paddy555
Well-Known Member
the text below came up on one of the PSSM FB groups so posting it in case it interests anyone. (I expect most will have seen it already)
(note for anyone new to this it does not relate to the PSSM 1 test which is not problematical)
From the text
I am sharing this in the interest of dispelling some dangerous misinformation that is floating around the equine community at the moment. Please feel free to share as well. It is critical for people to be able to understand what the facts are around this in order to make informed decisions.
As an advocate for the horse, I am always looking for ways that I can support the community towards bettering the lives of these magnificent animals. As an equine professional, it is my duty to keep up with the ever evolving science surrounding the animals we are so lucky to be guardians of.
When I was suggested to look into hair testing for PSSM2 a year ago, I jumped at the opportunity to learn about another way to ensure our horses health and well being. The more I researched, the more data I read, the more I immersed myself in the groups to learn about how people were using these tests - the more concerned I became.
As lovely as it sounds to have a simple test to tell us if our horse has this disease, the science simply doesn’t support that the genetics correlate to the disease or cause the disease - in the same way that a bay horse that has gone blind will test E_A_ if you test for his extension and agouti status, that doesn’t mean that those genes correlate to the blindness. That horse may have a genetic issue that has caused the blindness, and it may even be related to another color gene but that doesn’t mean the genes you tested for when you tested the agouti status caused the disorder in any way. Correlation does not equal causation.
This is one of the hardest things for people to understand, and perhaps why this test has gained such an unusual following despite being dismissed by every major equine medical institution (including ones who are well known for endorsing and utilizing controversial and experimental genetic tests). Your horse may have PSSM2 and a muscle biopsy may have confirmed it, but the genes being present doesn’t mean that they are what caused the disease. The data shows clearly that there isn’t enough correlation between the presence/absence of genes and the presence/absence of the disease to show any relation whatsoever.
Did you know the test failed to identify 57% of symptomatic horses confirmed to have PSSM2 through muscle biopsy? This means that a horse who tested positive through the gold standard for identifying the disease, didn’t have any of the genetic variants this test uses to “identify” the disease according to this test. That statistic alone should be enough to discredit the test as an unreliable way to identify the disease - especially when looking to utilize the test for prepurchase examination or when considering a horse for breeding purposes. Actually, it is enough to discredit the test among experts in the field who can actually understand the science, as every single veterinary professional and genetic specialist I have spoken to on the subject, in various institutions across the United States, has dismissed the test as unreliable and frankly dangerous in it’s misleading nature.
My primary problem with this test is the idea that it should be a part of a pre purchase exam or a breeding assessment - when it is completely unreliable in both being unable to identify horses confirmed to have PSSM2 as well as “identifying” horses who are completely healthy and will never exhibit and single symptom as being “positive”. Basing the purchase of a horse on the negative result of a test which has shown to falsely clear 57% of positive horses is just wildly unfair to everyone involved. To the buyer who expects to never deal with the disease, as well as (and more importantly) to the horse who has been “cleared” and thus won’t be properly tested further down the road if they show symptoms. Equally horrifying is the idea of basing the inclusion or exclusion of a horse in a breeding program based on the results of this test. In breeds in which PSSM2 is present (which is actually documented to only be a limited number of breeds), horse who actually do have PSSM2 could be incorrectly identified as a negative carrier and thus included in the breeding program despite experiencing symptoms, meanwhile horses who do not have PSSM2 and will never experience a single symptom could be culled from the gene pool for having one of these genes. Everyone promoting this test claims they are just looking out for horse welfare - what about the welfare of these horses? What about the owners? The breeders?How is it ethical in any way to be promoting a test that is so unreliable?
If this was a human test it would never be allowed to make it to the public, as it would cause dangerous misinformation and potential for misdiagnosis - however the equine world is vastly under regulated and so this is allowed to be promoted. Even with vets around the world dismissing it as scientifically unsound, it being proven over and over again to be scientifically unsound, people who are unable to understand the science of it are falling into the trap of self-serving biases. It baffles me that despite the fact that there is not a single peer reviewed study which supports this test, meanwhile there are several which completely debunk it - people still are pushing hard for this test. I thought that anyone who did independent research from a neutral perspective (as I did, even testing some of my horses - who came back negative by the way, for anyone who wants to say I am trying to discredit the test due to having “positive” horses) would be able to see the fallacies with this, but unfortunately in the age of anti-science and dangerous misinformation, people are being easily mislead. I am sharing my thoughts on this after a year of deep dives and reaching out to many many experts, and am including links to various sources for those of you who wish to learn more.
To anyone who is being told this is a necessary part of the pre-purchase examination, save your money. If you are concerned, speak with your vet. To anyone with a breeding program being told this is a necessary test to determine whether a horse is fit to breed or not, please consider the potential danger this misinformation could pose to your program if the test is wrong in either direction. This test has not stood up to the rigorous scrutiny needed to ensure accuracy and reliability. Do your research, speak with experts, if you have a horse who concerns you, take the necessary steps to identify and diagnose or clear that horse through scientifically founded methods.
Pictured: slides from a recent talk given by Dr Rebecca Bellone, the leading geneticist at UC DAVIS with 30 years experience in diagnostic markers across species.
