Genetic Welfare Problems of Companion Animals

An information resource for prospective pet owners

Rhodesian Ridgeback

Rhodesian Ridgeback

Dermoid Sinus

Related terms: Fistulous tract, dermoid cyst, pilonidal cyst

Outline: Rhodesian Ridgebacks are commonly affected with a developmental abnormality that results in tubular indentation (dermoid sinus) of the skin above the spine. These tunnels can run deep into underlying tissues as far as the spinal cord exposing the affected animals to risk of infections causing serious disease and pain.


Summary of Information

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1. Brief description

Dermoid sinuses are congenital abnormalities (present from birth) that consist of hollow tubular indentations of the skin that penetrate down into the tissue below. They create problems because they are prone to infections. They occur along the dorsal midline (centre of the back of the dog. The depth these penetrate into the tissue below varies between individuals. The type that pose the greatest (life-threatening) risk are those, called category IV sinuses, which connect with the spinal cord and risk infections of this delicate nervous tissue.

2. Intensity of welfare impact

The greatest welfare impact occurs with a category IV sinus becoming infected, leading to infection of the central nervous system. This can lead to severe illness and pain, and possibly death.

Other sinus infections lead to mild-moderate discomfort and pain. Currently, it is the policy of some breeders to euthanase affected puppies shortly after birth.

3. Duration of welfare impact

The presence of sinuses is life-long unless they are surgically removed. All have potential to become infected. Infections can last for days to weeks. However, if the spinal cord is involved and becomes infected animals may have permanent after effects, die or need to be euthanased.

4. Number of animals affected

8-10% of the Swedish population of Rhodesian Ridgebacks are affected and the prevalence is increasing (Salmon Hillbertz 2005). The prevalence in other populations is not known, though all Rhodesian Ridgebacks with a ridge are predisposed. The prevalence of different categories of sinus is not known.

5. Diagnosis

Preliminary diagnosis is made by visual examination and palpation (touch) and the category of sinus is confirmed using contrast radiography (x-rays).

6. Genetics

The ridge of Rhodesian Ridgebacks is caused by a mutation resulting in duplication of a length of DNA coding for three growth factor genes. This mutation, which has an autosomal dominant pattern of inheritance also causes dermoid sinuses.

7. How do you know if an animal is a carrier or likely to become affected?

Most affected individuals can be detected at birth, however, it is currently impossible to known if a healthy, ridged individual will produced affected puppies or not.

8. Methods and prospects for elimination of the problem

This condition could possibly be eliminated by only breeding from ridge-less individuals. (At present, the breed standards, which are based on appearance not welfare considerations, specify that there should be a ridge).  It is currently not known if the ridge-less population provides a large enough genetic pool for this to be carried out safely, within the breed, without narrowing the gene pool and exacerbating other genetically influenced conditions. It may therefore be necessary to outcross with other breeds in order to tackle this problem.


For further details about this condition, please click on the following:
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1. Clinical and pathological effects

A dermoid sinus is a congenital abnormality i.e. an abnormality that is present from birth. It consists of a tube-like tract progressing vertically down from the midline of the dorsum (centre of the back) into the underlying tissues. This tract is lined with skin which contains hairs, hair follicles, sebaceous glands etc. (Salmon Hillbertz 2005). Thus, over time, the sinus becomes filled with skin debris, secretions and hair, making it prone to infections caused by both overgrowth of bacteria and yeasts normally found in small numbers on the skin surface and by bacteria not normally present (Scott et al 1995).

Dermoid Fig 1

Figure 1.  A sinus extending from the skin surface down to the spinal column, which can lead to infections as a result of the culmination of hair, bacteria and skin secretions. This diagram roughly corresponds to a Type 1 sinus (see below). (Image property of Siv Hansen, to whom we are grateful for permission for its inclusion).

