Aphanomycesas a Cause of Ulcerative Skin Lesions of Menhaden from Chesapeake Bay Tributaries

Abstract

During the summer and fall of 1997, an unusually high prevalence of skin lesions in fishes from Chesapeake Bay tributaries as well as two fish kills in the Pocomoke River stimulated significant public concern. Atlantic menhaden Brevoortia tyrannus were the most frequent target of the acute fish kills and displayed skin lesions that were attributed to the presence of the toxic dinoflagellate Pfiesteria piscicida. Hence, the penetrating skin ulcers so commonly found in this species are now widely viewed by the general public and some scientists as Pfiesteria‐related and to be caused by exposure to Pfiesteria toxin. We examined, histologically, 121 menhaden with these ulcers collected from both Maryland and Virginia waters of the Chesapeake Bay in 1997 and 31 from the Pocomoke and Wicomico rivers in 1998. All of the deeply penetrating ulcers, as well as raised lesions (with or without eroded epithelium), were characterized by deeply penetrating fungal hyphae surrounded by chronic, granulomatous inflammation. These lesions had an appearance identical or similar to epizootic ulcerative syndrome (EUS), an ulcerative mycotic syndrome of fishes in other parts of the world caused by the fungal pathogen Aphanomyces invadans. They were also identical to ulcerative mycosis of menhaden previously reported along the Atlantic coast of the USA as associated with Aphanomyces spp. In 1998, using methods for isolation of A. invadans, we were able to culture from affected menhaden an Aphanomyces sp. that by preliminary tests is similar or identical to A. invadans. We believe these findings suggest that factors other than Pfiesteria toxin need to be considered as the cause or initiator of these lesions.

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

“…Whereas the menhaden ulcers are infectious and develop over an extended period (weeks) (14), Pfiesteria exposure of tilapia results in rapid development (hours) of acute widespread erosion of the epidermis, followed quickly by death. UM is an infectious disease characterized by one or more focal, deeply penetrating, often perianal ulcers.…”

“…UM is an infectious disease characterized by one or more focal, deeply penetrating, often perianal ulcers. The causative agent is now known to be an invasive, highly pathogenic Oomycete fungus called A. invadans (14,15). This fungus is also the cause of EUS in the IndoPacific region and has caused extensive losses to commercial fisheries there since the early 1970s (11)(12)(13).…”

“…Menhaden ulcers are characterized histologically by the consistent presence of deeply penetrating fungal hyphae, extensive myodegeneration, and a special type of host cellular defensive response called granulomatous inflammation directed at the fungal agent (6,14). This type of response is typically elicited in vertebrates by infections that are difficult for a host to kill and degrade and is composed predominantly of a mononuclear phagocyte called the macrophage (24).…”

“…Aphanomyces invadans, an Oomycete fungus, was recently implicated as the etiologic agent of EUS. Grossly and histologically, EUS in Indo-Pacific fishes and UM in menhaden from mid-Atlantic U.S. estuaries are essentially identical, and a recent investigation indicates that the fungal agent observed in menhaden UM is also A. invadans (14,15). Recent work in our laboratory indicates this fungal agent may be a primary pathogen in menhaden, capable of inducing formation of skin ulcers, with other environmental or biological factors, including Pfiesteria spp., playing no role whatsoever (16).…”

Differential Diagnosis of Ulcerative Lesions in Fish

“…These causative agents are closely interrelated and may act through several pathways to cause injury to host cells either directly or indirectly. Blazer and colleagues examined Atlantic menhaden from Chesapeake Bay and its tributaries in Maryland and Virginia (19). Of 121 lesioned menhaden from the Chesapeake Bay collected in 1997 and another 31 collected from the Pocomoke and Wicomico Rivers in Maryland in 1998, all had deeply penetrating fungal hyphae surrounded by intense granulomatous inflammation.…”