Bacterial kidney disease (BKD) in rainbow trout (Oncorhynchus mykiss)

Heindrich Snyman, Hugh Cai, Mykolas Kamaitis,  

Animal Health Laboratory, University of Guelph, Guelph, ON (Snyman, Cai); Upper Grand Veterinary Services, Guelph, ON (Kamaitis). 

AHL Newsletter, 2020;24(2):17. 

 
During early spring, a commercial aquaculture operation was experiencing a slow cumulative increase in the daily mortality rate of a group of rainbow trout (Oncorhynchus mykiss). On-farm postmortems were performed on the daily mortalities and it was noted that some of these fish contained pale gray and granular-appearing swollen kidneys. Formalin-fixed tissue samples and plated bacterial cultures of gill, spleen, and kidney from the affected fish were submitted to the Animal Health Laboratory for further analysis and testing.  

Histological evaluation of the head- and trunk-kidneys revealed numerous scattered, dense, coalescing interstitial aggregates of macrophages, fewer neutrophils, and occasional central cavitary regions of necrotic cellular debris and fibrin (Fig. 1A & 1B). Diffusely, macrophages contained abundant numbers of ~ 0.5 µm diameter intracellular gram-positive bacilli (Fig. 1C & 1D). Within regions not effaced by the histiocytic inflammation, there was widespread expansion of the interstitial hematopoietic precursors cells with separation of individual tubules, consistent with chronic systemic inflammation. Similar aggregates with intra-histiocytic coccobacilli were also present in the liver, spleen, and occasional gill filaments, while the same type of infiltrate also expanded the meninges within the brains (Fig. 1E). Within the heart there was widespread endocardial cell hypertrophy with acute myofiber degeneration and necrosis, and similar dense aggregates of macrophages expanded the epicardial surface (Fig. 1F). Both the epicardial macrophages and hypertrophic endocardial cells contained abundant numbers of the same intracytoplasmic coccobacilli. Bacterial cultures did not yield any pathogens; however, the histological changes were highly consistent with a diagnosis of disseminated Bacterial Kidney Disease (BKD). 

BKD is an important infectious disease of salmonid aquaculture worldwide. It can affect all species of wild and reared fresh- and salt-water salmonids, but is particularly important in pacific salmonid aquaculture (e.g. Rainbow trout, Chinook and Coho salmon). The causative agent is Renibacterium salmoninarum, a gram-positive, non-motile, intracellular bacterium of the Micrococcaceae family. Inflammation can vary from granulomatous to pyogranulomatous with varying amounts of accompanying necrosis. The bacteria typically infect primarily macrophages and although the head and trunk kidney are a commonly-affected site (as indicated by the name), infections are often disseminated throughout multiple visceral organs and tissues. Vertical transmission through the egg occurs and although all ages are susceptible to infection, disease outbreaks usually tend to occur in older age classes.   
 
On standard bacteriological media, R. salmoninarum is very slow-growing and even with the use of specialized media and the addition of antibiotics and antifungals designed to preferentially enhance its growth, cultures often become overgrown by other faster-growing microorganisms. Routine bacteriology is therefore not recommend for detection of infection. There are very few gram-positive bacterial fish pathogens and therefore the presence of characteristic bacteria and consistent histological changes are considered pathognomonic for BKD. Further confirmation can however be sought through PCR testing, ELISA serology, or 16S rRNA sequencing.   AHL 

   Figure 1. Bacterial kidney disease in farmed rainbow trout. A&B. Trunk kidney with disseminated coalescing 

   interstitial histiocytic aggregates (asterisks). C&D. Lesions contain abundant intra-histiocytic gram-positive   

   bacilli. E. Meninges are expanded by the same dense histiocytic infiltrates (asterisks). F. The heart contains  

   widespread endocardial cell hypertrophy (arrows) with myofiber degeneration and necrosis, and intracellular   

   bacilli.  
 

References 
1. Ferguson H et al. Systemic Pathology of Fish. Scotian Press; 2nd edition. 2006. ISBN-10: 0955303702. 
2. Kent M, Poppe T. Diseases of seawater net-pen reared Salmonid fishes. Quadra printers Ltd. ISBN 0-920225-10-1. 
3. Noga E. Fish Disease: Diagnosis and Treatment. Wiley-Blackwell; 2nd edition. 2010. ISBN-10: 0813806976.