The abnormality occurs during the early development of the foetus. In these early stages the tissue that will form the spinal cord and brain, called the neural tube, forms from an invagination (inward folding) of the outer tissue layer called the ectoderm. This invagination fold then forms a complete tube, which breaks free of the surface tissue and sinks into the other tissues to form the neural tube and later the nervous tissue. Occasionally this process, of forming a tube and breaking off from the rest of the ectoderm (which goes on to form the skin), is incomplete and sinus tracts – points of connection - are left between the skin and the spinal cord. They are called dermoid sinuses.

Dermoid sinuses can penetrate the tissues below the skin to a variable extent and are classified according to the depth they penetrate (Gammie 1986) and as to whether they still have an opening with the surface skin (Booth 1998). Currently there are five categories in the classification by Salmon Hillbertz (2005:

  • I – Extending as deep as the supraspinous ligament (this is a ligament that runs along the top edge of the vertebrae – bones of the spinal column).
  • II - Not extending as far as the supraspinous ligament but connected to it by a fibrous strand
  • III - Not extending as far as, or connected to, the supraspinous ligament
  • IV - Attached to the dura mater (the outer membranes of the spinal cord)
  • V - No connection to the skin surface. No definite location in a specific tissue.

For types I, II, III and IV there are one or more openings on the skin although they may only be identified by close examination. They often have characteristic tufts of hairs protruding (Hathcock et al 1979). Type V has no opening onto the surface skin.

 Dermoid figure 2a

Figure 2a. Shows how dermoid sinuses are classified, from I-V, depending on the extent to which they penetrate beneath the skin and whether they open out on to the skin surface.

 Dermoid figure 2b

Figure 2b. Illustrates a cross section through the dorsal line (or back) of a dog. The type IV sinuses, those that penetrate as far as the spinal cord (G in the diagram), pose the greatest health and welfare risk.  (Image illustrated by Mr. Jeffrey A Leath, from the article “Dermoid Sinuses: Description, Diagnosis and Treatment” by Dr. Lisa Miller and Dr. Karen Tobias in Compendium 25 (4): 295-299 (2003). We are grateful to Dr. Karen Tobias and ‘VetLearn' (www.vetlearn.com) for the permission to reproduce the image here.)

Clinical signs are similar for categories I, II and III. There may be no signs, other than the tufts of hair and small skin openings, if the sinus is not infected. If the sinus becomes infected there may be swelling, discomfort and pain. If the area is touched it may be very painful.  An abscess may form which could burst releasing a purulent (pus) discharge.

 Dermoid figure 3  Dermoid figure 4
                         Figure 3                                               Figure 4

Figures 3 and 4. A small skin opening along the dorsal line indicates the presence of a dermoid sinus, though the absence of swelling indicates it is not infected. (Images property of Siv Hansen, to whom we are grateful for the permission to reproduce them here).

Dermoid figure 5

Figure 5. If the affected area was covered in fur, a characteristic tuft of hair may indicate the presence of a sinus, with a small skin opening apparent on closer inspection. (Image property of Elizabeth Akers at www.ridgeback.org, to whom we are grateful for the permission to reproduce it here). 

Type IV sinuses may cause no symptoms when the sinus is not infected, but there is a high risk of these tracts allowing bacteria and infection down into the spinal cord which can produce profound problems including myelitis, meningomyelitis or encephalitis (inflammation of the nervous tissue – spinal cord and/ or brain, or tissues surrounding them) (Angarano and Swaim 1993), possibly leading to death or necessitating euthanasia. Signs seen with infected type IV dermoid sinuses include the dogs being very unwell with a raised temperature and being off their food (Shell 2005). Affected animals can have dysmetria (abnormal gait when moving), ataxia (unsteadiness when moving), posterior paresis (loss of full use of the hindlimbs) and increased, excessive sensitivity to being touched near the affected area (hyperaesthesia) (Angarano and Swaim 1993). Spinal pain can occur. Symptoms can progress to complete paralysis of the hind legs or, sometimes, all limbs (Shell 2005). Treatment involves surgery to close the sinusesand treatment of the infection if present. The outlook for dogs with spinal cord/ brain infections is guarded.

