Histopathological findings in farmed rainbow trout (Oncorhynchus mykiss) naturally infected with 3 different Aeromonas species

Abstract

This study describes the macroscopic and microscopic lesions in farmed rainbow trout (Oncorhynchus mykiss) naturally infected with genetically identified Aeromonas salmonicida, A. hydrophila, and A. veronii species. The genus Aeromonas includes bacteria that naturally inhabit both waterways and organisms. At least 27 Aeromonas species have been identified to date, some of which can cause significant economic losses in aquaculture. As up to 68.8% of Aeromonas isolates may be misidentified in routine biochemical and phenotypic tests, however, reported cases of Aeromonas infection in fish may be wrongly identified. Our findings confirmed that the 3 Aeromonas species studied are associated with septicemia and dermal lesions in rainbow trout.

The genus Aeromonas of the family Aeromonadaceae includes bacteria that are pathogenic to fish and other animals. Aeromonas bacteria are also the etiologic agent responsible for a variety of infections in both immunocompetent and immunocompromised humans (1). In salmonids, A. salmonicida causes furunculosis, a disease characterized by skin ulcers and septicemia. Other Aeromonas species are involved in similar pathological conditions (2). Usually, the diagnosis is based on isolating a Gram-negative, motile or non-motile bacterium, identified mainly by routine phenotypic and biochemical tests.

To date, at least 27 species have been genetically identified and recognized in the genus Aeromonas (3–5). When the phenotypic identification of Aeromonas isolates is compared to the genetic identification, 35.5% to 68.8% of trout isolates may be misidentified (6,7). It is likely that the species could be incorrectly identified in some published reports of diseased salmonids, where Aeromonas was isolated and identified with phenotypic testing. The objective of this study was to describe the lesions observed in farmed rainbow trout that were naturally infected with 3 different species of Aeromonas, which were previously phenotypic and genetically identified (7,8).

Tissue samples from 5 farmed rainbow trout with lesions associated with Aeromonas infection were preserved in 10% neutral buffered formalin and processed, sectioned, and stained for routine histological examination. In all cases, no other bacterial agents or infectious pancreatic necrosis virus were identified. All Aeromonas species were genetically identified (7,8). In Case 1, the isolation of A. veronii, 2 fish, 16-to-18 cm long, raised in a concrete pond with spring water from a farm in Mexico State, were studied. Dermal erosion with exposition of premaxilar bone tissue was observed in both fish (Figure 1a). A. veronii was isolated from the gills of 1 fish and the dermal lesion of the other fish. In Case 2, isolation of the motile biovar of A. salmonicida, 1 fish, 17-cm long from a natural pond with spring water from a farm in Hidalgo State, was studied. A circular ulcer (13-mm diameter) was observed at the base of the right pelvic fin (Figure 2a). The isolation of A. salmonicida was obtained from the lesion. In Case 3, isolation of A. hydrophila, 1 fish, 18 cm-long, from a natural pond with spring water from a farm in Michoacan State, was studied. A circular dermal ulcer (7- × 8-mm diameter) was observed at the base of the left ventral fin, as well as petechias and desquamation around the affected area (Figure 3a). In Case 4, isolation of A. veronii, 1 fish, 16 cm long, from a natural pond with river water from a farm in Hidalgo State, was studied. A conspicuous epidermal erosion of the dorsal fin was observed. Aeromonas veronii was isolated from the lesion, kidney, and liver.

Figure 1.

Macroscopic and microscopic lesions in rainbow trout naturally infected with A. veronii. a — Erosion of dermis and exposition of premaxilar bone tissue. b — Premaxilar dermis with inflammatory infiltration, edema, and free-melanine (hemotoxylin & eosin). c — Liver, extensive hemorrhage in parenchyma (hemotoxylin & eosin). d — Liver, slight melanomacrophage center (MMC) infiltration in the centrilobular region, degeneration, and hepatocellular necrosis (hemotoxylin & eosin). e — Kidney, moderate MMC interstitial infiltration (hemotoxylin & eosin).

Figure 2.

