Myxobolus cerebralis is a myxosporean parasite of salmonids (salmon, trout, and their allies) that causes whirling disease, an important condition in both salmon and trout farming and wild fish populations. It was first described from rainbow trout in Germany a century ago, but its range has spread, and it has now been reported from most of Europe (including Russia), the United States, South Africa and other countries. In the 1980s, it was discovered that M. cerebralis needs to infect a tubificid oligochaete (a kind of segmented worm) to complete its life-cycle. The parasite infects its hosts by injecting them with some of its cells after piercing them with polar filaments ejected from nematocyst-like capsules. Though not transmissible to humans, it is one of the most economically important myxozoans in fish as well as one of the most pathogenic. It was the first myxosporean whose pathology and symptoms were described scientifically.
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