Sample of pink salmon infected with Henneguya salminicola, caught off the Queen Charlotte Islands, Western Canada in 2009
Sample of pink salmon infected with Henneguya salminicola, caught off the Queen Charlotte Islands, Western Canada in 2009

This article is about diseases and parasites in salmon, trout and other salmon-like fishes of the family Salmonidae.


Cnidarian parasites

According to Canadian biologist Dorothy Kieser, the myxozoan parasite Henneguya salminicola is commonly found in the flesh of salmonids. It has been recorded in the field samples of salmon returning to the Queen Charlotte Islands. The fish responds by walling off the parasitic infection into a number of cysts that contain milky fluid. This fluid is an accumulation of a large number of parasites.

Henneguya and other parasites in the myxosporean group have a complex life cycle where the salmon is one of two hosts. The fish releases the spores after spawning. In the Henneguya case, the spores enter a second host, an invertebrate, in the spawning stream. When juvenile salmon migrate to the Pacific Ocean, the second host releases a stage infective to salmon. The parasite is then carried in the salmon until the next spawning cycle. The myxosporean parasite that causes whirling disease in trout has a similar life cycle.[1] However, as opposed to whirling disease, the Henneguya infestation does not appear to cause significant incapacitation of the host salmon — even heavily infected fish tend to return to spawn successfully.

According to Dr. Kieser, a lot of work on Henneguya salminicola was done by scientists at the Pacific Biological Station in Nanaimo in the mid-1980s, in particular, an overview report[2] which states that "the fish that have the longest fresh water residence time as juveniles have the most noticeable infections. Hence in order of prevalence coho are most infected followed by sockeye, chinook, chum and pink." As well, the report says that, at the time the studies were conducted, stocks from the middle and upper reaches of large river systems in British Columbia such as Fraser, Skeena, Nass and from mainland coastal streams in the southern half of B.C. "are more likely to have a low prevalence of infection." The report also states "It should be stressed that Henneguya, economically deleterious though it is, is harmless from the view of public health. It is strictly a fish parasite that cannot live in or affect warm blooded animals, including man".

The myxosporean parasite Ceratomyxa shasta infects salmonid fish on the Pacific coast of North America
The myxosporean parasite Ceratomyxa shasta infects salmonid fish on the Pacific coast of North America

According to Klaus Schallie, Molluscan Shellfish Program Specialist with the Canadian Food Inspection Agency, "Henneguya salminicola is found in southern B.C. also and in all species of salmon. I have previously examined smoked chum salmon sides that were riddled with cysts and some sockeye runs in Barkley Sound (southern B.C., west coast of Vancouver Island) are noted for their high incidence of infestation."

Sea lice

Sea lice, particularly Lepeophtheirus salmonis and various Caligus species, including C. clemensi and C. rogercresseyi, can cause deadly infestations of both farm-grown and wild salmon.[3][4]

Sea lice on the back of an Atlantic salmon
Sea lice on the back of an Atlantic salmon

Sea lice are ectoparasites which feed on mucus, blood, and skin, and migrate and latch onto the skin of wild salmon during free-swimming, planktonic nauplii and copepodid larval stages, which can persist for several days.[5][6][7] Large numbers of highly populated, open-net salmon farms can create exceptionally large concentrations of sea lice; when exposed in river estuaries containing large numbers of open-net farms, many young wild salmon are infected, and do not survive as a result.[8][9] Adult salmon may survive otherwise critical numbers of sea lice, but small, thin-skinned juvenile salmon migrating to sea are highly vulnerable. On the Pacific coast of Canada, the louse-induced mortality of pink salmon in some regions is commonly over 80%.[10]


In 1972, Gyrodactylus, a monogenean parasite, spread from Norwegian hatcheries to wild salmon, and devastated some wild salmon populations.[11]


