Saprolegnia parasitica, an oomycete pathogen with a fishy appetite: new challenges for an old problem

Water moulds (oomycetes) of the order Saprolegniales, such as Saprolegnia and Aphanomyces species, are responsible for devastating infections on fish in aquaculture, fish farms and hobby fish tanks. Members of the genus Saprolegnia cause Saprolegniosis, a disease that is characterised by visible white or grey patches of filamentous mycelium on the body or fins of freshwater fish. Up till 2002, Saprolegnia infections in aquaculture were kept under control with malachite green, an organic dye that is very efficient at killing the pathogen. However, the use of malachite green has been banned worldwide due to its carcinogenic and toxicological effects and this has resulted in a dramatic re-emergence of Saprolegnia infections in aquaculture. As a consequence Saprolegnia parasitica is now, economically, a very important fish pathogen, especially on catfish, salmon and trout species, and warrants further investigation to develop new alternative control strategies.