Health testing: time to see the bigger picture?

This article was first published in Our Dogs as part of Ian Seath’s “Best of Health” series.

The latest edition of the KC sponsored Journal of Canine Genetics and Epidemiology has a paper by researchers from the Roslin Institute in Edinburgh.  “The challenges of pedigree dog health: approaches to combating inherited disease” is one of the best papers I’ve read for a long time.  This is an open-access paper, so anyone can read the full paper. [cgejournal.org/content/2/1/3]

The review discusses the background to inherited genetic diseases in pedigree dogs and how breeding strategies and genetic testing can be helpful in combating and reducing disease frequency. It also highlights the importance of maintaining genetic diversity within each breed. The strengths and weaknesses of the various approaches to health improvement such are also discussed.

The loss of genetic diversity in pedigree dogs has been caused partly by breeding practices such as line-breeding, over-concentration on certain physical traits and over-use of so-called popular sires.  Interestingly, the paper says that loss of diversity and inbreeding does not always mean an increased incidence of poor health.  The authors suggest this might be because current inbreeding hasn’t depressed genetic variation significantly from the “genetic load” present in the founding population, or harmful recessives may actually have been bred out.

Health conditions not related directly to Breed Standards account for over 75% of all inherited conditions in dogs according to a 2010 paper by Asher et al.  That rather makes a nonsense of the calls being made by the RSPCA for Breed Standards to be overhauled and reviewed by an independent authority.  The increasing prevalence of diseases is more likely to be a result of the way breeds have developed, i.e. breeding practices, than Breed Standards themselves.

Farrell et al in the Roslin Institute paper says that clearly a relationship exists between morphologies and diseases; we only have to look at the brachycephalic breeds to recognise this. But, directly apportioning the contribution of “shape” to the prevalence and severity of health issues is not straightforward.

As an example from my own breed, Dachshunds, we are currently conducting a “lifestyle” survey where we are gathering data on the length and height of dogs as well as information on their diets and exercise.  The emerging data (from 1700 dogs) show 16% have had some degree of back disease, but the prevalence varies from 8% in Standard Wires to 24% in Standard Smooths. I’ve not yet analysed the body proportions or weights for each variety of Dachshund, but the average length to height ratio across all 1700 dogs is exactly 2:1, which just happens to be what our Breed Standard specifies.  It will be fascinating to see if we can find a correlation between body proportions and back disease from this large sample of dogs, and to compare our findings with those of Rowena Packer who published a paper on this subject in 2013.  She concluded that longer, lower Dachshunds had a higher risk of back disease.

Farrell et al go on to say that 396 disorders have been identified in pedigree dogs that are known, or suspected to have a genetic root cause.  They don’t explain the prevalence of these, nor do they describe their severity/impact on a dog’s wellbeing.  They go on to describe the importance of screening schemes and the increasing availability of DNA tests.  Some screening schemes have yet to be successful or are, at best, making slow progress towards improvements.

A further insightful piece of analysis of the Farrell data has been done by Dr. Carol Beuchat of the Institute of Canine Biology.  She looked at the raw data supporting the Farrell paper and showed that 47 breeds have more than 20 identified genetic disorders. GSDs top the list with 77, closely followed by Boxers with 63.  However, there are only tests for 11 of the GSD disorders and only 4 for those in Boxers.  The stark conclusion from this is that there is a huge gap between genetic problems and available DNA tests and even if there was a test for every issue, breeders simply could not afford to use every one and what on earth could they do if they had all these results?

What is more worrying is that, while there are only a few tests available, but many more inherited diseases, breeders may be making things worse by removing dogs from the gene pool on the basis of a single DNA test.  Removing all “Affected” dogs for a particular condition risks increasing the frequency of other harmful recessive mutations.  Not only that, but removing dogs from the gene pool further reduces the Effective Population Size with its associated risks of more inbreeding and genetic bottlenecks (especially if everyone rushes to use a popular “Clear” stud dog).

Carol Beuchat says “A health-tested puppy with a Coefficient of Inbreeding of 30% is an oxymoron. Preventing the 25% risk of a known disorder, then breeding with a 30% risk of producing a new one is not a responsible breeding strategy“.

Health-testing is doomed to fail as there will never be tests for all the mutations and using tests to remove dogs from the gene pool will make things much worse.  We’d probably make more progress by discouraging “popular sires” than seeking the next generation of DNA tests.

I think much of what the Farrell paper says completely overturns what so many people are saying they want the KC to do, such as only register puppies from health-tested litters and make health-testing mandatory.  Do they mean “every possible health test” now and in the future? Yes, health-testing is important and should certainly be used to avoid producing puppies that will be “Affected” by serious health conditions.

Screening schemes and DNA tests are valuable tools available to breeders, but they are not “the answer” and must be seen in the wider context of effective breeding strategies.  As with the “events, dear boy” at Crufts, health tests are simply events and we mustn’t lose sight of the bigger picture.

Note: Carol Beuchat’s ICB blog post which builds on the Farrell paper is worth reading:

http://www.instituteofcaninebiology.org/blog/what-does-health-tested-really-mean

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