Interdigital furunculosis in dogs
Short, bristly-coated dogs (Labradors, bulldogs, pitbulls, Great Danes, etc) are vulnerable to developing a condition called furunculosis. This condition is marked by the development of nodules in between the dog’s toes. These nodules can rupture and drain. The nodules, or “furuncles,” are the dog’s own hair follicles, broken off inside the skin, and surrounded by inflammatory cells.
Causes of Furunculosis
Furunculosis is most commonly caused by allergic inflammation. When the dog is exposed to something it is allergic to, the skin becomes inflamed. Inflammation of the feet causes them to swell, though this swelling can range from subtle to severe. Subsequently, walking on the swollen feet can cause short bristly hairs to break off inside the skin due to frictional forces. The immune system then treats the broken off hair follicle as foreign material, much like a wood splinter. The immune system walls the hair follicles off by surrounding them with white blood cells—forming the skin nodules that eventually rupture and drain. Bacteria can proliferate in the damaged tissue, leading to secondary infection.
Furunculosis can also result from endocrine (internal) disorders that lower the immune system, such as Cushing’s disease or hypothyroidism.
It can be exacerbated by factors that cause inappropriate or unusual weight bearing of the limbs, such as orthopedic issues/lameness and obesity. The latter simply causes the animal to bear more weight on the limbs, exacerbating the frictional forces on the feet.
Treatment Overview
To treat furunculosis, the underlying cause must be identified and treated. Bloodwork can evaluate for endocrine disorders. If allergic dermatitis is suspected, anti-inflammatory allergy medications are the mainstay of treatment. In addition, exacerbating factors such as lameness and obesity should be addressed.
Because secondary infection is common, microscopic evaluation of the furuncles (cytology) to detect infection should be performed. A culture may be indicated to target antibiotic selection. Topical therapy to reduce inflammation and fight infection is often helpful.
Treatment Options for allergy-driven folliculitis/furunculosis
Prednisone: Prednisone is an oral steroid. It is the best anti-inflammatory medication and most powerful treatment at inducing remission of furunculosis. Potential of side effects prednisone are increased thirst, urination, and drinking. Long-term use of prednisone is reserved for severe cases not responding sufficiently or not tolerating Atopica (see below) because prednisone can be hard on the liver, kidneys, and gastrointestinal tract and increase insulin resistance, making the patient at risk for diabetes, amongst other potential side effects. Topical steroids and steroid injections can also be helpful.
Atopica/cyclosporine: This medication is the long-term treatment of choice for furunculosis because it is our best anti-inflammatory allergy medication that safe for long-term use. It is a dose-dependent immunomodulatory medication. At the dose used for furunculosis, it decreases inflammation and does not suppress the immune system. Long-term, it can be tapered to the lowest effective dosage. It is slow to take effect, building in the system over 2-4 weeks, so it is generally coupled with prednisone initially. The prednisone is tapered off as the Atopica takes effect.
Atopica causes upset stomach (vomiting, diarrhea, decreased appetite) in about 10% of patients, especially when first prescribed. This can be mitigated by freezing the capsules, giving it with food, and slowly building the dose. Other potential side effects of Atopica include initial shedding, followed by copious hair growth and gingival hyperplasia (gum overgrowth).
Like all chronic oral medications, Atopica is processed by the liver and kidneys. Baseline bloodwork is recommended, and then bloodwork is generally checked again after 2-3 months and then every 6 months long-term, to ensure that the patient is safely processing the medication.
Unfortunately, Atopica is quite expensive. Several new canine generics are now on the market, though, such as Sporimmune (capsules) and Cyclavance (liquid), and can be used to help save cost.
It is important to note that our allergy long-term allergy medications—Cytopoint, Apoquel, and Zenrelia—are generally not as effective as Atopica for managing furunculosis. This is because these medications are not as good at controlling inflammation as Atopica is.
Allergen-specific intervention
Long-term administration of Atopica is an excellent treatment option for treating furunculosis. However, identifying allergens for avoidance and to potentially build tolerance to the allergens may reduce the dependency on medications long-term.
There are 3 general types of allergy: flea bite allergy, food allergy, and environmental allergies. If a food allergy is suspected, we recommend performing a strict elimination diet trial using prescription hypoallergenic diet exclusively for 8 weeks and then monitoring clinical response. If environmental allergies are suspected, we can consider serum allergy test or intradermal allergy testing to identify major allergens. From these results, allergen avoidance can be attempted and allergen specific immunotherapy can be considered. With allergen specific immunotherapy, the dog will be exposed to miniscule amounts of their allergens on a regular basis, either orally, transdermally (applied to the skin), or through subcutaneous injections. This therapy aims to slowly train the immune system to tolerate the allergens. On average, immunotherapy will take 1 year to take effect. It benefits about 70% of patients by reducing or eliminating the need for long-term medication.
Laser ablation of furunculosis
Because furunculosis causes such severe inflammation, chronic cases can experience severe scar tissue accumulation and remodeling of the foot pads. The latter condition is marked by the development of pseudopads where haired skin used to be on the underside of the foot. Scar tissue and pseudopads are permanent and do not respond to anti-inflammatory medications. For these dogs, we may consider laser paw revision surgery, where we use our surgical laser to remove the damaged tissue. This procedure usually requires anesthesia and extensive bandaging post-op, but it a great option for dogs whose chronic damage makes them unable to respond to medical management.
