Anal Furunculosis in Dogs (Perianal Fistulas)
Perianal fistulas are deep wounds and small holes that appear in the skin around the anus. They form small channels that drain, smell strongly, and do not heal on their own. The mechanism is primarily immune-mediated: the body's defenses at...
What is it?
Perianal fistulas are deep wounds and small holes that appear in the skin around the anus. They form small channels that drain, smell strongly, and do not heal on their own.
The mechanism is primarily immune-mediated: the body's defenses attack the skin in this region on a genetic background. The condition mainly affects German Shepherds and their crossbreeds.
It is a chronic disease: the wounds last for weeks or months, and relapses are common. Starting treatment early prevents the lesions from spreading to the muscle that closes the anus.
Commonly observed signs
These signs are neither exhaustive nor specific. Only a veterinarian can establish a diagnosis.
Related signs — each one links to its page:
Wounds or small holes around the anus, often multiple, draining purulent or bloody fluid.
Unpleasant odor from the rear end.
The dog licks or nips persistently under the tail.
Straining and pain when passing stool; some dogs hold back and become constipated, others develop diarrhea.
Stool leakage without the dog's awareness in advanced cases.
Loss of appetite and weight loss.
Change in temperament due to pain: irritable dog that resists having the tail or rear end touched.
The wounds may appear to improve and then return.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
The exact cause is not completely known.
An immune system dysfunction on a genetic background is the most accepted explanation.
The shape of the tail, wide at its base and carried low, promotes moisture and dirt accumulation in the anal region; this is a predisposing factor, not a cause.
Bacterial infection often becomes established in the wounds and perpetuates them.
German Shepherds and German Shepherd crossbreeds: these are by far the most commonly affected.
Irish Setters and Labradors are also noted.
Middle-aged dog; risk increases with age.
Tail wide at the base and carried low.
History of skin disease, hypothyroidism, or immune system disorder.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
Your veterinarian examines the anal area, sometimes under sedation because the area is very painful.
They assess the extent and depth of the wounds and perform a rectal examination.
They rule out other causes of wounds near the anus: anal gland abscesses, rectal tumors, or tumors of the anal region. A sample may be analyzed if there is doubt.
They check for wound infection and possible infestation by fly larvae.
Treatment
The main treatment is medical and lasts a long time: medications that suppress the immune system response, given by mouth or applied to the wounds, prescribed and monitored by your veterinarian.
A special diet, based on a new protein source for your dog, is often used along with medication.
An antibiotic may be added when wounds are infected.
The area is clipped and cleaned regularly, according to your veterinarian's instructions.
Surgery is reserved for lesions that do not respond to medical treatment.
Do not apply ointment or disinfectant to the wounds on your own without veterinary advice.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
There is no known prevention.
In a German Shepherd or German Shepherd mix, examine the area under the tail regularly and consult your veterinarian as soon as a small draining wound appears.
Keep the area clean and dry, especially in hot weather.
After healing, follow the monitoring plan given by your veterinarian: relapse is common and responds better to treatment when caught early.
Possible complications
Spread of wounds into the muscle that closes the anus, with loss of bowel control.
Narrowing of the anus from scarring, making bowel movements difficult.
Wound infection.
Infestation of wounds by fly larvae (myiasis), especially in summer.
Relapse when treatment is stopped.
When to see a veterinarian
You see small white worms moving in the wounds: consult your veterinarian the same day.
Your dog is unable to have a bowel movement: consult promptly.
The wounds are spreading or bleeding, or your dog is not eating: consult without delay.
Your dog is leaking stool without realizing it.
You see one or more small openings or draining wounds around the anus.
Your dog is licking constantly under the tail, has pain during bowel movements, or there is a foul smell from the rear end.
Prognosis
The disease is well controlled in many dogs with consistent treatment, but it rarely heals permanently: relapse is common and requires long-term monitoring.
Treatment started early, when wounds are not extensive, gives the best results.
The outlook is more guarded when the anal sphincter muscle is affected.
Frequently asked questions
Perianal fistulas or anal sac abscesses: what's the difference?
Anal sac abscess most often forms a single swelling on one side of the anus, appearing over several days, which ruptures and then heals with treatment; it mainly affects small breeds. Perianal fistulas form multiple wounds or small holes around the anus that discharge for weeks and recur; they mainly affect German Shepherds. The veterinarian can determine which condition it is on examination.
Is this an infection that can spread to other dogs?
No. Bacteria settle in the wounds, but the disease comes from a malfunction of the dog's own immune system. It is not transmitted to other animals or to humans.
Why does the treatment take so long?
Because the immune system reaction needs to be calmed durably. Wounds may appear healed before the disease is under control: stopping too early risks a relapse. The duration and discontinuation of treatment should be decided with the veterinarian.
Does changing your diet really work?
It is part of the treatment in many dogs, as a complement to medications. To be effective, the prescribed food must be the only food given, with no treats or table scraps, for the duration recommended by your veterinarian.
References
Content reviewed by the TunisieVet editorial team · Updated 11 October 2026
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