Bacterial gill disease in a research colony of lake whitefish

Heindrich Snyman, Marcia Chiasson, Calvin Kellendonk, Patricia Bell-Rogers, Qiumei You, Lisa Ledger, Jason Eidt, Hugh Cai

Animal Health Laboratory, University of Guelph, Guelph, ON (Snyman, Kellendonk, Bell-Rogers, You, Ledger, Eidt, Cai); Alma Aquaculture Research Station, University of Guelph, Elora, ON (Chiasson). 

AHL Newsletter 2020;24(3):13.

A research colony of lake whitefish (Coregonus clupeaformis) held in flow-through tanks at 8.5oC were experiencing a sudden increase in daily mortality, losing ~ 93 fish out of a population of 574 over a 5 day period (~ 16% cumulative mortality rate).  Fish were exhibiting flared gills with foci of hemorrhage along the gill base and occasional fin erosions.  Dissolved oxygen was ~ 7.0 mg/L, which is within an acceptable range for salmonid fishes.  Feed was withheld and aeration was increased in an effort to alleviate the respiratory distress.  Although this did slow the mortality rate, fish were still exhibiting respiratory distress and a subset of four fish were submitted fresh dead to the Animal Health Laboratory for analysis.

Fish ranged from 28- 35 cm in length and all fish contained flared opercula and diffusely congested gill arches.  There were scattered hemorrhages throughout the gill filaments, along the base of the gill arches and the pectoral fins, and two of the fish contained intra-ocular hemorrhage.  Gill wet mounts of all four fish revealed widespread lamellar hyperplasia with interspersed dense meshed mats of filamentous bacteria (Fig. 1).  Tissues were collected for histopathology and bacterial culture, and acute viral hemorrhagic septicemia (VHS) was ruled out by PCR testing.

Histologically, there was widespread lamellar epithelial hyperplasia that extended along the entire length of the filaments, often filling the interlamellar spaces and resulting in lamellar fusion (Fig. 2).  Filling the remaining interlamellar spaces and lining the hyperplastic epithelium were scattered dense mats of long slender filamentous bacteria (Fig. 3).  Aerobic culture of gill surface swabs yielded Flavobacterium branchiophilum, which together with the histological changes were consistent with a diagnosis of bacterial gill disease (BGD).

BGD is caused by the gram negative, non-motile, slender filamentous bacterium Flavobacterium branchiophilum.  The disease is typically seen in young intensively reared salmonids and is considered one of the most significant infectious diseases affecting freshwater salmonid aquaculture worldwide. Rainbow trout aquaculture represents the majority of these cases; however, all Salmonidae are theoretically susceptible.  Despite rearing lake whitefish for four years and having typical historic outbreaks of BGD in rainbow trout, this is the first time this disease has been identified in lake whitefish in this facility and at the AHL. The affected tank of lake whitefish was effectively treated with a 1-hour static bath of 10 ppm Chloramine-T on three occasions with one day in between treatments.   AHL       

Figure 1. Gill wet mount with meshed mats of filamentous bacteria (arrows).

Figure 1. Gill wet mount with meshed mats of filamentous bacteria (arrows).

Figure 3. Higher magnification of the gill with fused gill lamellae (asterisks) with surface colonies of filamentous bacteria (arrow)

Figure 2. Histopathology of gill with fused gill lamellae (asterisks) and surface colonies of filamentous bacteria (arrow).  (H&E)

Figure 4. Surface colonies of meshed filamentous bacteria (arrows).

Figure 3. Higher magnification of surface colonies of meshed Flavobacterium branchiophilum bacteria (arrows). (H&E)



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