An interdigital cyst is an inflammatory reaction between the toes, and not a true cyst. It appears as a swelling that may open and drain pus. The causes are numerous, and include trauma, contact irritants, foreign bodies such as thorns and plant awns, and the bites of ticks and other external parasites.
Treatment: Treatment may involve long-term antibiotics. Try to remove any primary cause, such as a foreign body. Warm compresses may be helpful; alternatively, soak the affected feet in a medicated solution of betadine or nolvasan for 5 to 10 minutes. Further diagnostics may be needed to find the underlying cause.