Enteric redmouth disease is a bacterial infection of freshwater and marine fish caused by the pathogen Yersinia ruckeri. It is primarily found in rainbow trout and other cultured salmonids. The disease is characterized by subcutaneous hemorrhaging of the mouth, fins, and eyes. It is most commonly seen in fish farms with poor water quality. Redmouth disease was first discovered in Idaho rainbow trout in the 1950s.[12]

The bacteria Piscirickettsia salmonis causes the disease piscirickettsiosis, which has a mortality rate as high as 90% in certain salmonid fishes and is ubiquitous on Chilean salmon farms. Infected fish may or may not display external symptoms of infection, but they frequently display ulcers on the liver and kidney and often develop anemia. The disease was first reported in 1989 as coho salmon syndrome.[13]

Viral infectious diseases

In 1984, infectious salmon anemia (ISAv) was discovered in Norway in an Atlantic salmon hatchery. Eighty percent of the fish in the outbreak died. ISAv, a viral disease, is now a major threat to the viability of Atlantic salmon farming. It is now the first of the diseases classified on List One of the European Commission’s fish health regime. Amongst other measures, this requires the total eradication of the entire fish stock should an outbreak of the disease be confirmed on any farm. ISAv seriously affects salmon farms in Chile, Norway, Scotland and Canada, causing major economic losses to infected farms.[14] As the name implies, it causes severe anemia of infected fish. Unlike mammals, the red blood cells of fish have DNA, and can become infected with viruses. The fish develop pale gills, and may swim close to the water surface, gulping for air. However, the disease can also develop without the fish showing any external signs of illness, the fish maintain a normal appetite, and then they suddenly die. The disease can progress slowly throughout an infected farm and, in the worst cases, death rates may approach 100 percent. It is also a threat to the dwindling stocks of wild salmon. Management strategies include developing a vaccine and improving genetic resistance to the disease.[15]

Another infectious virus in salmon is Piscine orthoreovirus. It was first discovered in 2010, present in Atlantic Salmon farms exhibiting high levels of Heart and Skeletal Muscular Inflammation (HSMI) and Cardiomyopathy syndrome (CMS).[16] Since then it has been observed in areas across the globe, such as Great Britain, Chile and across the Pacific Northwest.[17][18][19] Whether or not it causes HSMI has been through extensive study. Injection of PRV from into healthy Atlantic Salmon has caused HSMI in Norway, but these results haven't been reproducible on the same species of salmon in farms in British Columbia, despite HSMI being directly linked to PRV in one BC farm.[20][21][22] Because of this, numerous researchers have suggested that PRV may be necessary for HSMI, but not sufficient.[21]

Interaction with humans

In the wild, diseases and parasites are normally at low levels, and kept in check by natural predation on weakened individuals. In crowded net pens they can become epidemics. Diseases and parasites also transfer from farmed to wild salmon populations. A recent study in British Columbia links the spread of parasitic sea lice from river salmon farms to wild pink salmon in the same river."[23]

The European Commission (2002) concluded “The reduction of wild salmonid abundance is also linked to other factors but there is more and more scientific evidence establishing a direct link between the number of lice-infested wild fish and the presence of cages in the same estuary.”[24] It is reported that wild salmon on the west coast of Canada are being driven to extinction by sea lice from nearby salmon farms.[10] Antibiotics and pesticides are often used to control the diseases and parasites, as well as lasers.[25][26]

These predictions have been disputed by other scientists[27] and there is much debate on whether the correlation between sea lice infestation and declining wild salmon stocks is driven by causal factors.

Other conditions

Gas bubble disease is caused by contact with supersaturated water.[28]



