Saturday, April 3, 2010

More on DSLD, Degenerative Suspensory Ligament Desmitis

After my last post on DSLD I got an email telling me the reference from Dr Deb Bennett was written before the latest study. That email went on to state that no horse with dropped fetlocks should ever be bred. The inference was because of the information in the study I had referenced.

While I appreciate someone trying to be sure I understand the information, I can't help but be frustrated because it is clear the person has not really done her homework. The scientific evidence does not conclude nor support that all horses with dropped fetlocks should be excluded from the breeding pool.

It's important when reading things on the internet, even if those are scientific studies published in medical journals, to be sure of all the facts. Everyone makes mistakes and that includes those doing studies. If you look closely at the most current study by BMD Veterinary Research you'll see it does indeed support my position about mistakes although that may not be obvious if you didn't verify the information you read with the references indexed.

The current study by BMC Veterinary Research does indeed state in the section "Abstract" under the subheading " Background" that The pathogenesis remains unclear, though the disease appears to run in families.

Then under the section labelled "Background" with supportive references it states Degenerative suspensory ligament desmitis (DSLD) is a heritable, debilitating syndrome recognized in Peruvian Pasos, Peruvian Paso crosses, Arabians, American Saddlebreds, American Quarter Horses, Thoroughbreds, and some European breeds [12].

There is a big difference between "appears to be" and "is heritable." The study contradicts itself on the subject. So which is it?

If you read the referenced work that is supposed to support the statement that the disease is heritable you will find there is no stated or implied proof that the disease is heritable in the stated reference. Reference 12 is the study by Janice Young done in 1993.

That study states under the heading " Prognosis" The question of heredity must be answered as there are several sire/son, dam/daughter and full sister combinations of affected individuals. Then again under the heading "Summary" it states The question of heredity needs to be addressed, as these horses are only returning to breeding soundness rather than to previous athletic ability.

While the Young study suggests this disease could be hereditary, it clearly states the possibility needs to be investigated. That's a far cry from proving the disease is hereditary. Dr Deb Bennett understood this when she made her statement she " would not remove a horse from a breeding population for dropped fetlocks alone."

Since the most current study has totally misrepresented the heritability issue, many believe that it is a proven scientific fact that DSLD is heritable when in reality that is not the case. There have been NO studies, referenced in any of the works, published either on the internet or in print, that have proved if indeed the disease is heritable or not. Until that is done this issue will remain nothing more than a question.

Yet there are those actively out there spreading the word DSLD is a heritable disease based on the error of the most recent study by BMC Veterinary Research. One only has to go to forums where breeding discussions are being held to see. Not only that, but breeders making informed decisions based on the material are being attacked by those misinformed individuals.

It is indeed unfortunate that not all people read the references. They assume the authors like those of the BMC Veterinary Research study has been careful in quoting the references to assure accuracy. They do not take it upon themselves to check those references to be sure they are indeed accurate before they make up their minds and declare themselves experts.

Another important piece of information is this study was not about the heritability of the disease at all. it was about what tissues are affected by the disease. Yet the statement about heritability is legitimized by its mention in the study. That is what many would call poor science.

Poor science isn't just about using improper techniques in reaching a conclusion. It also about stating things as fact that are not supported by the evidence, either the researchers or someone else's. In this case stating the disease is "heritable" is a gross error. An error that could even lead to the issue of heritability not being addressed because others believe that's already been done.

Another unfortunate side effect of this, besides the dissemination of incorrect information, it also affects the legitimacy of the study. True scientists tend to distrust a study that has misrepresented the truth whether on purpose or by error or even omission. Science wants clear facts and abhors muddy statements representing facts. It suspects those things mixed together and discounts their findings.

There's another thing that needs to be addressed here that would be this statement from the Young study. The question of heredity must be answered as there are several sire/son, dam/daughter and full sister combinations of affected individuals. .This is an important statement because the fact there have been related individuals who have manifested the disease does indeed suggest the possibility of a genetic link.

However, it takes more than "several" to do a valid study. Numbers are important in science because they rule out coincidence as a cause. They also rule out other unknowns. Bigger numbers are better. Smaller numbers are not to be trusted. Again, that is good science.

The reason for this is clear. When science jumps the gun before it has all the facts it can create a really big mess. There are many incidences of this in the past, some catastrophic ones, so it is wise to avoid jumping to conclusions before all the facts are in.

I might add that the observation of related individuals with the diagnosis of DSLD was within the breed of Peruvian Paso horses. To this point there have been no other familial relationships of affected horses that have been documented in the literature. That information also deserves study before a valid picture can be drawn.

Not only that but when you've looked closely enough to see that all of these "several" combinations diagnosed with DSLD came from the same family of horses . While that might "prove" to some heritability, the only thing it proves is that there is some form of commonality in that family. The "what" is still a big question mark.

Also to be considered is the fact that most of the other individuals diagnosed with the disease have no other affected family members at all. For a disease that is thought to be hereditary, this fact opens the door to lots of questions as well. Certainly there is enough information to suggest that heritability be considered but there's also information that suggests heritability may not be the case.

For me I'm going to stick to what I know and that is the science of the subject. I am going to do what I can to stay on top of that information because the last thing I want is to get behind it. But I don't want to make mistakes by getting ahead of it either.

I would like to add that the science does support some important characteristics about DSLD that have nothing to do with heritability. It's important to know what these are so affected horses can be properly diagnosed and not all horses with dropped fetlocks lumped into that group. There are indeed differences between DSLD affected and unaffected individuals.

"Unique to DSLD, however, is diffuse enlargement of the affected ligaments despite exercise restrictions [12,13] "

"Horses with DSLD typically develop an insidious onset of bilateral or quadrilateral lameness without a history of trauma or performance related injury [13].

Ultrasonography of affected ligaments is characterized by a diffuse loss of echogenicity and an irregular fiber pattern" [14-16].

12.Young JH: Degenerative Suspensory Ligament Desmitis.

Hoofcare and Lameness 1993, 6-19.

13.Mero JL, Pool R: Twenty cases of degenerative suspensory ligamnet desmitis in Peruvian Paso horses.

Abstract for AAEP, Orlando 2002, 48:329-334.

14.Dyson S: Diagnosis and prognosis of suspensory desmitis. In Proceedings of the 1st Dubai International Symposium. Edited by: Hauser ML, Matthew R. Rantanen Design, USA; 1996:207-225.

15.Dyson SJ, Arthur RM, Palmer SE, Richardson D: Suspensory ligament desmitis.

Vet Clin North Amer 1995, 11:172-215.

16.Gibson KT, Steel CM: Conditions of the suspensory ligament causing lameness in horses.

Equine Vet Ed 2002, 4:50-64.

These statements are from the BMC Veterinary Research study and the references stated do indeed support these statements making them a reliable resource in the diagnosis of this insidious disease.

1 comment:

  1. This is all very interesting so I sent it to my daughter who will probably understand it better than me.

    Hope you had a great Easter.