Whirling disease is caused by which type of organism?

• A. Bacterium
• B. Protozoan parasite
• C. Fungus
• D. Virus

Answer: (B)

Whirling disease is caused by a protozoan parasite. The disease agent, often named Myxobolus cerebralis, is a tiny, spore-forming parasite that cycles between an aquatic worm and a fish. In young salmonids it migrates into cartilage and bone, causing deformities and the characteristic erratic, whirling swimming behavior. This organism is distinct from bacteria, which are prokaryotes; from fungi, which have different cellular biology and life cycles; and from viruses, which are acellular and rely on host cells for replication. Recognizing it as a protozoan parasite helps explain its parasitic lifestyle and the way it spreads through aquatic environments via its worm host and waterborne spores.