SLV

Salmon Leukemia virus/syndrome (SLV)

In the early 1990s Fisheries and Oceans Canada (DFO) published a series of papers on a disease they named Plasmacytoid Leukemia (PL), reporting it is caused by the Salmon Leukemia virus. This disease was killing up to 100% of the Chinook in salmon farms in Sechelt Inlet, Discovery Pass, and the Broughton Archipelago through the 1990s. The industry switched to Atlantic salmon at this time. While DFO did not complete their research, never actually visualizing the causative agent, they did publish the evidence it is caused by a virus.

“Our study showed the presence of a retrovirus in chinook salmon with PL, which we are tentatively calling SLV [Salmon Leukemia Virus]. In addition, our results suggest that SLV is the etiological agent of the disease. ” (Eaton and Kent 1992)

See also: Kent and Dawe 1993. Further evidence for a viral etiology in Plasmacytoid leukemia of chinook salmon Oncorhynchus tshawytscha. DISEASES OF AQUATIC ORGANISMS. 15: 115-121.

Testifying at the Cohen Commission, Dr. Kent withdrew his support his work describing the viral nature of this disease, however there are defined symptoms and so currently this disease has been identified as a “syndrome” (see pg 77, first two paragraphs).

Transmission: Transmission research found 100% of sockeye and chinook challenged with Salmon Leukemia infected tissue homogenates became infected with 100% mortality in the sockeye and very high mortality in the chinook (Kent and Dawe 1990).

Dr. Kristi Miller’s Genomic Lab (Pacific Biological Station, DFO) found genomic evidence that the cells of Fraser salmon including sockeye, coho and Chinook are responding to a retrovirus with signals for Leukemia, tumours and cancer. In particular Miller has found this genomic profile is highly associated with the sockeye dying of pre-spawn mortality. She is hindered in identifying Plasmacytoid Leukemia because the causative agent was never confirmed. Miller’s work on the Parvovirus in BC salmon may discover salmon leukemia is a parvovirus.

Symptoms: Dr. Gary Marty, the BC fish farm vet who performed the histology on the farm salmon audits 2006-2010, assigned the acronym “ISH” to the diagnostic symptom of Salmon Leukemia: “hyperplasia of the interstitial cells of the caudal kidney” (Stephen and Ribble 1997). This acronym appears commonly in Dr. Marty’s records for both Pacific and Atlantic salmon, suggesting the disease is still prevalent in salmon farms today.

“Our results have shown that gross and histopathological information required to diagnose marine anaemia can be reliably observed in a significant proportion of dead fish recovered from seapens” (Stephen and Ribble 1997).

Plasmacytoid Leukemia disease is associated with tumours in the optic area of the salmon’s head. Samples were sent to the Registry of Tumours in Lower Animals, Smithsonian Institute, Washington DC (RTLA Accession Nos. 1022 and 1023).

Prevention: Salmon Leukemia is an immune suppressor, the fish dying of secondary pathogens such as Bacterial Kidney Disease and so may be managed on farms with vaccinations of the secondary pathogens. Chinook salmon farms were removed from the Fraser sockeye migration route in mid-2007. The first generation of Fraser sockeye to go to sea without exposure to chinook farms with a history of Salmon Leukemia retuned in 2010.

References:

  • Eaton and Kent 1992. A Retrovirus in Chinook Salmon (Oncorhynchus tshawytscha) with Plasmacytoid Leukemia and Evidence for the Etiology of the Disease. CANCER RESEARCH, 52: 6496-6500.
  • Kent and Dawe 1993. Further evidence for a viral etiology in Plasmacytoid leukemia of chinook salmon Oncorhynchus tshawytscha. DISEASES OF AQUATIC ORGANISMS. 15: 115-121.
  • Kent and Dawe 1990. Experimental Transmission of a Plasmacytoid Leukemia of Chinook salmon Oncorhynchus tshawytscha. CANCER RESEARCH (SUPPL) 50: 5679s-5681s.
  • Stephen and Ribble. 1997 Mortality surveys as a tool for studying marine anaemia in seapen-reared chinook salmon, Oncorhynchus tshawytscha (Walbaum). Aquaculture Research 28(4): 265-269.