Why are Dachshunds the shape they are?
Man and dog have been working together for a very long time. Archaeologists have found bones that appear to be from domestic dogs dating from 15,000 years ago, but genetic research suggests that the split between the dog and the grey wolf started about 40,000 years ago. There were about five separate times when the wolf came into the dog’s family tree, but by about 14,000 years ago the dog was probably very much a domestic animal, although they may still have looked a lot like wolves (Parker, 2010, Savolainen, 2002). Indeed, there are some dog breeds around that still look very wolf-like, even today.
About 10,000 years ago a mutation occurred in one puppy that fundamentally changed the shape of this animal and its descendants. This puppy was the ancestor of all the short leg dog breeds to be seen in the world today. This puppy had a duplicate, but somewhat abnormal, copy of a gene that codes for a growth-promoting protein called Fibroblast Growth Factor 4 (also known as FGF4) (Parker, 2009). This growth factor is important in determining when bones stop growing.
As puppies grow, the bones are initially made of cartilage and this is replaced by bony calcification later. The long bones of the legs grow in three parts, the main middle piece – the shaft – and the two ends where the joints are. The elongation of the bone happens as new cartilage is added to the ends of the shaft. The mutation, however, triggers excess production of the growth factor and this causes growth to stop prematurely; the ends of the bone fusing to the shaft before they should have done. The developing puppy therefore would have had rather short legs.
The mutation caused a form of dwarfing with short legs but a normal head, chest and body. The technical terms used for this are rather confusing as they have been borrowed from medical terminology:
Chondrodysplasia is a term often used in describing Dachshunds, but in humans the type of dwarfism this term is used for involves bones other than the legs and arms.
Hypochondroplasia is a better term for dogs as humans with this condition look relatively normal apart from shortened limbs, just like Dachshunds, although both the human conditions are results of abnormalities in a slightly different place in the bone growth control system (Fibroblast Growth Factor Receptor 3).
Achondroplasia is probably the best term for Dachshunds as it indicates a general abnormality in cartilage development (Stoppler, 2009).
Dogs had been domesticated for at least 5,000 years when the dwarfing mutation happened, although we don’t, of course, know if the dog with the mutation actually lived with humans. It survived with this abnormality, though (obviously), which may have been due to it being in a ‘domestic’ environment. From the human point of view, dogs with short legs would be useful in getting through dense undergrowth or for chasing game down holes, so there would have been advantages in retaining these dogs for hunting.
Today, the gene responsible for the short legs is found in more than 20 breeds. Interestingly, hunting breeds predominate in the current list of breeds that includes hounds, terriers and cattle dogs. There may well be other breeds to add as not all breeds have been investigated for this gene. Thus it can be seen that Dachshunds (and their cousins) actually haven’t got long backs, they have short legs.
So, why do they have problems with their backs?
The mutation that causes the short legs also causes an abnormality in the biochemistry of the intervertebral discs. The intervertebral discs are are disc-shaped fibrous sacs fully of jelly-like material. They are found between the vertebrae and these act as shock absorbers and allow flexibility in the back. As dogs get older, the discs degenerate in all breeds of dogs, but in Dachshunds this happens at a much earlier age than other breeds. The degeneration can be seen on X-ray as calcification of the gel in the centre of the disc in Dachshunds as young as 12 to 18 months (Jensen and Ersboll, 2000). Symptoms of disc disease can appear soon after this.
Not all dogs with the FGF4 mutation have a tendency to develop back problems though. Even within the Dachshund group, some lines seem to have back problems more than others (Lappalainen, 2001). All these dogs have the FGF4 mutation though, so there must be other factors involved in the predisposition to IVDD, and some would seem to have a genetic basis (Mogensen, 2011).
Because of this, work is currently going on to identify the genes involved in these but it is unlikely that a reliable genetic test to help reduce the incidence of IVDD will be available soon.
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The original version of this was written by Roger Sainsbury BVM&S, MRCVS.