DVM, PhD Anu Lappalainen
In dogs, there are two known FGF4 retrogenes which affect the length of limbs, making them grow short. One of these retrogenes is in chromosome 18 and is a so-called breed-defining mutation. Breeds that are homozygotic for this mutation have a typical, longish conformation as well as short and bent limbs, caused by a developmental disorder of the long bones in the limbs. Such breeds include e.g. the Dachshund, Pekingese, Welsh Corgi, Bichon Havanese, Coton de Tuléar, and Skye Terrier. It was previously believed that this mutation specifically is a predisposing factor for early degeneration of intervertebral discs, starting already during the first year of life. Limbs that are shorter than normal and early intervertebral disc degeneration are however connected with another FGF4 retrogene, located in chromosome 12. This mutation is likely very common in several Dachshund-type dogs but is found in several other breeds as well.
Retrogene FGF4 in chromosome 12 predisposes for disc degeneration and highly degenerated discs can be calcified. Degenerated discs are prone to damage and consequently lead to spinal disc herniation. Predisposition to calcification is hereditary, and the number of calcified discs is associated with the risk of disease. Of dogs with no calcified discs, only a few become ill. Calcified discs are visible on radiographs, which makes it possible to prevent the disease by means of radiographic screening. Studies have shown that most radiographically visible intervertebral discs are calcified by the age of two years. Some calcifications may disappear later and the recommended age for imaging is therefore 24 to 42 months. In order to reduce the occurrence of the disease, dogs with as few calcified discs as possible should be favoured in breeding.
A grading of IDD can be issued to a dog that is at least 24 months of age. International scale used in grading:
| IDD0, free | No changes |
| IDD1, mild | 1–2 partially or completely calcified intervertebral discs |
| IDD2, moderate | 3-4 partially or completely calcified intervertebral discs |
| IDD3, severe | 5 or more partially or completely calcified intervertebral discs |