Clinically, lupoid onychitis is an idiopathic condition which only affects the claws of dogs. They become very brittle, dry and separate from the underlying nailbed. They can regrow as misshapen claws. Multiple digits on multiple feet are affected but clinical signs are not noted in any other locations.
Histologically, this is seen as an interface type infiltrate of lymphocytes and plasma cells along the basement membrane of the claw bed and there is often pigmentary incontinence into the underlying connective tissue. In typical cases, apoptosis and vacuolation of basal cells in the epidermis is seen. There is separation of the epidermis from the dermis and the claw loosens and breaks free from the nailbed. The cause of the lesion is unclear but is thought to have an immune-mediated component. Although the condition is known as a lupoid onychitis, antinuclear antibody testing is negative.
Differentials can include trauma, bacterial or fungal infections, neoplasia or other immune-mediated diseases such as vasculitis and drug reactions. However, very few of these conditions typically affect only the digits, but they can affect multiple digits. If infection is present, it is typically secondary but culture and sensitivity will help to direct antibiotic therapy; however, it is important to make a diagnosis of the primary condition as repeated use of antibiotics can select for antibiotic resistance in these cases.
We often receive calls from clients concerning the correct biopsy procedure to make a definitive diagnosis. Biopsy in these cases is not easily obtained. A punch biopsy technique has been described in the literature in which 8mm punch biopsies are taken from affected claw beds; however, amputation of the distal portion of an affected digit, preferably an affected dew claw, is still the preferred sample. The digit can be submitted in normal fixative (10 percent neutral buffered formalin) although you may wish to submit a swab or sloughed claw at the same time for culture and sensitivity to help direct antibiotic therapy for any secondary infection.
Histological examination involves decalcifying the distal part of the digit and taking a longitudinal section through the sample so that we can see the nailbed. A negative histology result does not necessarily rule out the condition in a dog which has compatible clinical signs.
There are a number of therapies described in the literature and it can take several months to find out if they are successful because an entire new nail must regrow while on treatment. The prognosis for complete cure is guarded and long-term/lifelong treatment may be required.