Skin ulcers in estuarine fishes: a comparative pathological evaluation of wild and laboratory-exposed fish.

The toxic dinoflagellate Pfiesteria piscicida Steidinger & Burkholder has recently been implicated as the etiologic agent of acute mass mortalities and skin ulcers in menhaden, Brevoortia tyrannus, and other fishes from mid-Atlantic U.S. estuaries. However, evidence for this association is largely circumstantial and controversial. We exposed tilapia (Oreochromis spp.) to Pfiesteria shumwayae Glasgow & Burkholder (identification based on scanning electron microscopy and molecular analyses) and compared the resulting pathology to the so-called Pfiesteria-specific lesions occurring in wild menhaden. The tilapia challenged by high concentrations (2,000-12,000 cells/mL) of P. shumwayaeexhibited loss of mucus coat and scales plus mild petecchial hemorrhage, but no deeply penetrating chronic ulcers like those in wild menhaden. Histologically, fish exhibited epidermal erosion with bacterial colonization but minimal associated inflammation. In moribund fish, loss of epidermis was widespread over large portions of the body. Similar erosion occurred in the mucosa lining the oral and branchial cavities. Gills exhibited epithelial lifting, loss of secondary lamellar structure, and infiltration by lymphoid cells. Epithelial lining of the lateral line canal (LLC) and olfactory organs exhibited severe necrosis. Visceral organs, kidney, and neural tissues (brain, spinal cord, ganglia, peripheral nerves) were histologically normal. An unexpected finding was the numerous P. shumwayae cells adhering to damaged skin, skin folds, scale pockets, LLC, and olfactory tissues. In contrast, histologic evaluation of skin ulcers in over 200 wild menhaden from Virginia and Maryland portions of the Chesapeake Bay and the Pamlico Estuary, North Carolina, revealed that all ulcers harbored a deeply invasive, highly pathogenic fungus now known to be Aphanomyces invadans. In menhaden the infection always elicited severe myonecrosis and intense granulomatous myositis. The consistent occurrence of this fungus and the nature and severity of the resulting inflammatory response indicate that these ulcers are chronic (age >1 week) and of an infectious etiology, not the direct result of an acute toxicosis initiated by Pfiesteria toxin(s) as recently hypothesized. The disease therefore is best called ulcerative mycosis (UM). This study indicates that the pathology of Pfiesteria laboratory exposure is fundamentally different from that of UM in menhaden; however, we cannot rule out Pfiesteria as one of many possible early initiators predisposing wild fishes to fungal infection in some circumstances.

Renal lesions in estuarine fishes collected from the Elizabeth River, Virginia.

The renal lesions of five species of estuarine fishes from the heavily polluted Elizabeth River, Virginia, and the nearby, less polluted Nansemond River were studied grossly and microscopically. No gross lesions were observed but microscopic lesions occurred in renal tissue. Mesangial sclerosis occurred in hogchoker, and mesangiolysis occurred in both hogchoker and spot. Glomerular lesions in spot and hogchoker were more frequent in Elizabeth River samples than in Nansemond River samples. Tubular lesions occurred in both Nansemond River and Elizabeth River sciaenids and hogchoker; the prevalence of these lesions was higher in Elizabeth River sciaenids but equal in hogchoker. All lesions in toadfish were found only in fish from the Elizabeth River.

Host-parasite relationship of Ergasilus labracid Krøyer (Cyclopidea, Ergasilidae) and the striped bass, Morone saxatilis (Walbaum) from the lower Chesapeake Bay.

Epithelial hyperplasia was the most distinctive pathological change in gill filaments of striped bass, Morone saxatilis (Walbaum), from the lower Chesapeake Bay infested by Ergasilus labracis Krøyer. This tissue reaction initiated in the filament area most adjacent to the mouth of the attached parasite. Interruption of parasite egg sac production and eventual dislodgement associated with swelling of the gill filaments occurred unilaterally on gill arches of either side of the branchial basket. The largest number of parasites in all fish was found on the first outer hemibranch while abundance on successive hemibranches decreased in rank order. Copepods on the inner hemibranch, however, were least numerous on the first inner hemibranch and more numerous on the fourth inner hemibranch. On the whole, the number of parasites attached to the outer hemibranch outnumbered the number of parasites established on the inner hemibranchs. This differential settlement was most evident in low to moderate infections and became less apparent in heavy infections. Newly settled copepods showed highest abundance on the third arch and least abundance on the first and fourth arches. The causes for this differential distribution on the gill arches are discussed.