Dermoids in dogs are congenital growths made of misplaced skin-like tissue that forms before birth during embryonic development. They come in two distinct varieties that share a name but affect very different parts of the body: dermoid sinuses, which are tube-like tracts running from the skin surface toward the spine, and ocular dermoids, which are small masses of skin tissue growing on or near the eye. Both types are present at birth, both tend to cause increasing trouble as the dog matures, and both usually require surgery. What makes dermoids particularly interesting in veterinary medicine is that in at least one breed, the genetic mutation responsible has been pinpointed with unusual precision.
What a Dermoid Sinus Actually Is
A dermoid sinus is a tube or tract that forms along the dorsal midline of the body, typically running from a small opening on the skin surface down into deeper tissue. These tracts are lined with normal-looking skin epithelium and tend to fill with hair shafts, keratin debris, and sebaceous material over time. Think of it as a narrow tunnel of skin that burrowed inward instead of staying on the surface during fetal development. The defect arises from incomplete separation of the skin from the neural tube, the embryonic structure that becomes the spinal cord and brain.
Dermoid sinuses are classified into types based on how deep the tract extends. Types I through IV describe tracts of increasing depth, from those ending in the tissue just beneath the skin to those that attach to the ligament overlying the spinal column. Type V is the most serious, as these tracts penetrate through to the spinal cord or its coverings. A case report in a Shiba Inu documented a type V variant where the tract had a confirmed fibrous connection to the dura, the protective membrane surrounding the spinal cord, along with spina bifida at the same site.1PubMed. Complete surgical excision of a type V-variant dermoid sinus (V/VI overlap) associated with spina bifida in a Shiba Inu dog: A case report A type VI dermoid sinus, which connects directly to the spinal cord itself, was described in a French Bulldog that presented at four months old with acute pain and reluctance to move, likely from the tract tethering the spinal cord.2PubMed Central. Dermoid sinus type VI associated with spina bifida and tethered cord syndrome in a French Bulldog
From the outside, a dermoid sinus can be subtle. Owners sometimes notice a small dimple or pore on the dog’s back, occasionally with a tuft of hair protruding from it. In more advanced cases, the opening can become infected and discharge pus. A Saint Bernard presented at eleven months old with draining masses on the top of the head that turned out to be multiple dermoid sinuses filled with hair shafts and keratin, lined by normal squamous epithelium.3PubMed Central. Multiple dermoid sinuses of type Vb and IIIb on the head of a Saint Bernard dog That case is a reminder that dermoid sinuses don’t always present as a single neat tract on the back.
The Ridgeback Connection and the FGF Gene Duplication
Rhodesian Ridgebacks are the breed most closely associated with dermoid sinuses, and the reason is unusually well understood. The characteristic hair ridge running along the Ridgeback’s spine, the very feature that defines the breed, is caused by a 133-kilobase duplication of DNA that includes three fibroblast growth factor genes (FGF3, FGF4, and FGF19). This same duplication that produces the ridge also predisposes the dog to dermoid sinus formation.4PubMed. Duplication of FGF3, FGF4, FGF19 and ORAOV1 causes hair ridge and predisposition to dermoid sinus in Ridgeback dogs The Thai Ridgeback, which carries the same mutation, faces the same risk.
This puts breeders in an uncomfortable position. The ridge is a defining breed characteristic and a requirement in the show ring, yet the mutation that produces it is the same one that causes a potentially serious birth defect. Not every ridged dog develops a dermoid sinus, so the duplication creates a predisposition rather than a guarantee. But ridgeless Ridgeback puppies, which lack the duplication entirely, don’t develop dermoid sinuses at all. The genetics here illustrate a broader tension in purebred dog breeding: selecting for an appearance trait can drag along a disease risk that rides on the same stretch of DNA.
Breeds Beyond the Ridgeback
While Rhodesian Ridgebacks dominate the literature on dermoid sinuses, the condition has been documented across a surprising range of breeds. The Saint Bernard case with multiple head dermoids mentioned earlier is one example.5PubMed Central. Multiple dermoid sinuses of type Vb and IIIb on the head of a Saint Bernard dog A Dachshund with congenital limb and vertebral malformations was found to have a dermoid sinus in the upper thoracic region that had contributed to cervical pain, muscle rigidity, and difficulty walking.6PubMed Central. Spinal dermoid sinus in a Dachshund with vertebral and thoracic limb malformations Cases have also been reported in French Bulldogs, Shiba Inus, and Cavalier King Charles Spaniels, among others.
In non-Ridgeback breeds, the genetic mechanism isn’t as clearly defined. The FGF duplication is specific to the ridge trait in Ridgebacks and Thai Ridgebacks. When dermoid sinuses appear in other breeds, the underlying cause likely involves different developmental errors during neural tube closure, but the specific genes involved haven’t been identified with the same certainty. The inheritance pattern in these breeds remains debated, with suggestions ranging from dominant inheritance with incomplete penetrance to recessive inheritance.
