A new study, led by NOAA scientists and reported in the peer-reviewed journal Applied and Environmental Microbiology, provides strong evidence that the water mold known as Aphanomyces invadans is the pathogen responsible for seasonal outbreaks of skin ulcers and lesions observed in menhaden and other estuarine fish along the U.S. East Coast.
Large fish lesion events in the 1990s, initially linked to the dinoflagellate Pfiesteria piscicida, caused widespread concern over the safety of seafood and recreational waters. These concerns caused many people to avoid the coast and to avoid eating seafood. An independent study published in 2003 in the journal Ocean and Coastal Management estimated that lost revenues for the tourism, restaurant and seafood industries exceeded $100 million.
Scientists from NOAA's Center for Coastal Fisheries and Habitat Research in Beaufort, N.C., led the research along with experts from the Florida Fish and Wildlife Conservation Commission, the North Carolina Division of Water Quality, the Virginia Institute of Marine Science at The College of William and Mary, and North Carolina State University's College of Veterinary Medicine.
The scientists developed two very specific tests or assays to detect the A. invadans water mold -- one using sensitive polymerase chain reaction (PCR) procedures and the other using fluorescent in situ hybridization (FISH). They first validated these assays in the laboratory before applying them to Atlantic menhaden taken from the Pamlico and Neuse River estuaries in North Carolina.
The results of both assays were the same -- all lesioned menhaden tested positive for A.invidans.
"The FISH assay is the first molecular assay to provide unambiguous visual confirmation that water mold from the ulcerated lesions were exclusively A. invidans," said Mark W. Vandersea of NOAA, corresponding author of the study. Until this study, scientists had been unable to positively identify A.invadans as the only species of water mold responsible for causing the lesions in wild caught fish.
The new study supports a growing body of research evidence, including recently collected data by researchers in southern Asia and Australia, indicating that A.invidans, rather than Pfiesteria, is the major cause of ulcers and lesions in fish.
A. invidans, however, should not be considered the sole cause for ulcerative lesions. Stress, poor nutrition, and certain parasite, bacterial, and viral infections are all capable of producing similar lesions. Further studies will be needed to determine how natural A.invidans infections are caused.
"The PCR and FISH assays developed in this study now make it possible to screen the large numbers of environmental samples needed to identify alternative hosts and sources of A. invidans infections," said NOAA scientist Wayne Litaker. "Identifying the sources and the conditions promoting the growth and transmission of the pathogen will help resource managers better predict when lesion events are likely to occur and perhaps develop effective mitigation strategies."
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