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Does your dog have a tender, raised, round, red nodule between his toes that’s obviously causing him pain? This cyst on your dog’s paw is most likely what’s commonly called an interdigital cyst.

“Interdigital cyst” is a misnomer; cysts are thin-walled sac that contains fluid. The painful lumps found between a dog’s toes are more accurately called a furuncle – also known as a boil – and is caused by an infected hair follicle. Your veterinarian may refer to this condition as interdigital furunculosis.

A hair follicle is a shaft or opening in the skin through which a hair grows. An interdigital cyst develops when a hair follicle becomes infected. These cysts most often develop on the top side of the webbing between the toes but can also appear on the underside of the webbing between the paw pads.

Causes of interdigital cysts

The most common cause of an interdigital cyst is trauma from the very hair that sits inside the follicle. Some breeds of dogs, including the Chinese Shar Pei, Labrador Retriever, and English Bulldog, have short, stiff hairs on the webbing between their toes. Taking an awkward step can drive one of these bristly hairs backwards into the hair follicle. This is called an ingrown hair.

This may seem odd, but hair is very irritating to the deeper layers of skin. The primary component of hair is keratin. When keratin enters the skin around the hair follicle, it causes an inflammatory reaction. The skin around the follicle becomes swollen and closes off part of the follicle. Bacteria inside the follicle become trapped and grow, causing an infected follicle and the formation of an interdigital cyst.

A misstep is not the only cause of traumatic ingrown hairs. Dogs who have varus (where the forelimb is rotated so that the elbow juts outward) or valgus (where the front paws are turned outward from the wrist of their forelimbs) – either due to their breed or an angular limb deformity – are more likely to develop interdigital cysts. Dogs who have varus or valgus are more likely to bear weight on the webbing between their toes rather than just the paw pads. This increases the likelihood of driving hairs back into the hair follicle, resulting in an interdigital cyst.

Dogs who are overweight are more likely to bear weight on the webbing between their toes when walking or running. So are dogs whose gait has been altered by osteoarthritis.

Interdigital cysts can also be caused by stepping on a thorn or a burr. Grass awns and foxtails can enter the webbing between a dog’s toes and cause an interdigital cyst.

Any condition that causes your dog to excessively lick his paws can also lead to the formation of interdigital cysts. This includes demodicosis, atopic dermatitis, and bacterial and fungal infections. See the “Seriously Itchy Paws” for more information about what may be causing your dog’s paws itch.

Interdigital cysts typically require nothing more than a physical examination to diagnose and treat. But if your dog has more than one cyst, has recurring cysts, or cysts do not respond to conventional treatment, then your veterinarian may order diagnostics to determine the underlying problem. These diagnostics may include a biopsy of one or more cysts, a skin scrape cytology to look for Demodex mites, or a bacteria culture and sensitivity to determine the correct antibiotic.

Treatment for interdigital cysts

Single cysts may be treated with a topical antibiotic. Your veterinarian may prescribe a medicated cleanser, such as one containing chlorhexidine, to clean the cyst and the skin around it. Your dog may need to wear an Elizabethan collar (the cone of shame!) to prevent him from licking the cyst as it heals. An analgesic should be prescribed to reduce your dog’s pain and discomfort. Having multiple cysts or a single cyst that does not respond to topical treatment may require an oral antibiotic.

Photobiomodulation (also known as PBM or laser therapy) and fluorescence biomodulation (FBM) are therapies that may be used in addition to topical or oral antibiotics to treat interdigital cysts. Both therapies use specific wavelengths of light to reduce pain and inflammation and stimulate healing. PBM is available at many general practices whereas FBM may only be available at specialty hospitals or veterinary universities.

Dogs who do not respond to medical management may require surgical removal of their interdigital cysts. Removing a cyst with traditional surgical techniques requires removing the skin around the cyst and may alter the conformation of a dog’s paw. Alternatively, interdigital cysts can be vaporized with a carbon dioxide laser (CO₂ laser). This allows for removal of only the affected tissue and has a better outcome but may require multiple treatments to fully resolve the issue.

Prevention of interdigital cysts

There are a number of strategies you can implement to minimize the risk of your dog developing interdigital cysts. If your dog is overweight, talk to your veterinarian about the most effecive way to manage a weight loss program for your dog. Avoid walking through areas that contain thorns, prickers, grass awns, and foxtails. If your dog excessively licks his paws, make an appointment with his veterinarian to determine the underlying cause and appropriate treatment.

Keep your dog’s paws clean and dry; excess moisture can lead to bacterial and fungal skin infections. Your veterinarian may prescribe a medicated wipe that can be used daily. Inspect your dog’s paws regularly for any redness or small bumps so that issues can be addressed early before they become big, painful problems.