Aphanomyces invadans is a highly pathogenic oomycete fungi which infects the Atlantic menhaden Brevoortia tyrannus and many other species of fish from around the world. This water mould has been implicated in massive fish kills in North Carolina, resulting in hundreds of thousands of dead fish. On average, an infection resulting from less than 10 zoospores (the infective stage of this fungi) is enough to kill a fish, and even infection by just a single zoospore can result in ulcerous lesions that can lead to mortality. Fish infected with the fungi develops ulcerous lesions which ultimately lead to extensive tissue necrosis. This fungus develops extremely quicky, doubling its hyphal mass every ten days, and it is also highly invasive, extending its hyphae into various tissues including the liver, kidneys and spinal cord of the fish host. Interestingly, A. invadans outbreaks are associated with high rainfall. This is likely due to the fact that this water mould has a low salinity tolerance and will not grow in higher salinity waters.
Kiryu et al. (2003) Infectivity and pathogenicity of the oomycete Aphanomyces invadans in Atlantic menhaden Brevoortia tryannus. Diseases of Aquatic Organisms 54: 135-146.
(Photo from this paper.)
Contributed by Tommy Leung.