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Genetic Welfare Problems of Companion Animals
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Pug
Hemivertebrae
Related terms: Congenital vertebral anomalies, wedge vertebrae
Outline: Pugs commonly have deformities of the bones of the spine. These can lead to pressure on the spinal cord resulting in progressive pain and loss of hind limb function and incontinence.
Summary of Information
(for more information click on the links below)
1. Brief description
Vertebrae are the linked bones that together make up the backbone or spinal column of the dog. One of the functions of the spinal column is to protect the delicate spinal cord, which is the nervous tissue linking the brain to peripheral nerves supplying control and sensation to all the furthest parts of the body. The spinal cord runs through a bony tube formed by the vertebrae. Normal vertebrae are symmetrical, and generally brick-shaped or cylindrical in shape.
A hemivertebra is a vertebra that has developed abnormally so that rather than having a cylindrical shape, it takes on an abnormal wedge-shape which does not align correctly with the adjoining vertebra This can lead to instability and deformity of the spinal column, which in turn can lead to the spinal cord or the nerves arising from it becoming squashed and damaged.
Signs seen with hemivertebrae are a result of spinal deformity and damage to the spinal cord. They can include pain, wobbliness (ataxia) on the hind legs and worsening signs can include loss of hind leg function and incontinence (inability to control passing urine or faeces). NB. Chronic pain (such as that of sciatica in humans) is not always shown by crying or yelping in dogs.
2. Intensity of welfare impact
Pain from spinal cord compression (squashing) can be severe. Affected dogs can also lose function in their hind limbs and sometimes lose bladder and bowel control, causing distress. Not all animals with hemivertebrae develop these signs; some have milder signs of ataxia or no signs at all.
Dogs with severe signs may need major surgical interventions, which have their own welfare impacts, and, despite this, some may not recover and need to be euthanized on humane grounds.
3. Duration of welfare impact
Young dogs are most commonly affected when skeletal deformities deteriorate as their skeleton physically grows. The skeletal deformity is permanent without surgery. The clinical signs can develop rapidly over days, or gradually over weeks and months. Severely affected individuals would, without surgery, have permanent major disability. Even where surgery is possible, some animals may have unacceptable levels of disability necessitating euthanasia.
Thus this condition can severely limit both the quality and length of life.
4. Number of animals affected
The proportion affected are not known, but it is considered a common problem in Pugs and other brachycephalic (short-nosed) breeds that have screw-tails, as the deformity is connected to the screw-tail phenotype (physical shape) which is a breed characteristic. Screw tail describes a tail which,in its relaxed position, is coiled, usually to one side. The most severely affected tails cannot be straightened at all, others can be manually straightened but relax back into the coiled position. From data on estimates of total dog population in the UK and on the percentage of all micro-chip registered dogs that are pugs (Lucy Asher, 2011, personal communication), we estimate that the UK population size of this breed may be around 30,000.
5. Diagnosis
A diagnosis is made on examination and radiography (taking x-ray pictures).
6. Genetics
The exact genetics of this condition have yet to be worked out; however, Pugs are considered to be predisposed to hemivertebrae because of the breed having a screw-tail. The gene(s) causing the screw-tail deformity (which involves hemivertebrae in the tail) are thought also to be involved in producing hemivertebrae elsewhere in the spine.
7. How do you know if an animal is a carrier or likely to become affected?
All Pugs are prone to the problem because the breed has a screw-tail. That a dog has no hemivertebrae can only be confirmed by taking x-rays of the spine.
8. Methods and prospects for elimination of the problem
Not known, though whilst the breed standard includes a screw-tail the condition seems likely to persist. It seems likely, since the screw tail is caused by hemivertebrae in the tail, that out-breeding to dogs with straight tails, then selection for a straight tail might be a way forward to eliminate this welfare problem.
For further details about this condition, please click on the following:
(these link to items down this page)
- Clinical and pathological effects
- Intensity of welfare impact
- Duration of welfare impact
- Number of animals affected
- Diagnosis
- Genetics
- How do you know if an animal is a carrier or likely to become affected?
- Methods and prospects for elimination of the problem
- Acknowledgements
- References
1. Clinical and pathological effects
An understanding of the anatomy of the normal spine of the dog is useful in understanding this condition.
The spine or spinal column 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. In Pugs and other screw-tailed dogs there are relatively few coccygeal vertebrae. Vertebrae are numbered according to region of the spine, starting from the head and working towards the tail. Thus the first cervical vertebra is called C1, the second C2 and the first thoracic vertebra is T1 etc. Intervertebral discs are found between the vertebral bodies.
