Parasites and Trout

June 19, 2008

Question: We were fishing Shasta Lake recently and noticed a lot of the rainbow trout we caught and released had parasitic worms hanging off them. What can you tell me about them? (Jay S. via e-mail)

Answer: Without having a fish to examine or a description of the parasite, our best guess would be Lernaea. According to Dr. William Cox, DFG Program Manager for Fish Production and Distribution, this parasite is usually seen in warm water conditions and affects all species of fish, including trout. Since trout may move between the upper warm water areas of the lake and the cooler deep water sections, all trout, whether caught on the surface or at depth, are susceptible to carrying this Lernaea.

Another parasite called Salmincola is most often seen in gills, but may infect any part of the body. Lernaea usually is seen on skin or fins, but can also infect gills. The Lernaea are a longer threadlike parasite with double egg sacs at the end of their body. Salmincola are shorter, and often whiter in appearance (a common term is “gill maggott disease”) with similar paired egg sacs. Neither poses a threat or concern for human health.

These parasites and others that may be internal are killed during cooking, effectively eliminating any possibility of infecting humans eating the fish.

In the future if you catch diseased fish, our DFG Pathologists will examine them if you can deliver them fresh, or unfrozen but refrigerated, or on ice. Contact your local DFG office or contact our Statewide Fish Health Pathologist, Dr. Mark Adkison in Rancho Cordova, at (916) 358-2830.