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General · 17th May 2013
Ray Grigg
Strange things can happen when salmon eat chickens. Such a diet is unprecedented and bizarre, a violation of the biological order that has occurred over millions of years of evolutionary history. Nature, it seems, does the unusual when human ingenuity tampers with its traditions. And the consequences can be dire. But this is a complex subject that requires some context — an understanding of details first requires an understanding of principles.

Evolution is not as simple as we thought. Darwin's theory of natural selection only describes the slow “vertical” transfer of genetic material from parent to offspring used by large animals and plants. But the microbial world of bacteria and viruses also does a “horizontal” transfer of genetic material between similar and different organisms by a non-sexual means. This microbial capability — operating since early life on our planet billions of years ago — is a genetic free-for-all in which DNA is exchanged like goods at a swap-meet. These opportunistic organisms use this genetic process to optimize change in their individual traits and thereby accelerate evolution. Their only requirement is that they be brought together in close proximity.

We've already experienced the consequences. Many of our common human diseases have come to us from farmed animals through the horizontal transfer of novel genetic material — thanks to globalization and industrial agriculture, at least 30 have occurred since 1970. So the crowded conditions in poultry or salmon farms provide the perfect combination of density and stress that allows viruses to exchange genetic material with each other. The result can increase their virulence, allow them to infect a new species, or even create an entirely novel version of themselves — in taxonomy, a new genus. Which brings us to salmon and viruses.

The fish in the salmon farm in Norway that first began to exhibit strange symptoms in 1999 were infected with a new disease later identified as heart and skeletal muscle inflammation (HSMI). The symptoms were a pale and soft heart muscle, yellowish liver, swollen spleen and other swellings. Infection rates in pens were as high as 20%, with morbidity close to 100%. HSMI was extremely infectious, soon spreading to 417 other salmon farms in Norway, then to facilities in the United Kingdom. Indeed, HSMI was discovered to be so infectious that it threatened wild fish that came in contact with the farms or with infected fish that escaped from them. Tests indicate HSMI has arrived in British Columbia.

PLOS One published a scientific article on July 9, 2010, entitled “Heart and Skeletal Muscle Inflammation of Farmed Salmon is Associated with Infection with Novel Reovirus” (Gustavo Palacios, W. Ian Lipkin, et al.), linking HSMI with this “novel” piscine reovirus (PRV). The article's Abstract claims to “provide evidence that HSMI is associated with infection with piscine reovirus”, presumably the way AIDS is associated with HIV — one is a full-blown version of the other. The article claims that “PRV is a novel reovirus identified by unbiased high throughput DNA sequencing”, that “PRV is the causative agent for HSMI”, and that “measures must be taken to control PRV not only because it threatens domestic salmon production but also due to the potential for transmission to wild salmon populations.” Indeed, as Veterinary Research (4.06, Apr. 9/12) finds, “PRV is almost ubiquitously present in Atlantic salmon marine farms, and detection of PRV alone does not establish an HSMI diagnosis.”

If PRV is so prevalent and it does develop into HSMI as research suggests, this is a problem for salmon farming. But it strikes terror in those concerned about the health of wild salmon and the ecologies than depend on them. Indeed, PRV and HSMI may already be doing enough damage to be imperilling BC's wild salmon runs.

The clue to the origin and virulence of the PRV/HSMI virus and disease comes from the PLOS One article and the word “novel”. Two general kinds of the family of “Reoviridae” virus occur in the fauna community. One is an orthoreovirus, which includes both a mammalian and an avian strain. The other is an aquareovirus which is exclusive to aquatic animals. An analysis of the genetic material of the piscine reovirus identifies it as distinctly different from the two general groups, but situates it exactly between them, embodying half the attributes of the avian orthoreovirus and half the attributes of the aquareovirus. In other words, PRV is a new genus, designated GU994015 PRV, that has combined the traits of a bird virus and an aquatic virus. This probably explains why it is so infectious. But how did it become so “novel”?

The answer may be found in the chicken wastes that the salmon farming industry has been adding to its salmon feed — just the conditions that would provide viruses with the perfect opportunity to transfer genetic material horizontally. This would explain how the aquareovirus was able to exchange useful DNA with the avian orthoreovirus to develop a new virulent version of itself to infect fish, manifesting as the novel piscine reovirus and then with the clinical symptoms of HSMI. This suspicion is confirmed by a related article in PubMed (May, 2013) entitled “Piscine reovirus encodes a cytotoxic, non-fusogenic, integral membrane protein and previously unrecognized virion outer-capsid proteins”. According to the article, “Recent sequence-based evidence suggests that PRV is about equally related to members of the genera Orthoreovirus and Aquareovirus.” In other words, PRV seems to be a unique or “novel” virus created by combining the genetic material from two distinctly different viruses, one related to birds and the other related to aquatic animals — the first such amalgamation that has occurred since the divergence of the virus 49 to 52 million years ago (Journal of General Virology, Aug. 2002, vol. 83, no. 8, 1941-1951).

The discoverers of this virus, Gustavo Palacios, W. Ian Lipkin et al., are so confident of the causative relationship between PRV and HSMI that they have applied for a patent on the “immunogenic compositions and methods for inducing an immune response against Piscine reoviruses” (Pub. No.: US 2013/0058968 A1, March 7, 2013).

This preventative option might provide some hope for farmed salmon, but how exactly wild salmon would be immunized is a mystery. And a worrisome sentence occurs in another PLOS One article (June 5/12) entitled, “Atlantic Salmon Reovirus Infection Causes a CD8 T Cell Myocarditis in Atlantic Salmon (Salmo salar L.). “The etiology of myocarditis (cause of heart inflammation) in humans remains unknown in most cases but an association with a viral infection has attracted a lot of attention over the years.” (For other related information on piscine reovirus, please go to http://www.youtube.com/embed/ePGoTadmUO0.)