Growths and lumps on dogs are common, especially as dogs age. A growth on your dog that looks just like a toenail is rare. The growth is as hard as a toenail and clips like a toenail but contains no blood or nerves. The only fitting explanation found so far is a cutaneous horn. Possible diseases that present as cutaneous horns are extensive but typically benign.
A cutaneous horn is a general term used to describe a cylindrical mass of a-cellular keratin in which the height is greater than the diameter. Cutaneous horns are rare in dogs and cats and causes are speculative. Aetiology comes from an evaluation of the skin at the base or beneath it. Some cutaneous horns may derive from viral papillomas, actinic kerotoses, bowenoid in situ carcinoma, dilated pores or invasive squamous cell carcinomas.
Keratoma, corn or callus growth is a column of compacted keratin in the shape of a nail. The main cause of this condition is chronic pressure or friction. Paw pad keratomas typically occur in racing or retired greyhounds. Inorganic foreign items, such as sand or glass, are a possible cause as well as a papillomavirus infection.
Rarely observed in dogs, a dermoid cyst is a reduplication of the entire skin structure, including dermis and adnexa. Usually solitary and less than 1/2 inch in length, it often connects to a pore that connects the cyst of the skin surface. This type of cyst typically occurs along the dorsal midline and occasionally extends into the spinal canal where it attaches to dura mater. On rare occasion, a dermoid cyst can appear on the neck or shoulder.
Squamous papilloma growths typically appear on the face, eyelids and feet. It is a papillary, finger-like projection but is very small, less than 1/4 inch. A viral form of the disease, called viral papilloma, is a double-stranded DNA virus that infects many animal species. Records contain more than 130 strains of this growth since 2005.
Keratosis is a general term referring to a horny or warty growth or callus. This is thought to result from UV rays damaging DNA molecules. In dogs, it is common to find actinic keratosis near a squamous cell carcinoma. Lesions from actinic keratosis present as cutaneous horns on the abdomen, flanks and thighs of short-coated, white-haired breeds. Dalmatians, pit bull terriers, beagles and basset hounds have an increased incidence of actinic keratosis as well.