Lesions on this Dolly Varden or Arctic char are most likely furuncles caused by a common bacterial pathogen called Aeromonas salmonicida.
Observation: While on the Goodnews River fishing in early February, a local resident pulled in a trout and immediately noticed, what looked like pus coming out of a Dolly Varden (Salvelinus malma). Is this something that is a natural/unnatural event? Can we consume the fish or is there a recommendation and/or advisory about eating fish with pus coming out of it?
Alaska Department of Fish and Game (ADFG) Pathology Lab Consult Jayde Ferguson writes: "The fish with the pus-like lesions is a char (Dolly Varden or Arctic char) and although the photo is not of great quality, I would say that these lesions are most likely furuncles (boil-like) caused by a common bacterial pathogen called Aeromonas salmonicida. Here is a link to a chapter on this pathogen from our booklet on common fish diseases. Given that both fish have been frozen, this limits the type of testing and interpretation that can be performed. However, I would point out that if only 1 or 2 fish have been observed with these condition, then it is probably not a widespread issue caused by an environmental factor like pollution."
LEO says: According to the ADFG booklet, all salmonid species are susceptible, especially young fish when water temperatures exceed 8 degree C. Transmission of the bacteria between fish occurs and increasing temperature exacerbates the transmission. Both of these conditions (see related post: Dolly Varden with deformity) can be related to exposure to warm water temperatures during development. Is it possible that climate change and warmer summer water temperatures in the Goodnews River could be factor contributing to these disease? We put this question to the experts at the ADFG Fish Pathology Lab.
ADF&G Fish Pathology Lab Consult Dr. Ted Meyers writes: "Probably unlikely that water temperatures during incubation is a cause. Eggs incubate in the winter when water temperatures are low. The high temperatures we’ve been experiencing have been in the summer. The range of water temperatures in the winter are probably within the range for adequate incubation. Fish will also seek out refugia for the best locations. Also, if this was the cause then more fish would be affected. The only additional comment I would make is that because this fish was caught in February when the water is quite cold, Gram-negative bacterial infections are still possible but less likely. So a differential diagnosis for the lesions could also include a myxosporidian protozoan parasite."
There are no human health issues related to A salmonicida.
Resource:
Alaska Department of Fish & Game – "The Dolly Varden is one of the most widely-distributed salmonids in Alaska. It occurs throughout the coastal areas of the state from Southeast Alaska across the Gulf of Alaska and the Bering Sea into the Beaufort Sea to the Mackenzie River in northern Canada. It also occurs in streams in Interior Alaska and the Brooks Range. Freshwater habitats of Dolly Varden range from the smallest headwater stream (often less than 3 feet wide) to large deep lakes. Saltwater habitats range from brackish estuaries to fully-marine shoreline environments, and occasionally the open ocean. Entire populations of freshwater-resident Dolly Varden may spend their entire life cycle within a single stream, and during the winter, the entire population may reside at a single springs." Source: Species Profile - Dolly Varden (Salvelinus malma) ADF&G
- ADF&G Education Resources: Dolly Varden — Wildlife Notebook Series, (PDF47 kB) "Two forms or sub-species of Dolly Varden (Salvelinus mama Walbaum) occupy most of the coastal waters of Alaska. The distribution and range of each form has changed in recent years with knowledge gained from genetic analysis, but there is no absolute distinction in life histories between the two forms."