Rhodesian Ridgebacks are named after the ridge of hair found on the lower back, which stands up because the hairs are growing in an opposite direction to normal, with two whorls at the end nearest the head.  Salmon Hillbertz and Andersson (2006) established that the presence of the dorsal ridge in Rhodesian Ridgebacks is inherited in an autosomal dominant mode and that the gene for this predisposes dogs with the ridge to have dermoid sinuses. Not all Rhodesian ridgebacks are born with the ridge on their back, however it is written into breed standards around the world, including the UK’s updated standard, that the breed must have this ridge (UK Kennel Club - http://www.thekennelclub.org.uk/item/36).

In Rhodesian Ridgebacks the dermoid sinuses occur in the cervical (neck region), anterior thoracic (front half of the chest) or the sacrococcygeal regions (area over the pelvis and tail base) i.e. either in front of or behind the dorsal ridge which characterises the breed (Mann and Stratton 1966).

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2. Intensity of welfare impact

Dermoid sinuses that do not connect to the spinal cord have the risk, if infected, of causing localised pain, discomfort and possibly illness. Those that run deeper can lead to infections of the spinal cord resulting in severe illness and pain, with the possibility of disability, death or euthanasia. Salmon Hillbertz (2005) also reports culling of affected (but otherwise healthy) individuals as puppies by some breeders who discover the condition, along with culling of healthy ridge-less individuals who do not meet the breed standard.

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3. Duration of welfare impact

If not surgically removed, sinuses and the risk of infection will remain life-long. Infections and abscesses can last for days to weeks and may recur. For individuals, who have a category IV sinus, infections of the spinal cord can lead to life-long disability, death or euthanasia within days of the infection. Infections can occur at any age.Return to top

4. Number of animals affected

Salmon Hillbertz (2005) estimated 8-10% of the Swedish population of Rhodesian Ridgebacks to be affected, and that prevalence was increasing. The prevalence in other populations has not been determined, though many authors have reported Rhodesian Ridgebacks and their crosses having a predisposition towards developing the condition (Hofmeyr 1963, Hathcock and others 1979, Gammie 1986, Lambrechts 1996). The proportion of affected individuals that have category IV sinuses with risks of spinal infections is not known.

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5. Diagnosis

A preliminary diagnosis can usually be made on examination of the individual (though category V sinuses, below the skin, need to be palpated (felt) for carefully). The depth of the sinus is usually determined by contrast radiography (taking x-rays after injecting a fluid into the sinus which shows up on x-ray) (Scott et al 1995).

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6. Genetics

The genetics of dermoid sinus in Rhodesian Ridgebacks has been confused in the past and much literature pre-2005, contains outdated suggestions about its mode of inheritance. In the past it was suggested that the condition was inherited as a simple autosomal recessive gene (Scott et al 1995). However, further studies suggested that multiple genes were involved (Salmon Hillbertz, 2005). Salmon Hillbertz and Andersson (2006) established that the presence of the dorsal ridge in Rhodesian Ridgebacks has an autosomal dominant inheritance pattern and that the gene for this predisposes dogs with the ridge to have dermoid sinuses. Ridge-less Rhodesian Ridgebacks are free from the condition. Salmon Hillbertz and Andersson (2006) reported that no ridge-less individuals born between 1981 and 2002 in Sweden were affected by dermoid sinus.

It has now been shown that the mutation that causes the disease is a duplication of part of the chromosome involving three fibroblast growth factor (FGF) genes. These FGF genes play a crucial role in development, and it is thought that both the ridge and dermoid sinus development may be caused by the resulting abnormal function of one or more of these genes during development (Salmon Hillbertz et al 2007).

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7. How do you know if an animal is a carrier or likely to become affected?

