Saprolegnia in Freshwater Fish

Tetra with Fuzz on Skin

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Saprolegnia (also known as water mold, oomycete infection, or winter kill), is an opportunistic fungal pathogen in freshwater aquariums. It usually appears as a fuzzy patch on your fish's skin. Not considered primary invaders, these water molds take advantage of sick or injured fish to cause disease in the skin or gills. Severe infections can result in increased secondary diseases and/or death.

What is Saprolegnia?

Of the water mold class Oomycetes, Saprolengniaceae is a family that contains many species commonly called "saprolegnia." There are so many different species that every freshwater fish is susceptible to some Saprolegnia species. Typically, water molds feed on dead organic materials and replicate by forming spores. These spores seed a substrate and float about, waiting for a bit of dead tissue to inhabit and grow into. Dead skin cells on fish with injuries or weakened immune systems make ideal hosts.

Signs of Saprolegnia in Freshwater Fish

Saprolegnia will present itself as a fuzzy patch on the external skin or gills of a fish. Typically, the fuzzy patch is white or grey in appearance, but if your water has a lot of algae or other debris, they may be green, red or brown in appearance. Damage to the skin or gills opens the fish up to secondary attacks from bacteria and fungi, which can lead to death.

Predisposing Factors to Saprolegnia

Poor Water Quality

Fish swimming in bad water are predisposed to an increased incidence of many illnesses. The chronic stress caused from trying to maintain homeostasis in poor water leads to decreased immune function.This decreased immune function makes it much easier for bacteria, fungi and parasites to take hold and cause disease.

Inappropriate Temperature

Also known as "winter kill," severe or sudden temperature declines, or tropical fish kept at cooler temperatures, can cause increased incidence of Saprolegnia. This is most often caused by a malfunctioning heater. Make sure your aquarium has a functional thermometer to keep an eye on your tank's or pond's water temperature.

Immunosuppressed Fish

Just like people, cats, dogs and other pets, not all immune systems are created equal. Some fish will not have the robust immune capabilities of their counterparts and be more susceptible to different diseases. Some aquariums always have the same "canary" fish that gets sick first in every disease outbreak. By noting individual fish differences in disease susceptibility, you can catch early signs of disease by monitoring "weaker" fish. These fish with compromised immune systems may be more susceptible to opportunistic pathogens such as Saprolegnia.

How to Treat Saprolegnia

It is imperative that Saprolegnia be properly diagnosed by an aquatic veterinarian prior to treatment. Water mold can look very similar to Columnaris bacteria and will require a completely different treatment. Be a responsible pet fish owner by limiting medication mistakes. Your vet will do a skin mucus scrape or swab culture to determine the problematic pathogen. Dead fish cannot be used to make a diagnosis since it can create a false positive result. Remember, Saprolegnia loves dining on dead tissue.

Since it is a water mold, it is not responsive to antibiotics. However, there may be a bacterial infection that caused the fish to be weak initially, allowing the Saprolegnia to grow on it. You may need to treat for both bacterial infections and the Saprolegnia fungus, using different medications.

If you dump antibiotics into your aquarium, you may kill the beneficial bacteria in your biofilter and set off a New Tank Syndrome and have to restart your biologic filtration. Your veterinarian may prescribe injectable medication or medicated foods to effectively treat any bacterial infections. This will limit the effect of any antibiotic medications on your biologic filtration.

Due to its opportunistic nature, most Saprolegnia infections will go away when you fix the underlying predisposing condition (see above). Severe infectious may require treatment  specific to the water mold, using and antifungal medication in the water.

How to Prevent Saprolegnia

The best method of preventing Saprolegnia is to maintain your water quality and feed your fish a good diet. This is the best thing to promote their own immune function, which is the best deterrent of Saprolegnia. Keep up with your maintenance regimen, keep debris vacuumed out of the gravel, and have a hospital tank ready to quarantine any potentially sick individuals.

Does Saprolegnia Affect Humans?

Due to humans' non-aquatic nature, Saprolegnia is not of concern in humans. It will also not affect your terrestrial pets.