The information below is largely taken from a publication by the University of Florida: “Red Sore Disease in Gamefish by Reed and Floyd
Like any terrestrial animal, including man, fish are subject to many sources of infection and disease brought about by bacteria, virus, and many parasites. These organisms live within the water column, in the bottom detritus, on plants, and within other host animals such as snails and waterfowl.
Most fisherman have caught fish with evidence of disease such as infected eye sockets, fins, gills or body sores. This can often raise concerns about the health of the lake and its fish, yet seldom is this concern warranted. These organisms all exist in healthy ecosystems, just as they exist in the terrestrial environments, and there will always be some fish found with disease.
One of the most common disease problems encountered freshwater game fish is generically referred to as “red sore disease.” Red sore disease is a generic term that describes a physical condition of fish rather than referring to a specific disease agent. Fish most frequently affected are game fish, particularly bluegill (bream), largemouth bass, and striped bass and its hybrids. The condition is observed in fish from natural waters, recreational fishing ponds, and commercial aquaculture facilities. This problem usually occurs in the spring and fall, and fishermen and pond owners are often concerned by the appearance of red ulcers and sores on their fish. Typically, “red sore disease” is caused by two organisms, Aeromonas hydrophila , a bacterium, and Heteropolaria sp. (formerly Epistylis sp.), a protozoan.
Both of these organisms are found in healthy aquatic environments and are capable of causing disease. Red sore disease will often run its course, and fish may recover without treatment. The primary concern is often not mortality of fish, but rejection of the affected fish by anglers because of the diseased appearance. Occasionally red sore disease can reach epidemic proportions, contributing to significant mortality (more than 10 percent) of game fish, but this is rare.
Sores caused by Heteropolaria sp. can be characterized by white-grey, cotton-like patches on the body surface or the fins. Due to the irritation, the fish will “flash,” or rub, to rid itself of the parasite, causing scale loss and ulceration of the already damaged area. This allows the bacterium Aeromonas hydrophila to enter. In its mildest form, the condition is seen as red, raised “sores,” or lesions, on the tips of fins, particularly the dorsal fin. As the disease progresses, fish may be afflicted with fin erosion, and ulcers on the side of their body.
Other organisms that may be associated with ulcerative lesions on fish include a number of protozoans (eg. Icthyobodo sp. [formerly Costia sp.] and Trichodina sp.), the bacterium Flexibacter columnaris and fungi in the genus Saprolegnia . The presence of a red sore does not necessarily mean that the fish has Heteropolaria sp. or Aeromonas sp. These protozoans flourish and attach to the skin, where they cause unsightly, bloody, ulcerated areas.
Heteropolaria is almost everywhere in fresh water and sediments. It has a direct life cycle, requiring only the fish host. If the skin of a fish is breached by a bit, scrape, hook, etc, the open cut is immediately subject to infection by organisms in the environment. This is no different from a person’s skin being punctured or cut. When uncleaned, the abrasion will likely get infected producing a sore. The same thing happens with fish.
The protozoan reproduces by binary fission (dividing), and the resulting young forms are free swimming. The mature stage attaches to fish or other structures in its environment, including spawning containers and submerged logs, aquatic plants, etc. Heteropolaria thrives if there are high levels of organic matter in the water to provide nutrients. Stress, caused by poor water quality, crowding, water temperature variations, reduction in body condition, or spawning can increase the susceptibility of fish to red sore disease.
Wholesomeness of Fish with Red Sore Disease
Fishermen often ask whether it is safe to eat game fish that have sores on them. In most cases the sores are external only, and when the fish is cleaned, the damaged area is easily removed from edible tissue. Thorough cooking will eliminate any pathogens that might remain in tissue, resulting in a safe and wholesome product. Although the appearance of a fish with sores on it may be unappetizing, there is no reason to discard the fillets as long as they are thoroughly cleaned and cooked. For information on nine common fish diseases check out this site