A ridge-less individual will be free from the condition, however, it will not be able to be registered as a Rhodesian Ridgeback, to be shown or bred from as part of this breed in the UK as breed standards are currently based on appearance rather than welfare considerations.

It is known that a healthy ridged individual (ie one without sinuses) has the potential to produce affected puppies, and that the ridge predisposes both for the condition and to being a carrier of the tendency to have dermoid sinuses. Currently there is no genetic test to identify which ridged individuals may produce affected offspring. Affected individuals should not be bred from.

Puppies free from the defect should be chosen. It should be remembered, however, that the detection of category V sinuses, that do not open onto the surface of the body can be particularly difficult to detect. It is thus recommended that a vet examines the puppy for these prior to purchase.

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8. Methods and prospects for elimination of the problem

The obvious way to eliminate this condition would be to breed from ridge-less individuals only. However, due to the strong pressure that there has been from breeders to eradicate them, they now only constituent a small part of the population. They are estimated to make up 5.6% of the Swedish Rhodesian Ridgeback population (Salmon Hillbertz and Andersson 2006).

This ridge-less population may now be too small to form the basis of a breeding programme, within the existing breed, as excessive inbreeding could occur which could lead to the exacerbation of other hereditary conditions. Using out-crossing with other breeds could be beneficial. It would also mean alteration of the breed standard as, at present, the ridge is a fundamental characteristic of the breed.

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9. Acknowledgements

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 finding suitable illustrations for it.

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10. References

Angarano D and Swaim S (1993) Congenital Skin Diseases in Bojrab, M. Disease mechanism in Small Animal Surgery. 2nd Ed. London: Lippincott, Williams and Wilkins

Booth M (1998) Atypical dermoid sinus in a chow chow dog. Journal of the South African Veterinary Association 69: 102-104

Gammie J (1986) Dermoid sinus removal in a Rhodesian ridgeback dog. Journal of the South African Veterinary Association 27: 250-251

Hathcock T, Clampett G and Broadstone V (1979) Dermoid sinus in a Rhodesian ridgeback. Veterinary Medicine, Small Animal Clinician 74: 53-56

Hofmeyr C (1963) Dermoid sinus in the ridgeback dog. Journal of Small Animal Practice 4: 5-8

Lambrechts N (1996) Dermoid sinus in a crossbred Rhodesian ridgeback dog involving the second cervical vertebra. Journal of the South African Veterinary Association 67: 155-157

Mann G and Stratton J (1966) Dermoid sinus in the Rhodesian ridgeback. Journal of Small Animal Practice 7: 631-642

Salmon Hillbertz N (2005) Inheritance of dermoid sinus in the Rhodesian ridgeback. Journal of Small Animal Practice 46: 71–74

Salmon Hillbertz N and Andersson G (2006) Autosomal dominant mutation causing the dorsal ridge predisposes for dermoid sinus in Rhodesian ridgeback dogs. Journal of Small Animal Practice 47: 184–188

Salmon Hillbertz NHC, Isaksson M, Karlsson EK, Hellmén E, Pielberg GR, Savolainen P, Wade CM, von Euler H, Gustafson U, Hedhammar A, Nilsson M,  Lindblad-Toh K, Andersson L and Andersson G (2007) Duplication of FGF3, FGF4, FGF19 and ORAOV1 causes hair ridge and predisposition to dermoid sinus in Ridgeback dogs. Nature Genetics 39: 1318 – 1320. Published online: 30 September 2007 | doi:10.1038/ng.2007.4

Scott D, Miller W and Griffin C (1995) Muller and Kirk’s Small Animal Dermatology. 5th Ed. Philadelphia: W.B.Saunders Company

Shell L (2005) Dermoid sinus. (On-line). Available at http://www.vin.com/Members/Associate/Associate.plx?DiseaseId=347. Accessed on 23.7.10

© UFAW 2011


Credit for main photo above:

By Gulik (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons