presented similar lesions and clinical signs; consequently, the disease was diagnosed as nodular gill disease (NGD).

Grossly, NGD can be mistaken for BGD; the distinction is important because chloramine-T, routinely used in the treatment of BGD, is ineffective for the treatment of NGD. Static formalin baths of 170 ppm (37% formaldehyde) for 1 hour, supplemented with oxygen, may reduce amoebic infections but should be used cautiously as treatment can precipitate mortality in fish with gill disease. Cues for the onset of NGD are not completely understood, although poor water quality and elevations in temperature are likely to exacerbate infections. Similarly, the prevalence of this disease is not accurately known, but it is possible that it may be more common among salmonids in New Zealand than is presently recognised.