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.
Pathogenicity and cultural experiments described here provide futher evidence that a distinct species of Aphanomyces is responsible for much of the characteristic pathology of epizootic ulceration syndrome (EUS). Zoospores from 58 fungal isolates were injected intramuscularly in snakehead fish, Channa striata (Bloch). These fungi comprised: Aphanomyces strains isolated from EUS‐affected fish; saprophytic Aphanomyces, Achlya and Saprolegnia spp. from infected waters; and further saprolegniaceous fungi involved in other diseases of aquatic animals. Only the Aphanomyces strains isolated from fish affected by EUS, Australian red spot disease (already considered synonymous with EUS) or mycotic granulomatosis described from Japan were able to grow invasively through the fish muscle and produce the distinctive EUS lesions. In contrast to Aphanomyces astaci Schikora, the EUS‐Aphanomyces was shown to be unable to infect noble crayfish, Astacus astacus L. The snakehead‐pathogenic strains were further distinguished from all the other fungi under comparison by their characteristic temperature‐growth profile and inability to grow on certain selective fungal media.
The cause of deeply penetrating ulcers of Atlantic menhaden Brevoortia tyrannus has been the subject of significant research efforts in recent years. These lesions and the associated syndrome termed ulcerative mycosis have been observed along the East Coast of the United States since at least the early 1980s. Although Aphanomyces spp. were isolated from these lesions in the mid to late 1980s, similar lesions could not be reproduced by experimental infections of Atlantic menhaden with these isolates. The identical characteristic histologic appearance of granulomatous inflammation surrounding the penetrating fungal hyphae occurs in fish with epizootic ulcerative syndrome (EUS), as reported throughout South Asia, Japan, and Australia. Aphanomyces invadans has been found to be the causative agent of EUS in all of these countries. Using methods developed for the study of EUS, we successfully isolated an organism for which the DNA sequence, morphology, temperature and salinity growth characteristics, and infectivity of chevron snakehead Channa striata are identical to A. invadans. Using the polymerase chain reaction assay for A. invadans, we were able to demonstrate the presence of the organism from Atlantic menhaden lesions collected in U.S. estuarine waters from Delaware to South Carolina. In addition, the organism was present in lesions on a bluegill Lepomis macrochirus from a farm pond in Georgia and channel catfish Ictalurus punctatus from a farm pond in Louisiana.
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