Figure 1. The regions of the spine.
The body of each vertebra is positioned ventrally (towards the lower side). Dorsal (towards the back of the dog) to the vertebral body each vertebra forms a hollow tube of bone, called the vertebral canal. Through this tube created by the column of vertebral canals the spinal cord passes, connecting the base of the brain with peripheral nerves. This tube of bone protects the spinal cord. 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).
Figure 2. Diagram showing the shape of normally-formed vertebrae in the spinal column and how the spinal cord runs through them.
Each vertebral body develops in the foetus from two separate parts which fuse together and then ossify (turn to bone). Hemivertebrae occur when this fusing process fails to happen correctly so that the vertebral body of one or more vertebra is malformed and wedge-shaped (LeCouteur and Child 1995). Often the fusion or ossification is asymmetrical (Jeffery et al 2007). The malformations vary, with the apex of the wedge pointing dorsally, ventrally or medially (towards the middle of the animal) across the midline (LeCouteur and Child 1995). These abnormally-shaped vertebrae often lead to alterations and deformities in the shape of the spine such as kyphosis (curving upwards of the central spine), scoliosis (deviation to the side in the central spine) or lordosis (curving downwards in the central spine) (LeCouteur and Child 1995). This angulation of the spine is often moderate to severe and can lead to displacement of the vertebrae (LeCouteur and Grandy 2000). The deformity or instability in the spinal column can then narrow the vertebral canal, compressing and damaging the spinal cord, and this can lead to serious clinical signs (LeCouteur and Child 1995). This spinal deformity and the consequential clinical signs often progresses as the dog grows or, sometimes, happen secondary to mild trauma because of the spinal column instability (LeCouteur and Grandy 2000).
Figure 3. Abnormal development of vertebral bodies results in formation of hemivertebrae. Unlike the normal shape of vertebrae shown in Figure 2, in which the opposing faces of adjacent vertebrae are at right angles to the line of the spine, wedge shaped vertebrae as shown here can lead to distortion of the spine with health and welfare effects.
Figure 4. Radiograph of a hemivertebra in the thoracic region of a Pug. The wedge shaped vertebra has caused lordosis of the spine, which in turn can cause compression and damage to the spinal cord.
(Image property of Helen McDonald, to whom we are grateful for permission to reproduce it here).
Hemivertebrae can occur in any breed but are most common in brachycephalic, screw-tailed dogs such as Pugs (also English Bulldogs, French Bulldogs and Boston Terriers). This is because the kinked tails in these breeds result from hemivertebrae in the tail region of the spine and these breeds have genes that tend to cause the formation of hemivertebrae elsewhere in the spinal column as well as in the tail (LeCouteur and Grandy 2000). The commonest region of the spine to be affected by hemivertebrae, in Pugs, is the thoracic area (LeCouteur and Child 1995).
Figure 5. Hemivertebrae is the cause of kinked or screw tails in some breeds of dog, such as the Boston Terrier and Pug (shown here). This condition makes them prone to developing hemivertebrae elsewhere in the spinal column also.
(Image sourced from Wikipedia at www.wikipedia.org/Pug).
Hemivertebral abnormalities commonly occur without clinical sign of disease (Done et al 1975). However, if the spinal cord becomes damaged or compressed signs appear. Signs include posterior paresis (abnormal functioning of the hindlimbs), muscle wastage, abnormal shape to the back and back pain. If the problem is severe then the dogs can lose all use of their hind legs and can have urinary and faecal incontinence (inability to control passing urine or faeces). Signs can be gradual or rapid in onset and sometimes are progressive (Colter 1993). In a number of cases, however, signs may be mild and non-progressive and stabilise once the dog has stopped growing at about 9 months of age (Jeffery et al 2007), though the deformity will remain. In these cases no treatment may be necessary. Skin disease in skin folds can also occur if the tail is very tightly coiled.
In severely affected individuals the only hope is major surgery of the spine to attempt stabilisation and to stop the spinal cord being compressed. This is not straightforward, needs specialist veterinary treatment and may not always be successful. Some dogs which are completely paralysed in their hind legs may not recover use of them after surgery.
Paralysed dogs have long-term care needs which many owners find difficult and some dogs will be euthanized because of this. It is an ethical dilemma whether these permanently paralysed dogs should be euthanized or if their quality of life may be sufficient to justify supported living with paralysis and incontinence.