  1. ^ Crosier DM, Molloy DP, Molloy J (2012). "Whirling Disease – Myxobolus cerebralis" (PDF). Archived from the original (PDF) on 2008-02-16.
  2. ^ Boyce NP, Kabata Z, Margolis L (1985). "Investigation of the Distribution, Detection, and Biology of Henneguya salminicola (Protozoa, Myxozoa), a Parasite of the Flesh of Pacific Salmon". Canadian Technical Report of Fisheries and Aquatic Sciences (1450): 55.
  3. ^ "Sea Lice and Salmon: Elevating the dialogue on the farmed-wild salmon story" (PDF). Watershed Watch Salmon Society. 2004. Archived from the original (PDF) on 2012-07-13.
  4. ^ Bravo S (January 2003). "Sea lice in Chilean salmon farms". Bulletin-European Association of Fish Pathologists. 23 (4): 197–200.
  5. ^ Morton A, Routledge R, Peet C, Ladwig A (February 2004). "Sea lice (Lepeophtheirus salmonis) infection rates on juvenile pink (Oncorhynchus gorbuscha) and chum (Oncorhynchus keta) salmon in the nearshore marine environment of British Columbia, Canada". Canadian Journal of Fisheries and Aquatic Sciences. 61 (2): 147–57. doi:10.1139/f04-016.
  6. ^ Peet CR (2007). Interactions between sea lice (Lepeophtheirus salmonis and Caligus clemensi) and juvenile chum (Oncorhynchus keta) and pink salmon (O. gorbuscha) in British Columbia (Thesis). Victoria, Canada: University of Victoria.
  7. ^ Krkosek M, Gottesfeld A, Proctor B, Rolston D, Carr-Harris C, Lewis MA (December 2007). "Effects of host migration, diversity and aquaculture on sea lice threats to Pacific salmon populations". Proceedings. Biological Sciences. 274 (1629): 3141–9. doi:10.1098/rspb.2007.1122. PMC 2293942. PMID 17939989.
  8. ^ Morton A, Routledge R, Krkosek M (February 2008). "Sea louse infestation in wild juvenile salmon and Pacific herring associated with fish farms off the east-central coast of Vancouver Island, British Columbia". North American Journal of Fisheries Management. 28 (2): 523–32. doi:10.1577/M07-042.1.
  9. ^ Krkosek M, Lewis MA, Morton A, Frazer LN, Volpe JP (October 2006). "Epizootics of wild fish induced by farm fish". Proceedings of the National Academy of Sciences of the United States of America. 103 (42): 15506–10. doi:10.1073/pnas.0603525103. PMC 1591297. PMID 17021017.
  10. ^ a b Krkosek M, Ford JS, Morton A, Lele S, Myers RA, Lewis MA (December 2007). "Declining wild salmon populations in relation to parasites from farm salmon". Science. 318 (5857): 1772–5. Bibcode:2007Sci...318.1772K. doi:10.1126/science.1148744. PMID 18079401. S2CID 86544687.
  11. ^ Thomson M, Side JC (2002). "Environmental considerations and legislative control of marine salmon farming". In Stead SM, Laird LM (eds.). Handbook of salmon farming. Birkhäuser. p. 348. ISBN 978-1-85233-119-1.
  12. ^ Busch RA (March 1978). "MFR Paper 1296 Enteric Redmouth Disease (Haggerman Strain)" (PDF). Marine Fisheries Review. 40 (3). Retrieved 2014-01-05.
  13. ^ Rozas, M; Enríquez, R (March 2014). "Piscirickettsiosis and Piscirickettsia salmonis in fish: a review". Journal of Fish Diseases. 37 (3): 163–188. doi:10.1111/jfd.12211.
  14. ^ "Archived copy". Archived from the original on 2017-05-04. Retrieved 2018-08-09.((cite web)): CS1 maint: archived copy as title (link)
  15. ^ Fact Sheet - Atlantic Salmon Aquaculture Research Archived 2010-12-29 at the Wayback Machine Fisheries and Oceans Canada. Retrieved 12 May 2009.
