Saprolegnia is a natural condition affecting wild and farmed fish. The ultrastructure, biochemistry and molecular sequences of the Oomycetes show their phylogenetic roots with the Chromista, the chromophyte algae, and other Protista rather than the true fungi. Polymerase chain reaction (PCR) techniques are improving our understanding of taxonomic relationships within the Oomycetes, and developments such as random amplified polymorphic DNA (RAPD)-PCR are helping to resolve taxonomic problems and enable additional comparative studies within the Oomycetes.
Saprolegnia is generally observed as a superficial and chronic infection, exhibiting cotton-wool like tufts on the integument and gills of fish or eggs. This may spread over the whole body with up to 80% being covered. Early infections are characterised by skin lesions that are circular or crescent-shaped, which develop rapidly, causing destruction of the epidermis. Lethargy and loss of equilibrium occur as the infection proceeds. Early lesions are characterised by the growth of thin, white or grey threads, with microscopic examination revealing the characteristic, branched, coenocytic mycelium with many zoosporangia.