Muskie pox

From Fish

Piscirickettsia cf. salmonis are aerobic Gram-negative bacteria occurring intracellularly in fish, primarily salmonids. The bacteria are usually coccoid (but sometimes occur in paired rods or in rings), fastidious (with very specific growth requirements), and non-motile[1]. P. cf. salmonis bacteria isolated from muskellunge (Esox masquinongy) in Lake St. Clair only occur as curved rods and rings[2].

P. salmonis in salmonids can result in anemia, kidney necrosis, enlarged spleen, hemorrhages, nodules or crater-form lesions in the liver, dark coloration, skin lesions, lack of appetite, anorexia, and lethargic swimming activity. Symptoms vary in different populations and some fish display no external symptoms at all[3]. In Lake St. Clair, infected muskellunge can exhibit red skin lesions.

The geographic range of P. salmonis could be broad. It could be native to marine environments, including parts of the Pacific and Atlantic oceans; however, some of the marine regions in which it now occurs could constitute introductions. P. salmonis survives and replicates asexually through fission in the membrane-bound vacuoles inside the cytoplasm of host fish cells. It can rarely be passed from adult to young and is likely passed more frequently from adult to adult, entering through the skin or gills. It is inactive above 20°C and replication is optimal at 15–18°C. It survives better in saltwater than in freshwater, remaining infective for up to 14 days in the former but quickly becoming unstable in the latter[4].

Organisms that are piscirickettsia-like or rickettsia-like (including all fastidious, intracellular fish pathogens) are labeled PLOs or RLOs. PLOs and RLOs are associated with disease in many fish species. Some PLOs are genetically very similar to P. salmonis; a PLO that is 98% similar to typical P. salmonis has been isolated at 10 m depth in bacterioplankton from the coastal waters of Oregon[5].

P. salmonis causes mortality in salmonids, killing millions of hatchery farmed fish each year. As for many diseases that naturally occur in the wild, infection becomes more severe in crowded aquaculture settings. Coho salmon (Oncorhynchus kisutch) are highly susceptible while Atlantic salmon (Salmo salar) are less susceptible. P. salmonis has also been reported in rainbow trout (O. mykiss), cherry salmon (O. masou), chinook salmon (O. tschwaytscha), and pink salmon (O. gorbuscha), and can probably infect all salmonids[6].

In addition to the occurrence in freshwater muskellunge in Lake St. Clair, PLOs and RLOs have also been found in: - blue-eyed plecostomus (Panaque suttoni), a tropical freshwater fish shipped from Columbia to the USA[7]; - seabass (Dicentrarchus labrax) in the Mediterranean Sea[8]; - white sea bass (Atractoscion nobilis) off the coast of southern California[9]; - puffers (Tetrodon fahaka) from the Nile River in Egypt; - dragonets (Callionymus lyra) from coastal Wales; - tilapia (Oreochromis mossambicus and Sarotherodon melanotheron) in Taiwan, Jamaica, Indonesia, Florida, California, and Hawaii[10]; - and in groupers (Epinephelus melanostigma) in Taiwan.

P. salmonis was not isolated from salmonids in the freshwater stage of their life cycle until 1993 in Chile. The disease was very similar to P. salmonis occurring in fish held in marine aquaculture pens. It could have been transmitted to the fish being cultured in freshwater from brood stock that had survived an outbreak of P. salmonis in a marine environment, or it could have already been present in the freshwater culture environment [11]. The PLO infection of muskellunge in Lake St. Clair is unique, given that it occurred in wild fish. Other fish affected by P. salmonis and PLOs or RLOs have typically been cultured and many have been species that spend at least part of their life cycle in marine environments. Means of Introduction: Unknown

P. salmonis causes particularly severe problems for Chilean aquaculture. Many large mortality events occurred starting around 1989. Chilean aquaculture now raises more Atlantic salmon than coho salmon because the former is more resilient to the disease. Mortality events amongst various salmonid species in aquaculture facilities in different countries (i.e. Scotland, Norway, and Canada) have also occurred within the past two decades. Mortality events caused by PLOs and RLOs in some of the fish species listed above have also occurred, typically in association with aquaculture operations around the world[12].


  1. Mauel, M. J. and D. L. Miller. (2002) Piscirickettsiosis and piscirickettsiosis-like infections in fish: a review. Veterinary Microbiology 87(4):279-289
  2. Arkush, K. D., H. L. Edes, A. M. McBride, M. A. Adkison, and R. P. Hedrick. (2006) Persistence of Piscirickettsia salmonis and detection of serum antibodies to the bacterium in white seabass Atractoscion nobilis following experimental exposure. Diseases of Aquatic Organisms 73(2):131-139
  3. Fryer, J. L. and R. P. Hedrick. (2003) Piscirickettsia salmonis: a Gram-negative intracellular bacterial pathogen of fish. Journal of Fish Diseases 26(5):251-262
  4. Smith, P. A., et al (1999) Routes of entry of Piscirickettsia salmonis in rainbow trout Oncorhynchus mykiss. Diseases of Aquatic Organisms 37(3):165-172
  5. Mauel, M. J. and J. L. Fryer. (2001) Amplification of a Piscirickettsia salmonis-like 16S rDNA product from bacterioplankton DNA collected from the coastal waters of Oregon, USA. Journal of Aquatic Animal Health 13(3):280-284
  6. Rise, M. L., et al (2004) Microarray analyses identify molecular biomarkers of Atlantic salmon macrophage and hematopoietic kidney response to Piscirickettsia salmonis infection. Physiological Genomics 20(1):21-35
  7. Khoo, L., P. M. Dennis, and G. A. Lewbart. (1995) Rickettsia-like organisms in the blue-eyed plecostomus, Panaque suttoni. Journal of Fish Diseases 18:157-164
  8. Comps, M., J. C. Raymond, and G. N. Plassiart. (1996) Rickettsia-like organism infecting juvenile sea-bass Dicentrarchus labrax. Bulletin of the European Association of Fish Pathologists 16:30-33
  9. Arkush, K. D., H. L. Edes, A. M. McBride, M. A. Adkison, and R. P. Hedrick. (2006) Persistence of Piscirickettsia salmonis and detection of serum antibodies to the bacterium in white seabass Atractoscion nobilis following experimental exposure. Diseases of Aquatic Organisms 73(2):131-139
  10. Chern, R. S. and C. B. Chao. (1994) Outbreaks of a disease caused by rickettsia-like organism in cultured tilapias in Taiwan. Fish Pathology 29:61-71
  11. Gaggero, A., H. Castro, and A. M. Sandino. (1995) First isolation of Piscirickettsia salmonis from coho salmons, Oncorhynchus kisutch (Walbaum), and rainbow trout, Oncorhynchus mykiss (Walbaum), during the freshwater stage of their life cycle. Journal of Fish Diseases 18:277-279
  12. Larenas, J. J. et al (2003) Experimental vertical transmission of Piscirickettsia salmonis and in vitro study of attachment and mode of entrance into the fish ovum. Diseases of Aquatic Organisms 56(1):25-30