How can the veterinary pathologist beat the fish bacteriologist to a diagnosis?


At last week’s AAPSP Workshop on Fish Pathology, Dr Judith Handlinger provided a beautiful summary of the intricacies in identifying the bacterial disease agents based on observable microscopic lesions. I loved this section of the workshop the most. Imagine that you can beat the bacteriologist to the diagnosis! But in reality, pathologists rely on multiple lines of evidence for a definitive diagnosis.


Gram negative bacteria

Aeromonas salmonicida tend to occur as multifocal dense colonies of fine Gram negative bacteria, free within blood vessels, and in the interstitium of kidney in histology. The reason for them to be so dense is because they are non-motile and quite sticky. The atypical strains also show similar pathology, but clinically less severe with lower morbidity and mortality.


In Yersinia ruckeri infection, splenic lesions may include hyperplasia of the macrophage cuffs, fibrin deposits and oedema, and some intralesional, singular, Gram negative, bacterial rods. There is also mild, multifocal, haematopoietic necrosis in the kidney.


Vibrio anguillarum causes severe septicaemia. The spleen can be enlarged and haemorrhage, ellipsoids degenerate (loss of confluent rim of macrophages) and become depleted of lymphocytes (little white pulp remains).


Vibrio harveyi causes enteritis in barramundi and extensive peritonitis. There is a special strain (Vibrio Phenon 53) where you can get the large form of the bacteria on primary cultures, and reversion to small form on subculture. This large and small form has been described in Vibrio cholera. Suspect it might be due to changes in salinity. The large form can be seen encapsulated in bunches, within host cells.


Tenacibaculum maritimum (formerly Flexibacter maritimus) starts as superficial skin erosion. Microscopically, they line up side-by-side and produce expanding waves of necrosis that precede the bacteria. Early stages of bacterial infection, you may see lymphocyte infiltration in the skin. These fish tend to fend of disease. Those that don’t, develop spongy epithelial hyperplasia, epithelial loss, necrosis. In chronic cases, floccular proteinaceous material collect in scale pockets and there is scale necrosis (see osteoclastic resorption). Where it occurs in the gills and fins, there will be exposed cartilage rays.


Flavobacterium columnare (formerly Flexibacter columnaris) infection causes deep skin necrosis, extending into the musculature. Can also get under-running of damage, with bacteria present under the epithelium. Where it occurs in the gills and fins, there will be exposed cartilage rays.


Flavobacterium branchiophilum causes bacterial gill disease, and the lesion they incite is epithelial hyperplasia instead of necrosis. The less toxic the bacteria, the more hyperplasia and secondary lamellar fusion there is. You can see fine bacterial rods on the surface. There is also a lot of mucus production. Beware not to confuse thee bacteria with mucus strands. Perhaps use a Gram stain or Giemsa stain.


Piscirickettsia causes concentric waves of infection/necrosis in the liver and so grossly, you see targetoid patterns. But typically, you see non-specific petechiation of serosa. Microscopically, you may see intracytoplasmic, Gram negative, coccobacilli in histiocytes.


Infections with Photobacterium damselae subsp. Piscicida, you can see aggregates of gram-negative coccobacilli surrounded by Splendore-Hoeppli material.


Epitheliocystis (Chlamydia) occur as dense, intracytoplasmic colonies, in the gill epithelium. It does not seem to cause pathology in the Barramundi, however, there is focal host reaction in the salmon.



Gram positive bacteria

Lactococcus garvieae (previously Enterococcus seriolicida) are Gram positive cocci. They commonly cause peri-orbital congestion, haemorrhage and oedema (hence exophthalmia/pop-eye). Hagemann strain occurs in trout. Cause enlarged ellipsoids in the spleen, reflect increased phagocytic response. On Gram stains, the bacteria are demonstrated within macrophages in splenic ellipsoid cuffs.


Streptococcus iniae you see clumps of bacteria in heart, surrounded by mixed inflammatory cells. The Gram positive cocci are also found in the brain, gills and other blood vessels.


Renibacterium salmoninarum causes bacterial gill disease. It is a Gram positive, diplobacillus, rod shaped, non-motile, non-spore forming, intracellular bacterium. It causes granulomatous interstitial nephritis with myriad intralesional bacteria.


Carnobacterium (Gram-stain-positive rods) and Vagococcus (Gram-stain-positive cocci) infection you get peritonitis. Common in salmonids, post-spawning, due to retrograde infection up the oviduct.



Acid-fast bacteria

Granulomatous responses seen in mycobacteriosis, however, in seahorses it tends to be less granulomatous, and more histiocytic.


Nocardia (Gram positive, acid-fast, filamentous organisms) also evoke a granulomatous response. The bacterial colonies can also be surrounded by Splendore-Hoeppli material.

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