Genetic welfare problems of companion animals
Intervertebral Disc Disease
Condition: Intervertebral Disc Disease
Related terms: Ruptured disc, prolapsed disc, slipped disc, herniated disc, disc protrusion, disc extrusion, intervertebral disc displacement, disc disease
Outline: Many Dachshunds (approximately 25%), at some points in their lives, suffer from damage to the discs in their spines (rather like ‘slipped discs’ in humans). This can cause serious pain that can be of prolonged duration and may lead to severe damage to the spine and paralysis.
Summary of Information
(for more information click on the links below)
1. Brief description
Abnormal intervertebral discs are a feature of Dachshunds which is linked to selection in the breed for the characteristic feature of short legs. These abnormal discs press into the spinal cord causing pain and spinal cord damage in around 25% of Dachshunds at some stage of their lives.
2. Intensity of welfare impact
Severe: intervertebral disc disease occurs frequently in the Dachshund breeds and causes pain which can be severe and prolonged. It is a disabling condition that may lead to death or requires euthanasia.
3. Duration of welfare impact
The pain caused by this condition can last for days to weeks and may recur. Pain control using drugs can be difficult. If paralysis occurs, this can last for days to weeks or can be permanent and can lead to euthanasia.
4. Number of animals affected
Around 25% of all Dachshunds will develop this condition to the extent that they require veterinary treatment. At any time, a greater percentage may be suffering from less severe back pain that does not get recognised and treated. From data on estimates of total dog population in the UK and on the percentage of all micro-chip registered dogs that are Dachshunds (Lucy Asher, 2011, personal communication), we estimate that the UK population size of this breed may be around 60,000. It follows that about 15,000 of these have, or will develop this disease. If, from the above, we estimate lifetime prevalence to be 25%, it follows that about 15,000 dachshunds in the UK have or will develop this disease.
Veterinary surgeons will have a strong suspicion that a Dachshund is affected just from the signs of back pain and spinal dysfunction. Imaging the neck or back using x-rays, MRI or CT will confirm the diagnosis.
The condition is strongly linked to the genes that cause Dachshunds to have short legs (chondrodystrophy) but additional genes may also be involved. All the genes have not been determined and so genetic tests are not available to detect animals susceptible to the condition or those likely to have affected puppies. It is unlikely that the disease can eliminated from the breed without a fundamental change in its physical appearance.
7. How do you know if an animal is a carrier or likely to become affected?
Taking radiographs of the dog’s back between the ages of 2 and 4 years gives an indication as to how likely that individual is to get IVDD. However, some individuals will develop the disease before reaching this age. If parents have been radiographed and they show less than 3 calcified discs at this age then their offspring may be less likely to be affected than if there are more than 4 discs. However, this has not yet been scientifically proven and it is likely that all Dachshunds are at an increased risk of IVDD compared to most other breeds.
8. Methods and prospects for elimination of the problem
It has been recommended that dogs with more than 4 calcified discs at 2 years of age should not be used for breeding (Jensen et al 2008). Selective breeding programs based on this recommendation have been initiated in Norway, Denmark and Finland (Rohdin et al 2010). Though it is too early for evidence to have emerged to confirm that this will decrease the incidence of the disease, this seems a sensible way forward.
Ideally dogs used for breeding should have no calcified discs. However, it is possible that restricting breeding in this way would result in the breeding population being so a small that other inherited conditions may increase in frequency. In this case, out-crossing with dogs of other breeds would appear to offer a way forward.
For further details about this condition, please click on the following:
- Clinical and pathological effects
- Intensity of welfare impact
- Duration of welfare impact
- Number of animals affected
- How do you know if an animal is a carrier or likely to become affected?
