Ichthyophthirius multifiliis (Ciliophora) Invasion of Gill Epithelium

The sequence of morphologic events associted with Ichthyophthirius multifiliis invasion of gill epithelium began in the theront with differentiation of secretory mucocysts and the perforatorium. After escaping from the cyst the theront, which stained intensely with Mallory's stain, sought a host. As it approached the host epithelium, the contents of the mucocysts were discharged, enveloping the ciliate in sticky material, which made initial contact with the host epithelium. Rapid penetration by the theront caused disruption of one or more host cells and resulted in a focal necrosis associated with the anterior margin of the ciliate. Within five minutes postexposure, the parasite completed its invasion of the epithelial layer and stained less intensely. The remnants of host cells disrupted during its entry surrounded the trophont until they were ingested by the parasite. Within 40 min postexposure, synthetic activity of the parasite appeared to increase as evidenced by an abundance of organelles, particularly ribosomes and crystalline mucocysts. At this point, the overlying host epithelium appeared normal.