Question: We were fishing Hot Creek in Mono County last weekend, and my friend caught a rainbow trout that looked unhealthy. We thought it might have whirling disease. We found a picture, and based on that we wanted to know what is a likely case of whirling disease? Have fish with this disease been found in Hot Creek before?
Assuming this was whirling disease, what should we have done? We never keep fish and Hot Creek has zero-kill regulations, but it would seem wise to remove a whirling-diseased fish from the stream to give to the Department of Fish and Game (DFG) for examination. Since we were unsure, we released it.
In the future, what would be the best practice for maintaining the health of the fisheries in the watershed if we knew this was a diseased fish? Could we have collected this fish to turn over to the DFG for evaluation?
— Ray F.
Answer: This might be a case of whirling disease, but it's impossible to know based on a photo alone. Whirling disease afflicts juvenile fish causing neurological damage and skeletal deformation. Afflicted fish often can't swim in a normal manner. When startled, they "whirl" rather than darting away as a normal fish would. Survival rates for infected fingerlings are very low, and those that do survive have difficulty feeding and become easy prey for predators.
Whirling disease does not affect humans.
Dr. Mark A. Adkison, a senior fish health coordinator with DFG, said whirling disease is carried by the aquatic tubifex worm and infects fish when its spores are released into the river. These spores infect fish through the skin and then travels through the nerves and spinal cord, eventually emerging from the nerves into the cranial cartilage where it grows and develops into its final spore stage (myxospores).
The parasite consumes and deforms the cartilage and this causes the cranial deformities such as a sloped head and crooked jaw so commonly seen in WD-infected fish. Since the fish in the photo you sent does not have the characteristic cranial deformities, they might be due to some other cause.
Other possibilities include nutritional deficiencies or coldwater disease (CWD), which can cause spinal deformities similar to those in this fish. Flavobacterium psychrophilum is a bacteria present in most, if not all trout waters, and causes CWD. This disease is not a problem in the wild. It is a disease of concern in hatcheries, and it's fairly easy to control by reducing fish densities and antibiotic treatment.
Whirling disease is probably present in Hot Creek since it flows into the upper Owens River, and the upper Owens is positive for WD. The only way to tell for sure if a fish has WD is to test the fish for the WD parasite. The test kills the fish and is not something you could do streamside.
Dr. Garry O. Kelley, a DFG associate fish pathologist, says that once the parasite is established in a natural system, it's there for good. There's strong evidence that suggests WD might be reduced by eliminating susceptible or infected salmonids and by reducing habitats for the worm.
Reducing WD prevalence will help recruitment efforts since the parasite prefers the young. If regulations allow bag limits, then removing any deformed fish would be welcomed. Just keep in mind that a fish that grossly appears WD-positive may actually be negative, even in WD-positive waters. Specifically, the deformities could be genetic, an injury or some other pathogen.
As far as what to do with a diseased fish, if the fish was infected with WD to the point where it had severe deformities, it would probably be good to remove it from the creek to decrease the WD spore load in the environment. However, from the enforcement side, if it's a no-kill zone, it's up to enforcement as to whether they would cite the fisherman for not returning the fish to the stream.
Carrie Wilson is a marine biologist with the California Department of Fish and Game. Contact her at CalOutdoors@dfg.ca.gov.
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