Saprolegniosis (water mold disease) is an infection in fish caused by Saprolegnia fungi. It is particularly common in recently purchased and shipped fishes, especially those species that are prone to damage by nets, contact with other fish and handling. Other predisposing factors for saprolegniosis include trauma and recent environmental aberrations, such as temperature drop, dramatic pH change, and skin "burn" from a toxic chemical.
Affected fish have fungal tufts, which appear like wisps of cotton, on the skin and or fins. These raised lesions may range in color from pure white to tan, gray, brown or even green. The "cottony" appearance is only present when the fish is in the water. Once the fish is removed from the water, the mass collapses, resembling a ball of wet cotton or soaked bread.
The most commonly identified fungal pathogens of fish are water molds (Class Oomycetes) of the Saprolegnia genus. These pathogens are found almost everywhere (ubiquitous), they take advantage of a compromised host (facultative) , and they are found primarily in fresh or lightly brackish water. Their growth is favored by moderately acidic pH, low temperatures and the presence of decaying organic material.
Saprolegnia fungi rarely penetrate deep into muscle, since they spread mainly along the surface of the body. Fungal disease in fish is nearly always secondary to a break in the integrity of the epidermis and associated mucus coating. Mucus is considered an important barrier to oomycete colonization. The majority of Saprolegnia spores which land on healthy fish skin are quickly sloughed or die. If you were to place some uneaten food or other organic matter in an aquarium without fish, it would be quickly (within forty-eight hours) colonized by water molds.
Diagnosis is quick, accurate and generally inexpensive. Your veterinarian can make a diagnosis of saprolegniosis based on microscopic examination.
A biopsy sample is examined using a microscope and the presence of thick (10-25 micron), non-septate, branching hyphae (fungal stalks) confirms saprolegniosis. A number of other pathogens and saprophytes (opportunistic pathogens) including algae, crustaceans, helminths (worms) and protozoa commonly colonize the fungal tuft.
Differential diagnoses include: Ichthyophthirius, neoplasia, Heteropolaria (a stalked ciliated protozoan) and lymphocystis disease.
After evaluating the environment and history of your fish, your veterinarian may or may not decide to implement a treatment protocol. Several chemotherapeutic (drug treatment) options are available including: copper sulfate, malachite green, formaldehyde and sodium chloride. Some lesions are treated topically with a disinfectant like povidone iodine after the water mold and necrotic tissue have been surgically removed.
If the infection is not severe, many fish will heal with supportive care (good nutrition and clean water). In fatal cases, death is frequently, due to impaired osmoregulation and the fish's inability to maintain fluid balance.
Environmental stressors should be removed if at all possible. Management practices must be improved and other pathogens present should be addressed. If practical, infected fish should be isolated and provided with clean water, proper temperatures and good food until a diagnosis can be confirmed.
Try and keep tankmate aggression to a minimum, since nips and bites from other fish can predispose a fish to saprolegniosis. When moving or relocating fish, plastic bags are much safer to use than aquarium nets, which can damage sensitive skin and mucus with their rough-edged mesh. Some fish like catfish can get their spines caught in nets which may strip the underlying bone of protective skin and mucus.