When a Dermoid Sinus Gets Dangerous
A shallow dermoid sinus that ends in the subcutaneous tissue may cause nothing more than occasional irritation or infection at the skin opening. The real danger comes when the tract extends deep enough to communicate with the vertebral canal or spinal cord. Infection tracking down a deep tract can lead to meningitis or myelitis, both of which can be devastating. Even without infection, a deep connection can cause neurological problems through direct mechanical effects on the spinal cord.
The Dachshund case illustrates how dermoid sinuses can interact with other congenital abnormalities. That dog had vertebral malformations and limb deformities alongside its dermoid sinus, and imaging revealed epidural gas within the vertebral canal, suggesting the sinus had allowed air to enter the space around the spinal cord.7PubMed Central. Spinal dermoid sinus in a Dachshund with vertebral and thoracic limb malformations The French Bulldog with a type VI sinus developed tethered cord syndrome, where the spinal cord was essentially anchored by the tract and couldn’t move freely within the spinal canal, causing pain and reluctance to move.8PubMed Central. Dermoid sinus type VI associated with spina bifida and tethered cord syndrome in a French Bulldog That dog recovered fully after surgical removal, with no clinical signs at three months post-surgery, which underlines the importance of catching the condition early.
Diagnosing a Dermoid Sinus
Many dermoid sinuses are found during routine physical examination when a veterinarian or breeder palpates a small cord-like structure under the skin along the dog’s back. In Ridgeback litters, experienced breeders often check puppies within the first few weeks of life by pinching the skin along the midline and feeling for a fibrous thread connecting the skin to deeper tissue. A skin opening with a tuft of hair or a dimple on the dorsal midline is another telltale sign.
When a dermoid sinus is suspected, imaging becomes important for determining its depth and planning surgery. MRI has proven useful for visualizing fluid-filled structures, the fibrous capsule surrounding the tract, and the sinus pathway itself. In two Rhodesian Ridgebacks with type I dermoid sinuses, MRI identified these features clearly, though it wasn’t able to pinpoint exactly where the tracts terminated.9PubMed Central. Magnetic resonance imaging in the diagnosis of type 1 dermoid sinus in two Rhodesian ridgeback dogs CT fistulography, where contrast dye is injected into the tract opening and then scanned, can also help map the sinus path. The Dachshund case relied on a combination of radiography, CT, and CT fistulography to characterize both the dermoid sinus and the associated vertebral malformations.10PubMed Central. Spinal dermoid sinus in a Dachshund with vertebral and thoracic limb malformations
Surgical excision is the standard treatment. The entire tract needs to be removed down to its deepest attachment point. If the sinus attaches to the dura or spinal cord, the surgery becomes considerably more complex and carries higher risk. For shallower sinuses, the procedure is relatively straightforward and typically curative. Incomplete removal leaves tissue that can re-form the tract or become a focus of chronic infection.
Ocular Dermoids Are a Separate Problem
Ocular dermoids are a completely different condition from dermoid sinuses, despite sharing part of the name. These are small masses of skin-like tissue, often bearing hair follicles, that grow on the surface of the eye or eyelid. They develop before birth when cells destined to become skin end up in the wrong location during embryogenesis. Unlike dermoid sinuses, which are associated with neural tube closure, ocular dermoids involve the eye’s surface tissues.
A retrospective study of 49 ocular dermoids in 44 dogs found that the most common location was the limbus, where the cornea meets the white of the eye, accounting for about 45% of cases. Eyelid dermoids made up roughly 29%, corneal dermoids about 16%, and conjunctival dermoids around 10%.11PubMed. Ocular dermoids in dogs: A retrospective study A Brazilian study covering both dogs and cats found a similar pattern, with the limbal region being most commonly affected, followed by the cornea and eyelid.12Pesquisa Veterinária Brasileira. Epidemiological findings of ocular dermoid in dogs and cats: 50 cases (2007-2024)
The breed profile for ocular dermoids looks quite different from the breeds prone to dermoid sinuses. Mixed-breed dogs, French Bulldogs, Shih Tzus, and Labradors appear frequently in case series. A one-year-old Shih Tzu presented with what its owner described as abnormal hair growth on the eye, which turned out to be a hairy, pigmented dermoid about 5 mm across on the cornea and bulbar conjunctiva.13Indian Journal of Veterinary Pathology. Corneal dermoid in a dog Ocular dermoids primarily affect young dogs and most often appear in just one eye.14Pesquisa Veterinária Brasileira. Epidemiological findings of ocular dermoid in dogs and cats: 50 cases (2007-2024)
Surgery and Recovery for Ocular Dermoids
Surgical excision is the treatment of choice for ocular dermoids, just as it is for dermoid sinuses. The good news is that recurrence after removal appears to be rare. In the retrospective study of 49 dermoids, 34 were surgically excised and none recurred.15PubMed. Ocular dermoids in dogs: A retrospective study The specific surgical technique depends on the dermoid’s location and depth. Corneal dermoids generally require a superficial keratectomy, where the surgeon shaves away the affected layers of the cornea.
