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

Abstract

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.

Occurrence of nodular gill disease in farmed brown trout (Salmo trutta L.)

“…The episodes of NGD described by Speare () in the Arctic char and by Tubbs et al () in the Chinook salmon showed that species of freshwater salmonids other than rainbow trout may be heavily affected by this disease. Tubbs et al () reported a daily mortality rate of 1.5% in Chinook salmon fry concomitant with non‐optimal water conditions because of the presence of suspended solids. Speare () described a case of NGD and BGD co‐infection in the Arctic char with high mortality rate.…”

“…In 1999, Speare described a mixed infection of bacterial gill disease (BGD), caused by Flavobacterium branchiophilum , and NGD in arctic char ( Salvelinus alpinus ) and rainbow trout farmed in eastern Canada. This was the first and only report of NGD in a salmonid species other than rainbow trout, until 2010, when Tubbs, Wybourne, and Lumsden () reported an episode of NGD in Chinook salmon ( Oncorhynchus tshawytscha ) farmed in freshwater raceways in New Zealand.…”

“…The episodes of NGD described by Speare (1999) in the Arctic char and by Tubbs et al (2010) and Carpione del Garda (Salmo carpio) for commercial and restocking purposes in addition to rainbow and brown trout. In the absence of epidemiological studies, the spread of NGD in these species of salmonids needs to be further investigated, in order to assess a possible impact of disease on wild fish populations when infected trout are restocked, since farming conditions can amplify amoeba gill load.…”

Testate amoeba Rhogostoma minus Belar, 1921, associated with nodular gill disease of rainbow trout, Oncorhynchus mykiss (Walbaum)

“…To date, naked amoebae belonging to five genera, Acanthamoeba, Vermamoeba (formerly Hartmannella), Naegleria, Protacanthamoeba and Vannella, have been reported as potential agents characterized by morphological and molecular data (Dykov a et al 2010). As other studies of NGD in freshwater salmonids reported just a general amoebic aetiology (Speare 1999;Buchmann et al 2004;Antychowicz 2007;Tubbs, Wybourne & Lumsden 2010), a broader spectrum of agents is conceivable. To better understand the aetiology of NGD, data from outbreaks should more effectively characterize the amoebae by culture, morphology, and genetic studies.…”

“…As other studies of NGD in freshwater salmonids reported just a general amoebic aetiology (Speare ; Buchmann et al . ; Antychowicz ; Tubbs, Wybourne & Lumsden ), a broader spectrum of agents is conceivable. To better understand the aetiology of NGD, data from outbreaks should more effectively characterize the amoebae by culture, morphology, and genetic studies.…”