- Methods and prospects for elimination of the problem
To understand intervertebral disc disease some knowledge of the anatomy of the spine (the backbone) of dogs is needed. http://digital.library.wisc.edu/1711.dl/Science.VetAnatImgs (image published before 1926)
The spine of dogs is made up of many bones, called vertebrae and it is divided into 5 different regions. There are 7 bones in the neck (cervical) region, 13 in the chest (thoracic) region, 7 in the lower back (lumbar) region and 3 fused together to form the sacrum (at the hips). Lastly, there are the tail (coccygeal) bones, and their number varies. Vertebrae are numbered according to the region of the spine they reside in and starting from the head, working towards the tail. Thus the first cervical vertebra is called C1, the second C2, etc and the first thoracic vertebra is T1 etc. The intervertebral discs, implicated in this condition, can be found, as the name suggests, between the vertebra. Intervertebral discs are found between the vertebrae, from between the second and third cervical vertebrae (C2-3) and extending between the seventh lumbar and first sacral vertebrae (L7-S1). The 3 sacral vertebrae are fused and therefore do not have discs. Intervertebral discs are found between the coccygeal vertebrae also (Luttgen 1993) but the tail area of the spine is not relevant to IVDD (as there is no spinal cord in this area).
Each intervertebral disc is a highly specialised structure, designed to act as a mini shock absorber for the spinal column. Normal intervertebral discs consist of two parts: a central gelatinous area called the nucleus pulposus (NP) and an outer fibrous layer called the annulus fibrosus (AF). The jelly-like NP is virtually incompressible and helps dissipate and spread forces acting on the spine while the surrounding AF gives structure to the disc and adds to the shock absorbing effect whilst containing the NP (Braund 1993).
Intervertebral discs sit between the bodies of two vertebrae. On the upper, dorsal, side of the main vertebral body each vertebra forms a hollow tube of bone, called the vertebral canal. Through this tube, created by the adjacent column of vertebral canals, passes the spinal cord which is protected by it. The spinal cord is made up of delicate nerve tissue which transmits the messages between the brain and other parts of the body (excluding the head).
It is usual for intervertebral discs to degenerate with age in dogs. The NP gel is replaced by more mature fibrocartilage as part of the normal ageing process. This occurs gradually in most breeds of dog so that by 7 to 8 years of age the whole NP has changed (Braund 1993). As the dogs continue to age, further degeneration sometimes occurs in the discs, though calcification (abnormal deposits of calcium in tissues) is rare (McKee 2000).
Degeneration in intervertebral discs causes IVDD. Degenerative changes lead to the NP becoming harder and unable to act as a shock absorber. This leads to splits and tears in the AF. The AF layer is naturally thinner on the side nearest the vertebral canal and spinal cord. In IVDD either the whole disc gets pushed upwards into the vertebral canal and presses on the spinal cord (this is called a disc protrusion) or the AF splits and the NP material gets pushed out into the vertebral canal (this is called a disc extrusion). In either case this leads to the vertebral canal being narrowed and the spinal cord being squashed and sometimes damaged (Braund 1993). It is not uncommon for a jump or fall to initiate the degenerative disc to protrude or extrude (Braund 1993). IVDD is more likely to occur in some areas of the spine than others. In a retrospective study of 2395 cases of IVDD, at Auburn University, USA, 85% of cases were in the thoracolumbar region of the spine (between the thoracic and lumbar vertebrae) and 15% were in the cervical region of the spine (Hoerlein 1987). The lumbosacral area of the spine can also be affected (McKee 2000a).
Figure 3a: Vertebral disc protrusion. The disc is displaced causing pain by pressing on the spinal cord.
Figure 3b: Vertebral disc extrusion. In these cases, the outer layer of the disc ruptures and the disc contents are extruded into the spinal canal causing pain by pressing on the spinal cord. This is more common than protrusion in small breed dogs.
(Images property of Phil Witte to whom we are grateful for permission to reproduce them here).
Figure 4. A myelogram of the lumbar region of the spine indicating damage to the spinal cord (arrow on the diagram). Compression between the second and third lumbar vertebrae (L2 and L3) may have caused either intervertebral extrusion or protrusion which in turn has caused compression on the spinal cord.