The cornea does go through a healing phase after surgery. Temporary opacity was seen in all corneal dermoid cases in one surgical case series, with transparency beginning to return between weeks four and five.16Kafkas Universitesi Veteriner Fakultesi Dergisi. Surgical Correction of Ocular Dermoids in Dogs: 22 Cases Some surgeons use tissue grafts to speed healing and reduce scarring. When amniotic membrane transplantation was used after corneal dermoid excision, the corneal surface healed within two weeks and five of the treated eyes regained normal transparency within five weeks. At eight weeks, none of the dogs showed scarring, abnormal blood vessel growth, or impaired vision.17PubMed. Canine amniotic membrane transplantation for corneal reconstruction after the excision of dermoids in dogs
Many owners worry about whether their dog will have normal vision after surgery. For most corneal and limbal dermoids, the prognosis is encouraging as long as the surgery is performed before the dermoid damages deeper layers of the cornea. Delaying surgery gives the dermoid more time to irritate the eye, which can lead to secondary problems like chronic inflammation or pigmentary keratitis that may persist even after the mass is removed.
Dermoids in Unusual Places
While the dorsal midline and the eye are the classic locations, dermoids have turned up in some genuinely unexpected spots. A German Shepherd presented with recurring swelling under the jaw and in the tongue itself. Fluid from the mass showed signs of an infected cyst lined by epithelium, and when the tract was surgically explored, a dermoid cyst was found embedded in the frenulum and body of the tongue.18Australian Veterinary Journal. Dermoid cyst in the tongue of a dog These atypical locations make dermoids worth considering on the differential list even when a mass doesn’t appear in the expected spot. Histopathology is the definitive diagnostic tool, since the hallmark finding of a cyst lined by stratified squamous epithelium containing hair and keratin is distinctive regardless of location.
Breeding Decisions and Folic Acid
For Ridgeback breeders, the dermoid sinus question is a practical reality with every litter. Because the ridge and the predisposition to dermoid sinuses share the same mutation, eliminating the risk entirely would mean breeding ridgeless dogs, which most breed communities are unwilling to do. The pragmatic approach has been to screen puppies early, remove affected dogs from breeding programs, and avoid mating two dogs that have both produced affected offspring.
An intriguing angle involves folic acid supplementation. There is evidence suggesting that Rhodesian Ridgebacks may have an inherent difficulty in folic acid absorption or utilization, and that supplementing breeding females before and during pregnancy could reduce the occurrence of dermoid sinuses in their puppies. This parallels what’s known in human medicine, where folic acid supplementation before conception reduces the risk of neural tube defects like spina bifida. The logic makes sense given that dermoid sinuses are themselves a neural tube closure defect. While the data in dogs is not as robust as the massive human studies that established folic acid’s role in preventing birth defects, many Ridgeback breeders have adopted supplementation as a precautionary measure.
Current recommendations are that affected dogs and their close relatives should not be bred. In practice, this means that a puppy diagnosed with a dermoid sinus should be neutered, and its parents should be evaluated carefully before being bred again. Some breeders genetically test for the ridge duplication and use this information alongside physical screening of puppies to make more informed pairing decisions, though testing for the presence of the ridge duplication alone doesn’t predict which individual dogs will actually develop a sinus.
When Dermoid Sinuses Coexist with Spinal Defects
Several reported cases highlight how dermoid sinuses can be part of a broader pattern of developmental abnormalities rather than an isolated defect. The Shiba Inu with a type V/VI overlap dermoid also had spina bifida at the same vertebral level, suggesting that the same disruption in neural tube closure caused both the open vertebra and the sinus tract.19PubMed. Complete surgical excision of a type V-variant dermoid sinus (V/VI overlap) associated with spina bifida in a Shiba Inu dog: A case report The French Bulldog with tethered cord syndrome had a similar combination, with the dermoid sinus extending from the skin surface all the way to the spinal cord.20PubMed Central. Dermoid sinus type VI associated with spina bifida and tethered cord syndrome in a French Bulldog And the Dachshund had limb deformities and multiple vertebral anomalies alongside its dermoid sinus.21PubMed Central. Spinal dermoid sinus in a Dachshund with vertebral and thoracic limb malformations
These cases matter because they change the diagnostic workup. When a dermoid sinus is found, especially a deep one, the veterinarian should look for accompanying skeletal or spinal abnormalities that might affect the surgical plan and the overall prognosis. A straightforward sinus excision becomes a much more complicated proposition when there’s an underlying vertebral defect or when the spinal cord is tethered. Advanced imaging before surgery, ideally MRI or CT, helps catch these associated problems before the surgeon opens the site and discovers something unexpected.
For brachycephalic breeds like French Bulldogs, the association with spinal defects like spina bifida and hemivertebrae adds yet another concern to an already long list of congenital problems common in these breeds. The French Bulldog case resolved well with surgery, but not all dogs with deep dermoid sinuses and concurrent spinal anomalies are good surgical candidates, and the decision requires careful imaging and honest conversations between the surgeon and the owner about risks and expected outcomes.