ISA scare
Dr. Stephen Newman, Seafood Source, November 10,2011
During seminars and discussions with clients, I often explain that disease is a natural thing. In fact the absence, while highly desirable, is not “natural." Entropy ensures that this will be the case. Pathogens that cause disease are simply facilitating this. Evolutionarily it makes little sense to kill off the host thus the tug of war which we see as disease. In many respects disease can be looked at as entropy at work. Whether it is a virus or a bacterium, how the host reacts to it and how it spreads through the host can function to ensure that the host is recycled before we want. Production of a single species of animal in high numbers in close proximity to one another is an invitation for disease outbreaks. Aquaculture is no different than agriculture in this respect except that the medium through which the pathogens move is water rather than air.
Recently there were reports of a virus that has seriously impacted some salmon farming operations over the years and only last year decimated the industry in Chile having been isolated from a few wild fish in BC. No virus was found in any farmed fish. The presence of the virus in wild fish, if it were verifiable, would of course be cause for concern. Subsequent screening of larger numbers of fish has failed to confirm the finding. These types of results must be treated with a healthy amount of skepticism and not fear mongering. If they were found to be real, it does not mean that there will be inevitable problems in farmed fish or that the virus originated in farmed fish and that this, which some groups would have you believe, will invariably impact wild fish. The technique employed to determine the presence of the virus is a very powerful tool which amplifies the amount of viral DNA found so that it can be visualized (literally using gels and stains). It is so sensitive that it is prone to false positives.
Some years ago, I was commissioned by the Shrimp Growers of Belize to conduct a survey of the presence of the virus that causes what was unfortunately coined the Taura Syndrome in their dozen shrimp farms. This virus, the Taura Syndrome Virus, appears to have been originally isolated in Ecuador and then through the movement of animals is now virtually ubiquitous everywhere that shrimp are farmed. As a part of the survey I also tested a few animals for the presence of another seriously problematic virus, the White Spot Syndrome Virus (WSSV). One of the samples came back positive. This could have been viewed as a disaster in the making and a panicked reaction would have sent the industry into a death spiral (which has unfortunately happened in recent years although for much different reasons). We retested the sample in another lab and the sample came back negative. Since there was no evidence of active disease and no other positives, it was written off as a false positive. The lesson here is that these things can happen and that in the absence of symptoms and acute disease, spurious positives need to be carefully evaluated. There is no evidence that the reports on the salmon samples were anything but a false positive at this time (at least based on the information that I have available to me).
There has been a rush to condemn salmon farmers in some parts of the world for a variety of studies based on what many would agree is poor quality or at best contentious science. Some groups earnestly believe that farming salmon has damaged wild salmon runs rather than considering that in the big picture, this is probably far more complicated than a few studies suggest. It appears to me that it is far from certain that farming salmon plays any role in impacting salmon runs. As a fish pathologist (one of the many hats I have worn over the years) in the early days of salmon farming in the Pacific North West, we noticed that the diseases we were seeing in farmed fish typically were present in wild fish first. Bacterial Kidney Disease was one example. Many “wild” runs that were being “helped” by rearing juvenile salmon to smolts prior to being released were heavily infected with this bacterium. While things are different today as salmon farming is a mature agribusiness, I still believe that the reservoirs of pathogens and potential pathogens that are present in wild stocks will be the source of many of the disease problems we see in the future.
The take home lesson in all of this is that when we see things that appear anomalous, such as these reports of ISA, we need to tread very carefully. The test results need to be confirmed and if they are confirmed then a systematic look would be worthwhile trying to ascertain the prevalence. No one should be pointing fingers at farmed stocks as the culprit in this. If the finding is real it would imply a reservoir of the virus in the wild.
A prudent path to follow as well at this time is to ensure that all reasonable precautions are taken to lessen the potential of the virus wreaking havoc if it were to be present. This is not a new disease and every effort should be made to heed the biosecurity measures that have been proven to be effective in controlling and limiting the spread of the disease.
Some would say that I have a bias towards aquaculture. While I can understand this, it is actually the truth that I am biased towards. Nothing is all good and humanity has elected to propagate beyond what I (and many others) would characterize as wise. People need to eat and aquaculture is an important source of high quality protein and will remain so. To ensure that this is done in a responsible manner, we need to learn how to live with disease by using the tools of science and remaining vigilant. We shouldn’t rush to judgment based on anomalous test results.