(Image property of Southern Counties Veterinary Specialists, to whom we are grateful for permission to reproduce it here).
Any breed of dog can get IVDD; however, IVDD is much more common in chondrodystrophic dog breeds such as Dachshunds, Pekinese, Basset Hounds etc. Chondrodystrophy gives these breeds their characteristic short limbs in relation to their bodies – it is a form of short-limbed dwarfism related to genetic defects in certain bone and cartilage formation and growth (endochrondal ossification). It is sometimes called hypochondroplasia (de Lahunta and Glass 2009)
In chondrodystrophic breeds, such as the Dachshund, the intervertebral discs degenerate in a different way compared to other dog breeds. The NP chemical constituents are different, making them prone to a type of degeneration, called chondroid metaplasia, where the gel turns rapidly into hyaline cartilage and then calcifies and hardens (de Lahunta and Glass 2009). These undesirable degenerative changes occur very rapidly. They start as early as 4 months of age in chondrodystrophic breeds and the discs can be completely degenerated by 12 to 18 months of age (Braund 1993). By one year of age, approximately 90% of chondrodystrophic dogs have changes usually affecting multiple discs. Partial or complete calcification of the disc is common. Changes also occur in the AF with loosening of the fibres and fragmentation of its structure (McKee 2000).
In a Norwegian study by Stigen (1991), 327 young Dachshunds between 12 and 18 months of age were x-rayed and 24% of them were found to have calcified intervertebral discs. The average number of affected discs was more than two. Standard-sized Dachshunds were more likely to be affected, with the occurrence higher in the wirehaired variety (27.1%) than in the smooth coated (16.4%) or longhaired (9.1%) varieties. However, within the longhaired variety the occurrence was higher in miniatures (36.0%). Jensen et al (2008) have found that the number of calcified discs in Dachshunds is a good predictor of IVDD in individuals.
About 80% IVDD cases in Dachshunds occur between 3 and 7 years of age (as with other chondrodystrophic breeds); with males and females being equally affected (Braund 1993).
The onset of clinical signs varies from rapid - occurring over minutes to hours, to gradual - occurring over several days to weeks (Braund 1993). The speed with which signs appear and the severity of those signs depends on whether only pain is being caused, without damage to the spinal cord or if the spinal cord is also being damaged. If the spinal cord has been damaged then the severity of signs depends on: the speed of the injury, the extent to which the spinal cord has being compacted and the length of time it has been squashed (McKee 2000). Thus, when signs of IVDD occur, the speed with which a dog gets treatment can greatly affect the outcome.
Signs of pain are often the first sign of IVDD. These signs include the dog being reluctant to climb eg steps or stairs or jump eg into cars, or unprovoked yelping (McKee 2000). Alternatively, what may be seen is the dog taking up a typical stance with its head low and its back arched (McKee 2000). Other signs of pain reported by McKee (2000) include: reluctance to lower its head to eat, reluctance to look upwards, reluctance to turn in tight circles, having a tense neck, back and/or abdomen, restlessness or panting for no apparent cause.
Other signs of IVDD can be seen depending on where the disc protrusion/ extrusion has occurred along the spine. The most common sign of cervical disc disease is neck pain. The dog may also seem weak or incoordinated due to damage or pressure on the spinal cord affecting the functioning of its nerves. These signs of weakness and incoordination usually come on gradually. Only rarely is a dog with cervical IVDD incontinent (McKee 2000).
Thoracolumbar disc disease often causes both pain and spinal dysfunction. This is characterised by weakness and incoordination in the hind limbs, or the dog seeming to be lame or the dog being totally unable to move or use its back legs ie having hindlimb paralysis. In severe cases the dog may also be incontinent (unable to control its bowels or bladder). Dogs whose lower back is affected, by lumbosacral disc disease suffer pain and often have difficulty in getting up and show a reluctance to jump. Occasionally they exhibit limping in a hind leg, self-mutilation such as chewing at the tail or hind feet, holding the tail low or incontinence.
