Epitheliogenesis imperfecta (aplasia cutis) is a congenital discontinuity of squamous epithelium. It is seen in cattle (autosomal recessive trait), horses, swine, sheep, cats, and dogs, although it is rare in the latter three species. In cattle, affected breeds include Holstein-Friesian, Hereford, Ayrshire, Jersey, Shorthorn, Angus, Dutch Black Pied, Swedish Red Pied, and German Yellow Pied. It is common in swine, in which large lesions are obvious at birth as glistening red, well-demarcated discontinuities in the skin or mucous membranes. Infection and ulceration are early consequences. One or more hooves or claws may be deformed or absent; in some affected animals, there are other associated congenital anomalies. The condition is fatal when extensive, but small defects can be surgically corrected. Ultrastructural evaluation of this condition in American Saddlebred foals has demonstrated a relationship with junctional epidermolysis bullosa.
Focal cutaneous hypoplasia and subcutaneous hypoplasia are congenital, circumscribed hypoplastic defects of multiple or deeper skin layers in swine. The lesions manifest as skin depressions in which all skin layers or the subcutaneous fat layers fail to develop normally.
A nevus is a circumscribed developmental defect of the skin, whereas a hamartoma is a hyperplastic mass formed as a result of a developmental defect in any organ. Both nevi and hamartomas have been described as congenital skin defects, but the problem may not become obvious until later in life. In dogs, sebaceous nevi, pigmented epidermal nevi, inflammatory linear verrucous epidermal nevi, nevi comedonicus, linear organoid nevi, and follicular hamartomas are known to occur. In horses, cannon keratosis and linear epidermal nevi have been described. Doubtless, similar defects occur in all species. Mixed, or organoid, nevi consist of circumscribed collections of densely packed adnexal structures (pilosebaceous nevus and pilosebaceosudoriferous nevus). Collagenous nevi are nodules composed of focal collagen hyperplasia that displace the normal structures of the skin. Most lesions are alopecic, with pigmented, pitted surfaces. When not extensive, nevi can be excised; otherwise there is no known effective treatment.
Dermoid sinuses or cysts are seen in Thoroughbred horses and Rhodesian Ridgebacks (in which they are inherited) and occasionally other breeds of dogs. These are cystic structures lined with skin in which exfoliated skin, hair, and glandular debris accumulate. They are caused by failure of complete separation of the neural tube from the epidermis during embryogenesis; cysts are found on the dorsal midline and are rarely associated with spinal cord neural deficits. They can be removed by surgical excision. Follicular cysts are common clinical findings.
Follicular cysts develop by abnormal hair follicle morphogenesis and by retention of follicular or glandular products. They may be congenital when caused by the failure of the follicular orifice to develop normally. Congenital cysts are most commonly identified in Merino and Suffolk sheep. Periauricular (dentigerous) cysts are seen in horses and, although present at birth, may not be recognized until adulthood. Wattle cysts are seen in Nubian goats; these arise from the branchial cleft. Porcine wattles are seen fairly frequently in all breeds of swine. These are teat-like growths on the lower jaw.
