Branchiomyces in carp gills is usually localized in the blood vessels, the efferent branchial vessels and the capillaries, producing branched coenocytic hyphae capable of producing aplanospores by endogenous cleavage. In eels branchiomycosis hyphae and spores spread to visceral organs. Infection is probably by spores liberated from the necrotic tissue, but the exact route by which fish contract infection is unknown.

Infection in the blood vessels of the gill causes blockage, haemostasis and thromboses which consequently cause extensive necrosis of the gill filaments. Areas of the gill filaments turn brown, due to haemorrhages and thromboses, and grey as a result of ischemia. The process is fast and is accompanied by proliferation of the gill epithelium with resulting adhesions of the filaments. In eels, lesions containing hyphae and spores occur in the epicardium and the spleen.

Branchiomycosis occurs in eutrophic ponds with a high load of organic matter, ponds fertilized by organic manure, and water temperatures above 20°C. During the hot season, when ambient water temperatures are above 25°C, infection may spread to most fish in the pond and cause heavy mortalities.

One of the most common fungal infections of Koi. The fungal spores will grow anywhere on the Koi, including the gills, initially germinating on dead tissue. Their threadlike hyphae release digestive juices which break down the tissue so the fungus can absorb it, as the fungus grows these juices start breaking down living tissue.

Fungus on the body appears as cotton wool like growths, it is hard to tell if a Koi has it in the gills, but if it hangs at the surface gulping for air it is likely.

Carp pox. A virus that produces solid waxy lumps on Koi. It will not kill Koi and is generally harmless, but can look unsightly. It is most often present in small Koi and in cold weather, clearing up disappearing when Koi grow and in the spring when water temperatures rise.