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What is Black Spot?

Joel La Follette - Wednesday, May 03, 2017

This photo of a Deschutes Bull Trout infected with Black Spot was taken by Nick Wheeler on May 1st on the Warm Springs to Trout Creek section of the Deschutes. 

A few weeks ago I mentioned that a parasite that infects salmonids was becoming more prevalent in the Deschutes with many anglers reporting catches infected with the telltale “black spots.” To answer some the questions floating around I turned to biologist Greg McMillan and asked “Just what is Black Spot and why are we seeing an increase in cases in the Deschutes?”

Greg responded, "Black spot disease is caused by a flatworm (trematode) parasite known in the scientific community as Uvulifer ambloplitis, and also known as “neascus”.  This parasite has a complicated life cycle that starts with eggs in water, which hatch and become juveniles known as miracidia, which in turn infect aquatic snails.  In snails this form of the parasite matures into the next life form, known as cercariae.  Cercariae are shed by the snails and become free swimmers, which attach to fish.  Once the cercariae have attached to the flesh of fish, the fish develops an immune response that causes the dark spot.

Kingfishers are the next host, which become infected when they ingest infected fish.  The cercariae develop into adult flatworms.  The parasite then produces eggs, which are shed in feces by kingfishers, and deposited in water where the life cycle is reinitiated.

These flatworms do not appear to be fatal to fish, or other hosts.  There are scattered reports of fish stressed from other sources dying while infected.  No human infections have been reported, but there is no real surveillance mechanism to detect human infections.  Although probably safe for human consumption after thorough cooking, there is no study data to confirm that.

None of us who have fished the lower Deschutes River for decades can say that we’ve seen many, if any fish with this condition until a year ago.  There are reports indicating there have been infected fish in the lower Deschutes River and tributaries in the past, but they aren’t common.  So what has changed?  Is this random?  Or linked to the ongoing ecological changes we are all seeing in the lower river?

This might be related to an increase in the snail population in the lower Deschutes River. Portland General Electric’s Year 1 Data Summary Report from their Lower Deschutes River Macroinvertebrate and Periphyton Report Study published in 2014, indicates that there has been a significant increase in snail populations in the lower Deschutes River.  This increase in population in the intermediate host (snails) might be related to the increase in black spot disease noted in fish.  The snail population increase is likely linked to the increase in algae in the lower river.

Is this a catastrophic occurrence?  Probably not, but it could be another indication of ecological change in the lower Deschutes River."

As Greg said, Black Spot is probably not catastrophic, but it is of concern. Finding out more information on the disease occurrence in Oregon is hard as it seems to not be of concern to ODFW at this time. Perhaps if more cases are reported by anglers, ODFW will finally take notice and look into the cause of this increase in cases. 

I would suggest that anglers fishing the Deschutes carefully photograph and report cases of Black Spot to ODFW. Take note of where the catch was made and how many cases were observed. Please make an effort to leave fish in the water when handling and photographing them. If you send a photo of infected fish to me I'll add it to this blog post. Again, please handle all wild fish with respect and care.
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