Catfish with PGD also may become anemic. Swollen gills can occur with several other diseases, but you should suspect PGD until you get an accurate diagnosis. PGD may occur throughout the year but is most prevalent in the spring and fall. Cause PGD is caused by a protozoan parasite that infects the gills and other organs. The exact classification of this parasite has not been made.
Scientists suspect a myxosporean, haplosporean, or microsporean.
Little is known about the life cycle of the PGD parasite. The initial infecting stage is a single-celled organism invading the gills. It subsequently reproduces and spreads to other organs in the fish, including the brain, liver, kidney, and spleen. In laboratory experiments the PGD organism will survive in fish for up to two months. Gills will become necrotic, and infected parts may drop off, possibly releasing the parasite.