(note for anyone new to this it does not relate to the PSSM 1 test which is not problematical)
From the text
I am sharing this in the interest of dispelling some dangerous misinformation that is floating around the equine community at the moment. Please feel free to share as well. It is critical for people to be able to understand what the facts are around this in order to make informed decisions.
As an advocate for the horse, I am always looking for ways that I can support the community towards bettering the lives of these magnificent animals. As an equine professional, it is my duty to keep up with the ever evolving science surrounding the animals we are so lucky to be guardians of.
When I was suggested to look into hair testing for PSSM2 a year ago, I jumped at the opportunity to learn about another way to ensure our horses health and well being. The more I researched, the more data I read, the more I immersed myself in the groups to learn about how people were using these tests - the more concerned I became.
As lovely as it sounds to have a simple test to tell us if our horse has this disease, the science simply doesn’t support that the genetics correlate to the disease or cause the disease - in the same way that a bay horse that has gone blind will test E_A_ if you test for his extension and agouti status, that doesn’t mean that those genes correlate to the blindness. That horse may have a genetic issue that has caused the blindness, and it may even be related to another color gene but that doesn’t mean the genes you tested for when you tested the agouti status caused the disorder in any way. Correlation does not equal causation.
This is one of the hardest things for people to understand, and perhaps why this test has gained such an unusual following despite being dismissed by every major equine medical institution (including ones who are well known for endorsing and utilizing controversial and experimental genetic tests). Your horse may have PSSM2 and a muscle biopsy may have confirmed it, but the genes being present doesn’t mean that they are what caused the disease. The data shows clearly that there isn’t enough correlation between the presence/absence of genes and the presence/absence of the disease to show any relation whatsoever.
Did you know the test failed to identify 57% of symptomatic horses confirmed to have PSSM2 through muscle biopsy? This means that a horse who tested positive through the gold standard for identifying the disease, didn’t have any of the genetic variants this test uses to “identify” the disease according to this test. That statistic alone should be enough to discredit the test as an unreliable way to identify the disease - especially when looking to utilize the test for prepurchase examination or when considering a horse for breeding purposes. Actually, it is enough to discredit the test among experts in the field who can actually understand the science, as every single veterinary professional and genetic specialist I have spoken to on the subject, in various institutions across the United States, has dismissed the test as unreliable and frankly dangerous in it’s misleading nature.
My primary problem with this test is the idea that it should be a part of a pre purchase exam or a breeding assessment - when it is completely unreliable in both being unable to identify horses confirmed to have PSSM2 as well as “identifying” horses who are completely healthy and will never exhibit and single symptom as being “positive”. Basing the purchase of a horse on the negative result of a test which has shown to falsely clear 57% of positive horses is just wildly unfair to everyone involved. To the buyer who expects to never deal with the disease, as well as (and more importantly) to the horse who has been “cleared” and thus won’t be properly tested further down the road if they show symptoms. Equally horrifying is the idea of basing the inclusion or exclusion of a horse in a breeding program based on the results of this test. In breeds in which PSSM2 is present (which is actually documented to only be a limited number of breeds), horse who actually do have PSSM2 could be incorrectly identified as a negative carrier and thus included in the breeding program despite experiencing symptoms, meanwhile horses who do not have PSSM2 and will never experience a single symptom could be culled from the gene pool for having one of these genes. Everyone promoting this test claims they are just looking out for horse welfare - what about the welfare of these horses? What about the owners? The breeders?How is it ethical in any way to be promoting a test that is so unreliable?
If this was a human test it would never be allowed to make it to the public, as it would cause dangerous misinformation and potential for misdiagnosis - however the equine world is vastly under regulated and so this is allowed to be promoted. Even with vets around the world dismissing it as scientifically unsound, it being proven over and over again to be scientifically unsound, people who are unable to understand the science of it are falling into the trap of self-serving biases. It baffles me that despite the fact that there is not a single peer reviewed study which supports this test, meanwhile there are several which completely debunk it - people still are pushing hard for this test. I thought that anyone who did independent research from a neutral perspective (as I did, even testing some of my horses - who came back negative by the way, for anyone who wants to say I am trying to discredit the test due to having “positive” horses) would be able to see the fallacies with this, but unfortunately in the age of anti-science and dangerous misinformation, people are being easily mislead. I am sharing my thoughts on this after a year of deep dives and reaching out to many many experts, and am including links to various sources for those of you who wish to learn more.
To anyone who is being told this is a necessary part of the pre-purchase examination, save your money. If you are concerned, speak with your vet. To anyone with a breeding program being told this is a necessary test to determine whether a horse is fit to breed or not, please consider the potential danger this misinformation could pose to your program if the test is wrong in either direction. This test has not stood up to the rigorous scrutiny needed to ensure accuracy and reliability. Do your research, speak with experts, if you have a horse who concerns you, take the necessary steps to identify and diagnose or clear that horse through scientifically founded methods.
Pictured: slides from a recent talk given by Dr Rebecca Bellone, the leading geneticist at UC DAVIS with 30 years experience in diagnostic markers across species.
. I told him as a fellow scientist that his inability to explain his science easily and clearly meant he didn't really understand it at all 