Spinal cord function following degenerative intervertebral disc injury can improve spontaneously but usually veterinary medical or surgical treatment is necessary (McKee 2000b). Many cases of IVDD in Dachshunds need rapid specialist treatment which often involves major surgery on the spine. The outlook for dogs suffering from IVDD depends greatly on the site of their problem, how severe the signs are and, when spinal cord damaged has occurred, how severe that damage is and how longstanding it has been. Some dogs which are completely paralysed in their hind legs do recover use of them after surgery. However, some do not and remain paralysed and incontinent. In a study of 238 canine cases that had surgery at a German veterinary school, 85% of the patients showed a good response to treatment and 15% had a poor long-term outcome (Bull et al 2008). Jensen et al’s (2008) studied 61 Dachshunds until they were 8 years old; 22 (36%) dogs, developed IVDD and of these 8 were euthanatized due to their IVDD, 8 had surgery and were alive, 5 had medical treatment only and one had spontaneously recovered. In Mayhew et al’s study (2004) 25% of Dachshunds who had had surgery for thoracolumbar IVDD had a reoccurrence; 96% of recurrences developed within 3 years of surgery.
Paralysed dogs have long-term care needs which many owners find difficult and some dogs will be euthanased because of this. It is an ethical dilemma as to whether these dogs should be euthanased or if their quality of life is sufficient to justify supported living with paralysis and incontinence.
The commonest welfare impact of IVDD is pain. McKee (2000b) states this can be significant pain that is often not controlled by routine pain killers. This pain is apparent to owners as it affects the dog carrying out normal movements of its daily life such as climbing up steps, jumping and rising from a lying position. The pain can be of a highly debilitating level affecting the animal’s quality of life.
Along with pain, IVDD can also result in spinal cord damage, leading to the need for rapid veterinary treatment including major surgery. Though some dogs recover, surgical treatment is a major intervention and many animals have a prolonged recovery during which veterinary staff and owners need to carry out both invasive procedures and nursing which interfere with the dog’s life and daily routine. Reoccurrences of damage and pain can occur. At its severest, IVDD is life-threatening, causing permanent paralysis and requiring euthanasia (McKee 2000). As it is commonly seen in Dachshunds as young as 3 years old (Braund 1993), this condition can considerably impact on life expectancy in Dachshunds.
Signs of IVDD can appear very rapidly or gradually over longer periods of up to several weeks (Braund 1993). For some dogs it may mean a period of significant pain before diagnosis and a further period during which veterinary investigations and treatments are undertaken and then, hopefully, recovery. Major surgery may be necessary which has its own welfare implications. Some dogs will have reoccurrences, and for those with severe signs it can be life-threatening, particularly for those dogs with thoracolumbar disease (McKee 2000b).
The incidence of IVDD in all breeds presenting to a group of 13 North American Veterinary Schools was 23 cases per 1000 dogs per year (Priester 1976). The risk has been estimated to be 10 to 12.6 times greater in Dachshunds (Priester 1976; Goggin et al 1970). This means the incidence of IVDD during the lifetime of Dachshunds is about one in four (Luttgen 1993). Ball et al (1982) suggested an incidence of 19% generally in Dachshunds, but found a much higher incidence in some Dachshund families where 62% of individuals had had IVDD. Jensen et al (2008) stated the estimated lifetime incidence of disc herniation, where the disc protrudes/extrudes which produced obvious signs of discomfort was 18%, though they felt subclinical disc herniation, where obvious signs of discomfort were absent, in Dachshunds is likely to be even more common, although the true incidence is unknown and some of these dogs will have back pain that goes undetected by their owners.
