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Saprolegnia in Freshwater Fish – How to Make a Happy Home for Your Pet

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 spot on your fish’s skin. Not considered primary invaders, these forms of water take advantage of sick or injured fish to cause skin or gill diseases. Serious infections can lead to increased secondary diseases and / or death.

What is Saprolegnia?

Saprolengniaceae belongs to the aquatic class of the Oomycetes and is a family that contains many species commonly referred to as “Saprolegnia”. There are so many different species that every freshwater fish has a susceptible species. Typically, aquatic forms feed on dead organic materials and replicate by forming spores. These spores sow a substrate and float around while waiting for a piece of dead tissue to be consumed. Dead skin cells on fish with injuries or a weakened immune system are ideal hosts.

Signs of Saprolegnia in freshwater fish

Saprolegnia presents itself as a blurry patch on the outer skin or gills of a fish. Usually it looks white or gray. If your water contains a lot of algae or other debris, it can be green, red, or brown. Skin or gill damage leads to secondary attacks from bacteria and other fungi that can lead to death.

Predisposing factors for saprolegnia

Bad water quality

Fish that swim in poor water are predisposed to an increased incidence of many diseases. The chronic stress caused by trying to maintain homeostasis in poor water results in decreased immune function. This decreased immune function makes it easier for bacteria, fungi and parasites to seize and cause disease.

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Inappropriate temperature

Also known as “winter kill”, a sharp or sudden drop in temperature or tropical fish kept at colder temperatures can lead to an increased incidence of Saprolegnia. This is mostly caused by a defective heater. Make sure your aquarium has a working thermometer to keep an eye on the temperature of your aquarium or pond.

Immunosuppressed fish

Just like with humans, cats, dogs, and other pets, not all immune systems are created equal. Some fish do not have the robust immune abilities of their counterparts and are more prone to various diseases. Some systems always have the same “Canary” fish that gets sick first of every outbreak. By identifying individual fish differences in susceptibility to disease, you can spot early signs of disease by monitoring “weaker” fish. These immune system-compromised fish 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 shape can look very similar Columnaris spp. and require a completely different treatment. Be a responsible pet fish owner by limiting medication errors. Your vet will conduct a skin scrape or swab culture to determine the pathogen causing the problem. Dead fish cannot be used for diagnosis as this can lead to a false positive result. Remember, Saprolegnia loves to eat on dead tissue.

Since it is a form of water, it is not in response to antibiotics. However, secondary bacterial infection can occur due to the weakness of the fish from Saprolegnia. Adding antibiotics to your tank can trigger a new tank syndrome and have to restart your biological filtration. Your veterinarian may prescribe injectable drugs or medicated foods to effectively treat secondary infections. This limits the effects of drugs on your biological filtration.

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Because of its opportunistic nature, most Saprolegnia infections go away when you correct the underlying predisposing condition (see above). Serious infections may require treatment that is more specific for the particular aquatic form species. Differentiating between different species must be made by growing the sample in a laboratory.

How to prevent saprolegnia

The best way to prevent saprolegnia is to maintain the quality of the water and give your fish good nutrition. This is the best way to boost your immune function. This is the best remedy for saprolegnia. Stick to your maintenance program and have a hospital tank handy to quarantine potentially sick people.

Does saprolegnia affect humans?

Due to the non-aquatic nature of humans, Saprolegnia is of no concern to humans. It also doesn’t affect your land animals.

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