What are interdigital cysts?
Interdigital cysts (also known as follicular cysts or furuncles) are inflamed bumps or nodules located between the toes (interdigital area). These painful cysts can rupture and leak clear or blood-tinged fluid. If the cyst becomes infected, it will leak cream-coloured fluid.
Many factors contribute to the development of interdigital cysts, making them frustrating to manage.
How do interdigital cysts develop?
Interdigital cysts develop as a reaction to inflamed hair follicles between the toes. The inflamed hair follicles rupture into the surrounding tissue, creating more inflammation and swollen “cysts” that can open (ulcerate) and drain. This process is irritating and itchy, causing the dog to lick or chew at the area, which further traumatizes the hair follicles, continues the cycle of inflammation, and potentially introduces infection. If not treated quickly, cellulitis and scarring may occur.
What causes interdigital cysts?
Allergy-induced paw inflammation is the most common cause of interdigital cysts. Allergic dogs often have itchiness and recurrent skin infections in other areas, such as their ears. Allergy triggers can include pollen, mites, molds, fleas, and dietary proteins.
Other factors that increase the risk of your dog developing interdigital cysts (especially between the outside two toes) are mechanical in origin. An abnormal amount of friction in the interdigital space can cause inflammation of the hair follicles. This is more likely to occur for:
- large-breed dogs with short or coarse hair, such as pit bull terriers, bulldogs, German shepherds, and Labrador retrievers;
- dogs who experience abnormal trauma to their feet from rough ground or cage surfaces;
- obese dogs who put excessive pressure on their paws;
- dogs with arthritis or other pain that causes them to chronically shift their weight, creating abnormal friction between their toes; and
- dogs with large, webbed paws or deep interdigital pockets.
Some dogs will have interdigital cysts secondary to both allergic and mechanical factors.
Less common factors that can contribute to the development of interdigital cysts include skin mites called demodex, foreign bodies (e.g., splinters, grass awns), and endocrine disease (e.g, hypothyroidism).
What are the signs of interdigital cysts?
If your dog has interdigital cysts, you will see raised red bumps between your dog’s toes and your dog may be licking or chewing at their feet more than usual. Your dog’s paws may also be discoloured – especially if they have a secondary infection. You may even notice small blood spots on the floor if a cyst has ruptured. Dogs who are in pain may limp or be reluctant to walk.
How is the underlying cause of interdigital cysts diagnosed?
After a complete history and physical examination of your dog, your veterinarian may recommend testing the skin for demodex mites, bacteria, and yeast by performing skin scrapings, hair plucks, and/or tape preps of the area. If an allergic condition is suspected, a hypoallergenic diet trial may be recommended, followed by skin testing for environmental allergy. See handout “Allergies in Dogs” for more information.
Your veterinarian may take a sample from the cystic area for bacterial culture and sensitivity testing, especially if your dog is not responding to treatment. Your veterinarian may also check for endocrine diseases such as hypothyroidism or Cushing’s disease using blood and urine testing. A skin biopsy of the area may be recommended to rule out cancer or immune-mediated skin disease.
How are interdigital cysts treated?
Treatment is aimed at controlling the underlying cause, so it typically includes allergy treatments, hypoallergenic diets, and anti-inflammatory medications such as steroids (prednisone) and cyclosporine (Atopica™). Other allergy medications, including oclactinib (Apoquel™) or lokivetmab (Cytopoint™), can help control your dog's itching but aren’t considered to be effective enough to treat the degree of inflammation involved in interdigital cysts.
Secondary infections may require long courses of antibiotics, sometimes up to 8 weeks. Your veterinarian may recommend using pentoxifylline, a medication that can enhance the effects of the antibiotics. Topical treatments like steroid ointments and antimicrobial paw soaks (chlorhexidine) can be very useful to manage superficial inflammation and infection.
If arthritic changes and/or obesity have promoted cyst formation, these chronic conditions need to be managed more effectively with medications and nutritional therapy. Providing protective boots and controlling the surfaces your dog walks on can help manage the disease as well.
What is the prognosis for dogs with interdigital cysts?
Many cases respond well if an underlying disease is found and treated appropriately. If an underlying disease is not managed well, interdigital cysts can recur more frequently, causing chronic inflammation, scarring, and pain. In severe cases that don’t respond to recommended therapies, laser surgery may be required to offer comfort. Rarely, a surgery called fusion podoplasty may be recommended as a last resort, fusing the affected toes together.