Other factors have been implicated in increasing the risk of developing the condition. The risk of euthanasia due to the disease was significantly higher in Dachshunds that were found to have more than 4 calcified discs seen on radiographs taken between the ages of 2 and 4 years (Jensen et al 2008). Other factors that have been found to increase the risk of severe thoracolumbar IVDD in Dachshunds include: having longer backs (specifically the distance between T1-S1), being taller at the withers (shoulders), and having a smaller pelvic circumference (measurement taken around the pelvis) (Levine et al 2004). In Norway, Stigen (1991) found that standard-sized Dachshunds were more likely to be affected with calcified discs than miniatures, with the occurrence higher in the wirehaired variety than in the smooth coated and longhaired varieties being least affected. However, within the longhaired variety, the occurrence was higher in miniatures.
From data on estimates of total dog population in the UK and on the percentage of all micro-chip registered dogs that are Dachshunds (Lucy Asher, 2011, personal communication), we estimate that the UK population size of this breed may be around 60,000.
A confirmed diagnosis of IVDD is made using plain and contrast radiography (contrast radiography involves the injection into the spinal canal of a dye that shows up on x-rays) or using more advanced technology such as magnetic resonance imaging (MRI) or computed tomography (CT). Vets are likely to suspect IVDD in a Dachshund with the typical signs of back pain as it is so common but other causes such as trauma or neoplasms need to be excluded; MRI scans are very useful for this (Chang et al 2007). MRI or CT scans may also be helpful to give surgeons more detailed information prior to surgery (Schulz et al 1998, Naude et al 2008). These diagnostic methods all involve the dog being anaesthetised and some may only be available in referral centres. All these interventions incur a significant cost.
Ball et al’s study (1982) of 536 Dachshunds with IVDD found that the patterns of disease occurrence were consistent with a genetic model involving the combined effects of several genes. A straightforward dominant gene or sex-related gene was not found. In some families there was very high prevalence of IVDD with up to 62% of individuals being affected, as compared with the estimated breed prevalence of 19%. They did find that the disease was affected to some extent by environmental conditions. Ghosh (1990) concluded that IVDD development involved genetic and mechanical factors as well as the effects of aging. He also felt the strong relationship between chondroid metaplasia (the type of disc degeneration that Dachshunds have) and hypochondroplastic dwarfism suggests that a major genetic factor in IVDD is attributable to the hypochondroplasia gene(s).
In 2000, Jensen and Christensen reported finding calcification of intervertebral discs in 2 families of wire-haired Dachshunds with heritability around 0.60 to 0.87 based on offspring sire relationships. This means that well over half of the cause of IVDD in these dogs was due to their genes with a minority of influences being non-genetic. Previously, lower rates of heritability have been reported (Stigen and Christensen 1993) and the conclusion drawn then was that a genetic factor was essential for the occurrence of calcified discs in Dachshunds but that a common environmental factor was significant in determining the number of discs likely to undergo calcification in affected dogs. Jensen et al (2008) added that the high heritability of disc calcification within the breed indicates involvement of genes other than just the gene encoding hypochondroplasia.
The exact nature of the environmental factors that influence the development of IVDD have not been identified , although episodes may be induced by (normal) activities such as jumping and climbing (Braund 1993).
Thus, the general conclusion is that IVDD is caused by multiple genes, including the genes that code for chondrodysplasia, while natural behaviours also play a role in its development.
As noted above, it appears that all Dachshunds are at risk of this disease: typically about 1 in 4 develop it. Whilst IVDD is strongly inherited in Dachshunds, the exact genes involved have not been discovered and so providing advice on how to avoid the disease is not straightforward.
