Myxobolus cerebralis

 Myxobolus cerebralis is one of the best-known, pathogenic myxozoans and is the causative agent of whirling disease in salmon and trout. The parasite is of European origin, but has been distributed worldwide to at least 26 countries by human activities. Fish are infected after exposure to waterborne triactinomyxon spores (TAMs). The parasite burrows along the fish's nerves until it reaches cartilage (typically in the head and spine of young fish). Here the parasite develops into its myxospore stage. Spore development damages the fish cartilage and puts pressure on nerves, which leads to the classic signs of whirling disease: a darkened tail, deformed body and strange swimming behaviour. The parasite cannot be transmitted directly to other fish, instead it must develop into TAMs in aquatic Tubifex worms. The parasite is harmless to humans.