2. Intensity of welfare impact
Some dogs with hemivertebrae show no signs, however, others have major welfare problems such as pain, loss of function of the hindlimbs and incontinence. Pain from spinal cord compression can be significant and severe and difficult to control using drugs.
Along with pain, the spinal cord damage in severely affected individuals leads to the need for rapid veterinary treatments 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. Some animals may not recover and may need to be euthanized.
3. Duration of welfare impact
The condition is usually manifested in young dogs as the condition worsens as growth proceeds. In a case study by Jeffery et al (2007), all three dogs described were less than 7 months of age. Though some may be mildly affected, all affected animals will have permanent deformity unless surgical intervention is used. Signs can develop rapidly over hours or more gradually. Though some dogs with hemivertebrae do not develop signs, others develop severe problems which will not improve unless surgical intervention occurs. Some dogs with severe signs may not improve after surgery and may need to be euthanized.
4. Number of animals affected
The numbers affected are not known, but it is considered a common problem in Pugs and other brachycephalic (short-nosed) breeds that have screw-tails, as the deformity is connected to the screw-tail phenotype (physical shape) which is a breed characteristic. From data on estimates of total dog population in the UK and on the percentage of all micro-chip registered dogs that are pugs (Lucy Asher, 2011, personal communication), we estimate that the UK population size of this breed may be around 30,000.
5. Diagnosis
The condition may be suspected in any Pug with spinal deformity or with the characteristic clinical signs because it is commonly seen within the breed. A diagnosis can be made using plain and contrast radiography (x-rays, with or without dye, which shows up on the x-ray being injected around the spinal cord), however, Jeffery et al (2007) believe MRI (magnetic resonance imaging) scans are needed prior to surgery to help decide on exact surgical requirements.
The presence of hemivertebrae seen on x-rays may also be a confusing finding when older Pugs have spinal disease from other causes and can lead to a mistaken diagnosis.
6. Genetics
The exact genetics of this condition have yet to be worked out; however, Pugs are considered to be predisposed to hemivertebrae because of the breed having a screw-tail (Jeffery et al 2007). The, as yet unknown, gene/s that cause the screw-tail deformity (which involves hemivertebrae in the tail), are thought to also produce hemivertebrae elsewhere in the spine.
7. How do you know if an animal is a carrier or likely to become affected?
All Pugs are potentially prone to the problem because the breed has a screw-tail. Without x-rays of the spine, definite absence of hemivertebrae cannot be confirmed. However, a mature animal without signs has a lower chance of developing clinical problems than a puppy which has its growing stage ahead of it.
Currently no advice can be given on how to ensure a healthy individual, free from the condition, is not a carrier. This may become clearer when the genetics of the condition are better understood.
8. Methods and prospects for elimination of the problem
Not known, though whilst the breed standard includes a screw-tail the condition seems likely to persist. It seems likely, since the screw tail is caused by hemivertebrae in the tail, that out-breeding to dogs with straight tails, then selection for a straight tail might be a way forward to eliminate this welfare problem.
9. Acknowledgements
UFAW is grateful to Rosie Godfrey BVetMed MRCVS and David Godfrey BVetMed MRCVS for their work in compiling this section and to Hazel Bentall BVSc MRCVS for her comments on it.
10. References
Colter S (1993) Congenital anomalies of the spine. In Bojrab, M. (Ed) Disease mechanism in Small Animal Surgery. 2nd Ed. London: Lippincott, Williams and Wilkins
Done S, Drew R and Robins G (1975) Hemivertebra in the dog: clinical and pathological observations. Vet. Rec 96: 313
Jeffery N, Smith P and Talbot C (2007) Imaging findings and surgical treatment of hemivertebrae in three dogs. J Am Vet Med Assoc 230(4): 532-536
LeCouteur R and Child G(1995) Diseases of the spinal cord. In Ettinger, S and Feldman, E. (Eds) Textbook of veterinary internal medicine. 4th Ed. Philadelphia: WB Saunders Company
LeCouteur R and Grandy J (2000) Diseases of the spinal cord. In Ettinger, S and Feldman, E. (Eds) Textbook of veterinary internal medicine. 5th Ed. Philadelphia: WB Saunders Company
© UFAW 2011
Credit for main photo above:
By Beckilee (Own work) [CC0], via Wikimedia Commons
