Understanding Aeromonas salmonicida Infections in Salmonids

1. Ichthyophonus hoferi in salmonids (continued): Sometimes this pathogen is killed…

2. .. but the infection may become overwhelming, as in this kidney, where despite an obvious host response, the organisms are not contained. In such heavy infections the organisms are distributed throughout the body.

3. This is another salmon kidney showing giant cells and extensive destruction and replacement with inflammatory reaction. At higher magnification this material clearly has a basophilic core. A large focus of necrosis is present, outlined by eosinophilic material (arrows). Similar material is scattered through the remaining kidney interstitium.

4. Another area from the same kidney clearly shows the granulomatous nature of the reaction. These are Gram-positive filamentous rods, but were not identified. (No cultures undertaken. Nocardia is suspected on clinical history, and has been associated with Splendore-Hoppli reaction in mammals.) Splendore-Hoeppli protein is a response to continued exposure to certain types of antigen. It is relatively rare in fish, with the possible exception of a response to vaccines containing adjuvant. More examples can be expected here as adjuvant vaccines become more common in Australia. Oil emersion shows this reaction to be round bacteria, in this case. The eosinophilic material is Splendore-Hoeppli material (also known as club-colonies). This is comprised largely of antigen-antibody complex. The similarity to such material in mammals demonstrates the similarity of the immunology of all vertebrate classes.

5. Which brings us to..Inflammation & degeneration B. Focal reactions 2. Acute reactions - & more patterns of bacterial infection.

6. This slide shows large clumps of bacteria free within the interstitial kidney interstitial tissue of a Canadian salmon (arrows), with little host response apparent. The clumped nature implies non-motile bacteria (indeed quite “sticky” , a characteristic obvious in cultures). These characteristics are hallmarks of Aeromonas salmonicida. One strain of this bacterium, Aeromonassalmonicida salmonicida, isthe cause of “Furunculosis” - one of one of the oldest and most significant bacterial diseases of fish (particularly salmonid fish). The toxins from these large clumps of bacteria may cause death before an effective host response can be mounted. Other “Atypical” strains are known, particularly in marine fish but also Goldfish. The pathology is similar, though the course of the disease may be less dramatic, with more host response evident.

7. Classic or “typical” strain (Aeromonas salmonicida salmonicida) in a Gram stained section. From slide donated to EAFP teaching set by the Marine Laboratory, Aberdeen, of a moribund Scottish (farmed) Atlantic salmon (x 40 objective)

8. Tasmanian greenback flounder kidney with large focal lesion with faint basophilic masses at the margins. Higher magnification shows these to be similar to previous A. salmonocida slides: This is infection with an atypical A. salmonicida strain (of which there are increasing numbers being recognised). These tend to be overall less pathogenic than the typical salmonid strain, in that fewer animals are affected, but the pathology is similar, although animal may survive longer and mount more of a response.

9. More examples of flounder A. salmonicida showing the bacterial clumps and fibrin at the margin of expanding necrotic zone

10. Flounder isolate of atypical A. salmonicida in Atlantic salmon in interstitium and blood vessel (experimental infection). Similar natural infections have been seen in marine environment. Demonstrates that the various strains are not species-specific, though the susceptibility may vary. (Goldfish strains are also pathogenic to salmon.)