Macroscopic and microscopic lesions in rainbow trout naturally infected with the motile biovar of A. salmonicida. a — Ulcer at the base of the right pelvic fin. b — Dermis, infiltration of macrophages, hyperemia, and edema in the affected area (hemotoxylin & eosin). c — Liver, slight periportal lymphocyte infiltration and moderate hepatocellular degeneration (hemotoxylin & eosin). d — Kidney, slight melanomacrophage center (MMC) interstitial infiltration (hemotoxylin & eosin).

Figure 3.

Macroscopic and microscopic lesions in rainbow trout naturally infected with A. hydrophyla. a — Ulcer at the base of the left ventral fin. b — Dermis, macrophage infiltration and edema (hemotoxylin & eosin). c — Gram-negative rod resembling Aeromonas spp. in dermis (Gram stain). d — Liver, melanomacrophage center (MMC) infiltration in the periphery of biliary ducts and severe hepatocellular degeneration (hemotoxylin & eosin). e — Liver, lymphocyte infiltration in parenchyma (hemotoxylin & eosin). f — Kidney, severe interstitial MMC infiltration (hemotoxylin & eosin).

Case descriptions

Case 1

At the histological study, a predominant macrophage infiltration with scattered neutrophils, edema, a few melanomacrophage centers (MMCs), and free-melanine in the limits of the epidermal lesion were observed (Figure 1b). Severe hyperemia and hemorrhage in parenchyma (Figure 1c) with a few MMCs were observed in the liver (Figure 1d). Bacterial rods resembling Aeromonas spp., were observed in the perivascular space. A moderate interstitial MMC infiltration was observed in the kidney (Figure 1e).

Case 2

At the histological study, a predominant macrophage infiltration, blood vessel capillary hyperemia, and edema of the dermis were observed (Figure 2b). A slight lymphocyte infiltration at the periphery of the biliary ducts of the liver and moderate hepatocellular degeneration were observed (Figure 2c). A slight interstitial infiltration of MMC was observed in the kidney (Figure 2d).

Case 3

At the epidermis, histopathological findings included inflammatory infiltration of macrophages predominantly and edema (Figure 3b). At the Gram staining of the same tissue, a bacterial rod resembling Aeromonas spp. was observed (Figure 3c). In the liver, a slight MMC infiltration in the centrilobular region and severe hepatocellular degeneration (Figure 3d) and slight lymphocyte infiltration in the centrilobular region were observed (Figure 3e). A severe interstitial infiltration of MMC was observed in the kidney (Figure 3f).

Case 4

At the histological study, inflammatory infiltration of neutrophils, macrophages, and MMC were observed (Figure 4a). In the liver, moderate hyperemia in the centrilobular region, slight perivascular MMC infiltration, and slight hepatocellular degeneration were observed (Figure 4b).

Figure 4.

Macroscopic and microscopic lesions in rainbow trout naturally infected with A. veronii. a — Erosion of dorsal fin, moderate melanomacrophage center (MMC) infiltration (hemotoxylin & eosin). b — Liver, hyperemia in parenchyma, MMC infiltration, and hepatocellular degeneration (hemotoxylin & eosin).

In all the fish studied, the histopathological findings in gills included hyperplasia of epithelial cells located at the base and epical portions, loss of normal architecture with different degrees of severity in multifocal areas, telangiectasia, and edema. Lesions observed in gills may not be associated with Aeromonas infection, as similar lesions can be caused by other aquatic factors, such as water quality and toxins (9).

The histopathological findings of this study confirmed that several Aeromonas species are pathogenic to rainbow trout. In general, dermal lesions are associated with Aeromonas infection in fish (10–14). The ulcer observed at the base of the pelvic fin in 1 fish (Figure 2a) with isolation of the motile biovar of A. salmonicida appears to be a particularly common lesion in cases of furunculosis produced by this Aeromonas species (2). Similarly, vascular congestion and cytoplasmic vacuolization of hepatocytes have been reported in rainbow trout experimentally infected with A. salmonicida subsp. salmonicida (15). In Case 2 of the present study, the inflammatory lesions observed in the liver and kidney indicate a septicemic potential of the motile biovar of A. salmonicida. The species A. veronii and A. hydrophila have also been associated with diseases in ornamental and farmed fish (16,17).

In conclusion, our findings confirmed that the Aeromonas species, A. hydrophila, A. salmonicida, and A. veronii, are associated with septicemia and dermal lesions in rainbow trout. Our results may prove useful to future studies of rainbow trout where an Aeromonas species could be involved.