Amoebic gill disease (AGD) is a gill disorder found in marine fish, but primarily affecting salmon. AGD is caused by the protozoan amoeba species Neoparamoeba perurans. The parasite causes a proliferative response in the gill epithelium (this is generically known as proliferative gill disease, or PGD, of which amoebae are just one agent). Normally oxygen would diffuse through the thin epithelium, which it can’t do when the epithlium is thickened. In the parts of the gill affected, the fish effectively can’t breath.