Disease Spotlight: Interdigital Cysts

lay term: Interdigital Cysts

This is one of the most frustrating conditions seen in the veterinary dermatology practice.

What causes it?

We see this condition as a structural/functional problem with dogs who have a limb deformity; for example, bulldogs with bow legs. In Texas, we most commonly see these lesions secondary to atopic dermatitis (environmental allergy). Other common diseases that can lead to interdigital folic/furunc are food allergy or a demodex mite overgrowth in the hair follicles. Less likely diseases that contribute are contact allergy (with a caustic chemical or a weed such asCommelinaceae) or low thyroid function. A condition known as sterile pyogranuloma has been reported insmooth, short coated breeds like Dachshunds, English Bulldogs, Great Danes and Boxers but as dermatologists, we won’t call it immune mediated until allergy is ruled out. Likewise, it has been referred to as idiopathic (meaning we don’t know what causes it) but as dermatologist we want to rule out allergy before labeling is as such.

What happens to the skin to make it look this way?

Friction and inflammation leads to thickened skin and edema. The hair follicles become plugged and keratin is not exfoliated; thus, a comedome forms (think white head or black head, but only more severe). The follicles continue to dilate and a cyst forms. Rupture may occur and hair and keratin particles are introduced into the deeper layers of the skin and act as a foreign bodies. This causes an escalation in the inflammatory reaction and scar tissue forms. It is very common for the lesions to have a deep, secondary bacterial infection.

How are they treated?

We investigate and manage the underlying allergy, which takes time and patience. We manage the secondary infections.  Infections are typically deep and may require prolonged antibiotic therapy. We like to use other medications to manage the inflammation and the severity of the lesions will determine which anti-inflammatory medications are appropriate.

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