Saprolegnia is a common fungus-like organism found in aquatic environments. It causes problems for various aquatic organisms, including fish, often leading to saprolegniosis or “cotton mold” disease. This microorganism presents challenges for maintaining healthy aquatic ecosystems and aquaculture operations.
Understanding Saprolegnia’s Nature and Environment
Saprolegnia is not a true fungus, but an oomycete, a group of microorganisms often called “water molds.” While sharing traits with fungi like mycelial growth and osmotrophic nutrition, oomycetes are more closely related to golden-brown algae. Saprolegnia typically appears as white or grey fibrous patches, resembling cotton wool, on affected aquatic life or submerged organic matter.
This organism is ubiquitous in freshwater environments, including ponds, aquariums, and fish farms, and can also be found in moist soil and brackish water. Saprolegnia thrives in certain conditions, often acting as an opportunistic pathogen. It is more prevalent in lower temperatures, though it tolerates a range from 3–33 °C (37–91 °F). Poor water quality, temperature fluctuations, high fish density, and physical injuries to aquatic animals are risk factors that encourage its growth and infection.
Diseases Caused by Saprolegnia
Saprolegnia causes saprolegniosis, a skin infection damaging to fish, fish eggs, crustaceans, amphibians, and mollusks. While often acting as a secondary pathogen, taking advantage of stressed or injured aquatic animals, some species, like Saprolegnia parasitica, can be primary pathogens causing systemic disease. Salmonids, such as salmon and trout, are notably susceptible.
Visible symptoms of saprolegniosis include white or grey cotton-like growths on the skin, fins, gills, or eyes of infected fish. These lesions may initially appear as slightly elevated patches with a gelatinous consistency. As the infection progresses, patches can turn brownish with a red border and spread across the host’s surface, sometimes coalescing into larger areas. More virulent strains can penetrate deeper, damaging underlying muscles and potentially leading to respiratory difficulties if gills are affected. In fish eggs, the mold appears as a thick layer of mycelium that can spread from dead eggs to healthy ones, leading to suffocation and mortality.
Controlling and Preventing Saprolegnia Infections
Managing and preventing Saprolegnia outbreaks involves maintaining a healthy aquatic environment and addressing predisposing factors. Improving water quality through regular water changes and proper filtration reduces the organic matter Saprolegnia feeds on. Avoiding overfeeding also contributes to better water conditions. Reducing stress in aquatic animals by preventing overcrowding and rough handling minimizes their susceptibility to infection.
If an infection occurs, isolating affected individuals in a separate hospital tank helps prevent further spread. While mild cases may resolve with environmental improvements, stubborn infections might require treatment. Historically, malachite green was used, but it is now banned in many places due to its toxic effects. Alternative treatments include salt baths, formalin, hydrogen peroxide, and bronopol, though efficacy can vary, and some have safety concerns or limited practical use. Plant-based compounds and copper-based nanoparticles are also being investigated as potential future treatments.