Fish infected with M. cerebralis harbor trophozoites in the cartilage of the spine and skull, causing damage to the central nervous system. Repeated nuclear division occurs over several months, producing a large, multinucleated trophozoite. Some of these nuclei become sporoblasts and myxospores develop within them. Myxospores accumulate and are released when the fish dies.
Aquatic annelids ingest the myxospores. These penetrate the epithelial cells of the gut and undergo rounds of asexual multiplication. Eventually these produce gametes, which unite and form an actinospore, or triactinomyxon (also called a TAM). The TAM is long-lived and resistant to cold temperatures and even drying.
When a TAM contacts the skin of a young fish, filaments from the polar capsule evert rapidly and puncture the host's epithelium. The sporoplasm of the parasite enters the tissues, transforms into trophozoites, and migrates to the preferred site.