Cotton Wool Disease in Freshwater Fish

Guppy with fuzzy growth on dorsal fin

Flickr/Alice Chaos 

Cotton wool disease, also known as saddleback, fin rot and black patch necrosis, are all descriptive names for the same bacteria, columnaris (Flavobacterium columnare). This bacteria is commonly mistaken for a fungus, given its pale color and raised appearance. It can infect most species of freshwater fish, but is usually secondary to another primary stressor. Some strains are more deadly and contagious than others.

What is Cotton Wool Disease in Freshwater Fish?

Cotton wool disease is caused by the bacteria Flavobacterium columnare. It is not a fungus, despite its fungus-like appearance. It can infect the skin and gills and is of great importance to the commercial aquaculture market. It is rarer in the pet fish community. It is mostly an opportunistic pathogen that takes advantage of a stressed out fish with a compromised immune system. There are many strains within the species, some of which are significantly more deadly and spread more easily.

Symptoms of Cotton Wool Disease in Freshwater Fish

The most common clinical sign of cotton wool disease is a pale, raised patch on your fish's skin. It can be anywhere on the body, including the face and fins. Lesions typically have a fluffy appearance similar to a fungal growth. In some cases, it can spread to the gills, causing lethargy, difficulty swimming and loss of appetite. The gill tissue may appear necrotic or pale when examined.

Causes of Cotton Wool Disease in Freshwater Fish

Columnaris bacteria are commensal bacteria and occur naturally on healthy fish. Only when given a stressed out host or particularly nasty strain of bacteria, do the fish begin to show clinical signs. It commonly enters a healthy system when infected fish are added without proper quarantine. It is critical to keep a close eye on your tank's temperature, since columnaris bacteria love warmer water (around 80F) and will cause problems faster than in cooler water.

Diagnosis of Cotton Wool Disease in Freshwater Fish

It is critical to differentiate columnaris species from a fungal infection. In order to do so, your veterinarian will take a small biopsy from a sedated fish and place it under a microscope. If it is indeed cotton wool disease, the bacteria rods will form themselves into tiny haystacks, whereas fungus will not and the individual hyphae may be distinct. Additional testing may be warranted by sending a swab sample to a lab in order to identify its strain and understand which antibiotics will be most effective. Never "guess" with antibiotic treatment. You may end up wiping out your biologic filtration and using the incorrect product or dose, breeding more resistant strains in your system.

Treatment of Cotton Wool Disease in Freshwater Fish

All fish showing symptoms should be isolated into a hospital tank. Your veterinarian will prescribe an antibiotic to be added to the water or provide injectable treatment. Injectable antibiotics will not affect your biological filter and provide a stronger treatment option. Never attempt to inject your fish yourself.

Your fish may require different antibiotics if no sample is sent for culture and sensitivity testing. A detailed history of your fish including what treatments have already been attempted is critical to deciding which antibiotic is best for your situation.

Severely sick fish, including those with >50% of the gill tissue affected, may require euthanasia. It is very hard for fish to recover from such a severe infection. If your fish's gills have become infected, it is critical to give them additional oxygen support from an airstone.

Since it is a secondary invader, your fish system should be assessed for any potential stressors. This includes testing your water chemistry, reviewing your fishes' diet, comparing fish compatibility and any recent additions of fish or decor. If you do not remove the primary cause, your fish will get sick again after finishing treatment.

How to Prevent Cotton Wool Disease in Freshwater Fish

As with most diseases, cotton wool disease can be prevented with proper quarantine. This means a completely separate system with separate filtration and other equipment for 4-6 weeks. 2 weeks is not adequate and maintaining the correct temperature is very important. Watch your fish closely during this time period and watch for changes in their appearance and behavior. If you notice something wrong, it is critical to get it diagnosed fast and correctly to make sure it doesn't become a larger problem. By keeping your fish in isolation, you will protect your main system from infection.

It is also critical to keep your fish healthy in a low-stress environment. Make sure all fish have plenty of room, a high quality diet and good water chemistry.