Dermoid Sinus - A Summary by
E. Clough, V.M.D.
1010 Daniel Webster Highway
Merrimack, New Hampshire 03054

***Update: 12/4/09:Nature Genetics 39, 1318 - 1320 (2007)

Published online: 30 September 2007 | doi:10.1038/ng.2007.4

Duplication of FGF3, FGF4, FGF19 and ORAOV1 causes hair ridge and predisposition to dermoid sinus in Ridgeback dogs

Nicolette H C Salmon Hillbertz1, Magnus Isaksson2, Elinor K Karlsson3,4, Eva Hellmén2,5, Gerli Rosengren Pielberg6, Peter Savolainen7, Claire M Wade3,8, Henrik von Euler9, Ulla Gustafson1, Åke Hedhammar9, Mats Nilsson2, Kerstin Lindblad-Toh3,6, Leif Andersson1,6 & Göran Andersson1

The dorsal hair ridge in Rhodesian and Thai Ridgeback dogs is caused by a dominant mutation that also predisposes to the congenital developmental disorder dermoid sinus. Here we show that the causative mutation is a 133-kb duplication involving three fibroblast growth factor (FGF) genes. FGFs play a crucial role in development, suggesting that the ridge and dermoid sinus are caused by dysregulation of one or more of the three FGF genes during development.

Dermoid sinus (D.S.) was first used to describe the Rhodesian Ridgeback skin anomaly by Steyn, et al. This skin condition has also been called trichiasis spiralis, dermoid cyst, dermoid inclusion cyst and epidermal inclusion cyst. All of these terms have some applications; however, Dermoid, skin-like sinus, channel or fistula; (cyst means sac, i.e., not open to the surface) is most applicable. The Rhodesian Ridgeback Club of the U.S., Inc. prefers to use D.S. to describe the condition.


No reports have been located which substantiate the author's impression that D.S. has been found in other breeds.(Ed. Notes: DS are also found it in the Thai Ridgeback.) Dermoid sinus included in the ridge has been reported only once. They also occur in the sacral (rump) area and in this location are sometimes connected to the dura (spinal cord covering). This is not the case in the more common cervical (neck area) D.S. which connects the skin to the dorsal spinous ligament (the ligament which connects the top parts of the vertebrae). One or more D.S. may occur in the same animal. These sinuses are congenital (present at birth) and can be palpated (felt) as cords running between the skin and the spine. They form a small external opening which can be readily seen once the hair has been shaved.

Histologically (microscopically) the sinus is a thick-walled tube composed chiefly of fibrous tissue and lined with stratified squamous epithelium (skin cells). The surrounding connective tissue may or may not contain hair follicles, sebaceous glands and sweat glands which open into the lumen (hollow center) of the sinus. In uncomplicated cases the sinus is filled with sebum (oil), skin debris and hair. Once infected with bacteria the resulting inflammation and abscessation can lead to myelitis and encephalitis (swelling and infection of the spinal cord, its covering, and the brain). The reason for discrimination against animals with D.S. is the almost inevitable abscess, which forms.  


Although not well understood, transmission of D.S. seems to be a dominant, polygenic problem with inconstant penetration. Another geneticist thinks that D.S. is due to a single completely recessive autosomal gene. This is not likely because normal parents do produce pups affected with D.S. Because of the genetic complexity and the difficulty in arranging, coordinating and collating the breeding studies necessary to prove the heredity of D.S., it is unlikely that we will be able to substantiate the exact mechanism of genetic transmission. There is a widely held belief that breeding Ridgebacks with D.S. produces an increased prevalence of pups with D.S. The R.R. Club of the U.S., Inc. believes that this could be substantiated if breeders had accurate records which could be collated and computed. It is our belief that careful controlled breeding studies would prove the inheritance of D.S. to be not only complex but also inter-related with other characteristics. Therefore, the likelihood seems to be remote that we will ever have Ridgebacks which are entirely free of D.S.

Because selective breeding will unquestionably reduce the prevalence of D.S., but probably not eliminate its occurrence, and because the condition results in difficult to treat abscesses unless surgically removed, it is our opinion and strong recommendation that dogs which have D.S. not be acceptable as show or breeding candidates. Surgical correction can be accomplished; however, culling at birth is a more humane way to handle the affected pups.


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