Whirling disease in trout is caused by a protozoan parasite Myxobolus cerebralis. It affects the nervous system. Which disease is this?

• A. Bacteremia
• B. Whirling disease
• C. Fish lice
• D. Bacteria

Answer: (B)

Whirling disease is named for the distinctive swimming behavior it causes in infected trout. The parasite Myxobolus cerebralis invades the nervous system and associated tissues, leading to abnormal, whirling or corkscrew-like movements. That characteristic impairment is what gives the disease its name, so this choice directly matches both the causative agent and the symptom. The other options describe different problems: bacteremia is a bacterial infection of the bloodstream, fish lice are a separate external parasite, and bacteria is just a general term for bacteria, not a disease name. This disease mainly affects salmonids and can also cause skeletal deformities, with management emphasizing prevention of spread through infected fish and gear.