  16. ^ Løvoll M, Wiik-Nielsen J, Grove S, Wiik-Nielsen CR, Kristoffersen AB, Faller R, et al. (November 2010). "A novel totivirus and piscine reovirus (PRV) in Atlantic salmon (Salmo salar) with cardiomyopathy syndrome (CMS)". Virology Journal. 7 (1): 309. doi:10.1186/1743-422X-7-309. PMC 2994541. PMID 21067578.
  17. ^ Government of Canada, Fisheries and Oceans Canada (2018-04-03). "Piscine Orthoreovirus (PRV) and Heart and Skeletal Muscle Inflammation (HSMI)". Retrieved 2021-02-02.
  18. ^ Kibenge MJ, Iwamoto T, Wang Y, Morton A, Godoy MG, Kibenge FS (July 2013). "Whole-genome analysis of piscine reovirus (PRV) shows PRV represents a new genus in family Reoviridae and its genome segment S1 sequences group it into two separate sub-genotypes". Virology Journal. 10 (1): 230. doi:10.1186/1743-422X-10-230. PMC 3711887. PMID 23844948.
  19. ^ Siah A, Morrison DB, Fringuelli E, Savage P, Richmond Z, Johns R, et al. (2015-11-04). "Piscine Reovirus: Genomic and Molecular Phylogenetic Analysis from Farmed and Wild Salmonids Collected on the Canada/US Pacific Coast". PLOS ONE. 10 (11): e0141475. Bibcode:2015PLoSO..1041475S. doi:10.1371/journal.pone.0141475. PMC 4633109. PMID 26536673.
  20. ^ Wessel Ø, Braaen S, Alarcon M, Haatveit H, Roos N, Markussen T, et al. (2017-08-25). "Infection with purified Piscine orthoreovirus demonstrates a causal relationship with heart and skeletal muscle inflammation in Atlantic salmon". PLOS ONE. 12 (8): e0183781. Bibcode:2017PLoSO..1283781W. doi:10.1371/journal.pone.0183781. PMC 5571969. PMID 28841684.
  21. ^ a b Di Cicco E, Ferguson HW, Schulze AD, Kaukinen KH, Li S, Vanderstichel R, et al. (2017-02-22). "Heart and skeletal muscle inflammation (HSMI) disease diagnosed on a British Columbia salmon farm through a longitudinal farm study". PLOS ONE. 12 (2): e0171471. Bibcode:2017PLoSO..1271471D. doi:10.1371/journal.pone.0171471. PMC 5321275. PMID 28225783.
  22. ^ Garver KA, Johnson SC, Polinski MP, Bradshaw JC, Marty GD, Snyman HN, et al. (2016-01-05). "Piscine Orthoreovirus from Western North America Is Transmissible to Atlantic Salmon and Sockeye Salmon but Fails to Cause Heart and Skeletal Muscle Inflammation". PLOS ONE. 11 (1): e0146229. Bibcode:2016PLoSO..1146229G. doi:10.1371/journal.pone.0146229. PMC 4701501. PMID 26730591.
  23. ^ Seafood Choices Alliance (2005) It's all about salmon Archived 2015-09-24 at the Wayback Machine
  24. ^ Scientific Evidence Archived 2006-08-19 at the Wayback Machine.
  25. ^ Dumiak M. "Lice-Hunting Underwater Drone Protects Salmon With Lasers". IEEE Spectrum: Technology, Engineering, and Science News. Retrieved 2017-06-05.
  26. ^ "De har skutt 500 mill laserpulser mot millioner av lus – ikke en eneste laks er meldt skadet" (in Norwegian). Teknisk Ukeblad. 16 October 2017. Retrieved 20 October 2017.
  27. ^ Riddell BE, Beamish RJ, Richards LJ, Candy JR (December 2008). "Comment on "Declining wild salmon populations in relation to parasites from farm salmon"". Science. 322 (5909): 1790.2–1790. Bibcode:2008Sci...322.1790R. doi:10.1126/science.1156341. PMID 19095926. S2CID 7901971.
  28. ^ "Gas Bubble Disease (GBD)" (PDF). Retrieved 9 August 2018.
  29. ^ Agnew W, Barnes AC (May 2007). "Streptococcus iniae: an aquatic pathogen of global veterinary significance and a challenging candidate for reliable vaccination". Veterinary Microbiology. 122 (1–2): 1–15. doi:10.1016/j.vetmic.2007.03.002. PMID 17418985.

Further reading