Sebaceous cysts in dogs. Are they really sebaceous?
Introduction
Dogs often develop “lumps” under their skin, and they may or may not have a neoplastic origin. Non-neoplastic skin lesions include nodules (circumscribed, elevated solid lesions measuring over 1 cm in diameter) and cysts (clearly circumscribed cavities delimited by an epithelium that may contain fluid or solid material) of various origins.1 So, it is important for vets to be familiar with the different types of skin cysts found in dogs.
Types of skin cysts
Skin cysts are non-neoplastic structures classified according to the appearance of the epithelium lining the cyst and the structure from which it develops. Dogs can develop follicular, dermoid, epithelial, sebaceous and sweat gland cysts.2
Most skin cysts in dogs have a follicular origin and can be subdivided according to the type of follicle from which they develop. Thus, they can be classed as infundibular, isthmic, trichilemmal or matricial cysts and hybrids (combinations thereof), with the first two types being the most common in dogs.2
Follicular cysts usually appear as firm or fluctuant, round, smooth, well-circumscribed solitary structures located in the dermis or subcutaneous tissue. They usually appear on the head, neck, trunk and forelimbs and vary in diameter from 0.5 to 5 cm. However, the presence of multiple cysts, probably congenital, has also been reported along the dorsal midline and on pressure points (especially elbows) in young dogs due to chronic trauma, dermal fibrosis and obstruction of the follicular ostium.2
There is no evidence to suggest that the prevalence of these cysts is associated with sex or age. A yellowish/ochre or greyish fluid with a caseous or pasty consistency usually drains out of ruptured cysts.
- Infundibular follicular cysts. These are what owners, and very often general veterinary surgeons as well, typically call sebaceous cysts. They can be identified as a cystic cavity connected to a rudimentary hair follicle that is filled with keratin in a lamellar arrangement. 2
- Isthmic cysts are filled with amorphous keratin and the cyst wall shows evidence of trichilemmal differentiation.
- Matrix cysts have a wall composed of basophilic basaloid cells which rapidly keratinises to form an amorphous mass of eosinophilic keratin packed with ghost cells.2
- Dermoid cysts correspond to a rare developmental abnormality in dogs. They are usually congenital and hereditary and occur mainly in Boxers, Kerry Blue Terriers and Rhodesian Ridgebacks. They are found as single or multiple lesions along the dorsal midline. Histopathological characteristics include a cystic wall with small, well-developed hair follicles that undergoes epidermal differentiation, sebaceous glands and occasionally epitrichial sweat glands.2
- Apocrine sweat gland cysts are relatively common in the dog. There is no predilection between breeds or male and female dogs, but they are more frequent in dogs over 6 years. They occur when the gland becomes obstructed and appear as solitary,well-circumscribed lesions measuring 0.5–3 cm in diameter, generally on the head, neck and limbs. Sweat gland cysts usually contain an aqueous, transparent, acellular fluid.
Contrary to what one might think, genuine sebaceous gland cysts are extremely rare in dogs. They develop as firm, solitary dermal nodules of <1 cm in diameter and there is no predilection for sex, breed or age.2
Sebaceous cysts in dogs: treatment
The treatment options for sebaceous cysts in dogs include surgical excision or simply observing and monitoring their evolution.
- You should not try to drain cysts by squeezing them, because when the cyst wall ruptures it can release its contents into the dermis and trigger a foreign body reaction and secondary infection.
- Monitoring their evolution may be the only option in patients with multiple disseminated cysts for whom extensive surgery is probably not warranted given the benign nature of the lesions.
Conclusions
Based on the information available, we should avoid calling all “lumps” found on the skin of dogs sebaceous cysts. Without histopathological confirmation, it is probably more accurate to use the term skin cyst. Either way, it does not mean that a biopsy is required for all cysts detected on a dog’s skin. The decision to proceed with a biopsy or monitoring should be based on the specific characteristics of each case and agreed with the owners.
