DAO 54:135-146 (2003)  -  doi:10.3354/dao054135

Infectivity and pathogenicity of the oomycete Aphanomyces invadans in Atlantic menhaden Brevoortia tyrannus

Yasunari Kiryu1, Jeffrey D. Shields1,*, Wolfgang K. Vogelbein1, Howard Kator1, Vicki S. Blazer2

1Department of Environmental and Aquatic Animal Health, Virginia Institute of Marine Science, Gloucester Point, Virginia 23062, USA
2National Fish Health Research Laboratory, Biological Resources Division, US Geological Survey, 1700 Leetown Road, Kearneysville, West Virginia 25430, USA
*Corresponding author. Email:

ABSTRACT: Atlantic menhaden Brevoortia tyrannus develop characteristic skin ulcers in response to infection by the oomycete Aphanomyces invadans. To investigate pathogenicity, we conducted a dose response study. Juvenile menhaden were inoculated subcutaneously with 0, 1, 5, 10, 100, and 500 secondary zoospores per fish and monitored for 37 d post-injection (p.i.). Survival rates declined with increasing zoospore dose, with significantly different survivorship curves for the different doses. Moribund and dead fish exhibited characteristic ulcerous lesions at the injection site starting at 13 d p.i. None of the sham-injected control fish (0 zoospore treatment) died. The LD50 (lethal dose killing 50% of exposed menhaden) for inoculated fish was estimated at 9.7 zoospores; however, some fish receiving an estimated single zoospore developed infections that resulted in death. Menhaden were also challenged by aqueous exposure and confirmed that A. invadans was highly pathogenic by this more environmentally realistic route. Fish that were acclimated to culture conditions for 30 d, and presumably free of skin damage, then aqueously exposed to 100 zoospores ml-1, exhibited 14% lesion prevalence with 11% mortality. Net-handled fish that were similarly infected had a significantly higher lesion prevalence (64%) and mortality (64%). Control fish developed no lesions and did not die. Scanning electron microscopy of fish skin indicated that zoospores adhered to intact epidermis, germinated and penetrated the epithelium with a germ tube. Our results indicate that A. invadans is a primary pathogen of menhaden and is able to cause disease at very low zoospore concentrations.


KEY WORDS: Dose response · Epizootic ulcerative syndrome · LD50 · Lesions · Mortality · Secondary zoospores · Tissue repair · Ulcerative mycosis


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