Jensen et al (2008) recommended not breeding from Dachshunds with high numbers of calcified discs (greater than 4) seen on radiographs at 2 years of age; the number of calcified discs seen at this age being a strong predictor of later IVDD and calcified discs having been found to be inherited in the breed. They reported that, in 2008, the Danish Dachshund Club recommended breeding from dogs with 2 or fewer calcified discs and not from dogs with more than 4 calcified discs. Dogs with 3 or 4 calcified discs should only be used in strategic breeding (ie only allowed to produce 1 or 2 litters and with screening of the progeny before the parent is allowed to be bred further). Jensen et al (2008) agreed with this policy and recommended that radiographical screen should occur between 2 and 4 years of age, stating that before 2 years of age not all the calcification that is going to occur may have occurred and that after 4 years of age the appearance of calcified discs may change and become less obvious (Jensen et al 2008). It has been recommended that these radiographs should be taken at between 24 and 30 months of age (Jensen and Arnbjerg 2001).
Jensen et al (2008) suggested that their findings may also be used to identify individual dogs that are predisposed to IVDD so that preventive measures can be initiated (although it is not clear what these are). Furthermore, they recommended that the radiographic technique should be standardized and should include sedation of all dogs for optimal positioning.
Others have disagreed with the use of radiographic findings of calcified discs as a method for selecting breeding animals. Lappalainen et al (2001) felt selecting Finnish miniature Dachshunds for breeding on the basis of no radiographical calcifications was not possible as the number of animals passing this test would be so few. Rohdin et al (2010) stated that radiological studies detect only about 20-40% of the disc calcifications that can be identified by histopathology and suggested that CT is a more sensitive way of detecting calcified discs for breed screening programmes.
The absence of radiological disc calcifications does not exclude degenerative changes in the disc nor does it exclude disc calcification or disc herniation (Rohdin et al 2010).
As it is known that IVDD in Dachshunds has a high heritability and it has been recommended that dogs with more than 4 calcified discs on radiographs at 2 years of age should not be breed from, prospective owners should ask the breeder if the sire and dam have been radiographed at these ages and, if so, how many calcified discs they had. However, this procedure is not standard practice in much of Europe or the USA and such information will be unavailable for many dams and sires. Therefore, puppies from parents with a history of IVDD should not be purchased. In the future, x-raying or, better, CT or MRI scanning of breeding individuals may become more standard.
Other factors that have been found to increase the risk of severe thoracolumbar IVDD in Dachshunds include: having longer backs (specifically the distance between T1-S1), being taller at the withers (shoulders) and having a smaller pelvic circumference (measurement taken around the pelvic area (Levine et al 2006). Choosing animals without these characteristics would seem desirable, but not easy.
It is also recommended that any dog purchased with the intention of breeding should have radiographs or CT or MRI scans taken after 2 years of age and before the commencement of breeding to check their calcified disc status. Jensen et al.’s (2008) recommendation not to breed from dogs that have greater than 4 calcified discs on radiographs at 2 years of age could help decrease the incidence of the disease. Currently, there is no evidence to show that this policy has had an impact and there is no evidence it is being widely used across the world, however, selective breeding programs following this recommendation have been initiated in Norway, Denmark and Finland (Rohdin et al 2010). Though evidence is still lacking that this will decrease the incidence of disease, screening programmes seem a sensible way forward, though CT or MRI scans rather than radiography would probably be the ideal.
A stricter recommendation, not to breed from dogs with any calcified discs at 2 years of age, could be considered. This might have a more profound effect. However, it is possible that restricting breeding in this way would create too small a gene pool so that other inherited conditions increased in frequency. If this is the case then out-crossing with other breeds would appear to offer a way forward. However, to eliminate this problem, it would seem sensible not to breed from dogs that have any evidence of abnormal intervertebral discs.
If the genetics involved in causing calcified and degenerative discs are the same, or closely connected to, the genes causing chondrodysplasia – (the short-legged charachteristic of the breed) (Jensen et al 2008), it may be unlikely that the disease could be eliminated from the breed without a fundamental change in its body shape
UFAW is grateful to Rosie Godfrey BVetMed MRCVS and David Godfrey BVetMed FRCVS for their work in compiling this section and to Stephanie Kaufman for assistance in illustrating it.
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