Immune-mediated Polyarthritis (or IMPA) is a condition that causes joint pain, lameness, and swelling in multiple joints caused by an inflammatory response of your dog’s immune system. Erosive IMPA, also called rheumatoid arthritis, is more common in older small-breed dogs, and the joints affected are usually in the lower leg. Non-erosive IMPA is usually seen in young, large-breed dogs. IMPA may occur in any dog; however, it is more common in larger breed dogs, including:

German Wirehaired Pointer receiving an orthopedic exam.

German Wirehaired Pointer receiving an orthopedic exam.

•  American Eskimo
•  bull terriers
•  Doberman pinschers
•  German shepherd dogs
•  golden retrievers
•  Irish setters
•  Rottweilers
•  Labrador retrievers
•  pointers
•  spaniels

Signs of IMPA

Dogs with this condition often have a history of:

• Periodic walking issues: stiffness, difficulty standing to walk/reluctance to walk
• Swollen, warm, and painful joints
• Decreased appetite or anorexia (not eating at all for a period) and increased thirst
• Enlarged lymph nodes, fever, lethargy

Types of IMPA

Nonerosive IMPA

With this type of arthritis, there is inflammation in multiple joints (poly-meaning multiple), which can be triggered by drugs, vaccines, or autoimmune conditions that affect the whole body. The most commonly reported drug is one of the sulfonamide-containing drugs, which especially affect Doberman pinschers. Nonerosive IMPA has also been reported with other antibiotics (erythromycin, cephalosporin, penicillin). Phenobarbital, an anti-seizure medication, as well as erythropoietin, a red blood cell-boosting medication, may also include polyarthritis as a side effect. Vaccine-induced IMPA, or IMPA triggered by vaccines, generally happens within a month of a first vaccination or booster. These IMPA signs resolve in a few days without immunosuppressive therapy (drugs that reduce immune responses). Akitas may experience more severe cases of vaccine-induced IMPA.

In these cases, a dog has been on a medication long-term or perhaps restarted a medication. Steroid-responsive meningitis-arteritis, a condition where the dog’s immune system attacks blood vessels and the tissue coating parts of the nervous system, may also be a cause of IMPA. Additionally, dogs can get systemic lupus erythematosus or SLE, but SLE-related polyarthritis cases are rare as SLE is rare in dogs. Tick-borne diseases or diseases transmitted by ticks may cause polyarthritis.

For most of these dogs, it is not known what causes non-erosive polyarthritis, and it could be a combination of these factors.

GreatDane IMPA.jpg - Caption. [Optional]

Great Dane with fever, lameness, and visible joint swelling in the tarsal (ankle) area, carpals (wrists), and digits (toes). Radiographs and joint fluid analysis confirmed Type I idiopathic polyarthritis. Photo courtesy of Dr. Tina Wolfe, DVM, DACVS

Idiopathic IMPA

Idiopathic, in this sense, means that it cannot be classified into any other existing categories of polyarthritis in dogs. Most of these cases are seen in young, large-breed dogs. There are four subtypes of idiopathic polyarthritis:

• Type 1: We don't know the cause, and there is no underlying disease that can be found. This makes up 50 to 65% of idiopathic polyarthritis patients.
• Type 2: An infectious disease leads to an immune system response in the joints, causing inflammation in 10 to 25% of cases.
• Type 3: Associated with liver or gastrointestinal diseases, making up 5% of idiopathic polyarthritis cases in dogs.
• Type 4: Associated with cancer that exists outside of the joints, making up 2% of all dogs with polyarthritis.

Breed-Associated IMPA

Some breeds, including the Akita, Boxer, beagle, Bernese Mountain dog, spaniel breeds, and Weimaraner, may have immune-complex synovitis or inflammation of joint capsules.

Erosive IMPA

Erosive Polyarthritis can change joint surfaces by reshaping them and causing joint cartilage to have small lytic or punched-out lesions resembling dimples on a golf ball.

Rheumatoid Arthritis (RA) is the most described form of erosive arthritis in dogs. Standards used to diagnose RA in dogs have been adapted from human medicine, and there is not one specific diagnostic test for RA. There are 10 criteria used to determine if RA is present - a dog with seven out of 10 has RA, while having five out of 10 is suggestive. Small-breed, middle-aged dogs are often diagnosed with rheumatoid arthritis involving the wrist, knee, and digital (toe) joints.

Some dogs that initially appeared to have non-erosive IMPA occasionally can be diagnosed with erosive IMPA over time. They always were erosive, but the classic changes on radiographs weren’t seen initially. If clinical signs of polyarthritis persist, taking radiographs after time has passed is recommended.

Diagnosis

Your vet will conduct a thorough physical exam to identify signs of IMPA. Blood tests can reveal changes such as altered white blood cell counts, mild anemia or decreased red cell counts, and increased inflammation markers. Urinalysis is recommended to check kidney function. Tick-borne polyarthritis, which is non-erosive polyarthritis, may require specific antibody tests. Joint fluid testing through arthrocentesis (also known as a "joint tap"), a process where joint fluid samples are taken under anesthesia, is crucial. The timing is important as when dogs are showing clinical signs of IMPA, they are more likely to have positive joint taps. Imaging (X-rays, MRI) provides further insights into the condition. Non-erosive polyarthritis can result from immune reactions to infections, chronic inflammation, or cancers. Using imaging and immunological tests, your veterinarian will check for these, especially if the polyarthritis is being seen at an atypical age. 

Periodic physical exams will be recommended if your dog is diagnosed with any form of IMPA. It is a complex diagnostic process involving multiple aspects of your dog’s health, and you may be referred to a veterinary internist for additional testing and treatment.

Treatment

Treatment involves medications to help control your dog’s immune system responses, and the response to this therapy can help to confirm a diagnosis.

NSAIDs and Corticosteroids

For milder cases, nonsteroidal anti-inflammatory drugs (NSAIDs) might offer some relief, but most IMPA patients require corticosteroids like prednisone/prednisolone or other immunosuppressive drugs. Corticosteroids are often used alone, and dosing is adjusted based on your dog’s response. Corticosteroids are not compatible with NSAIDs as there is a significant risk of gastrointestinal bleeding and perforation. 

Doxycycline Trial

Tick titers (antibody levels against tick-borne organisms) have limitations, and a trial with doxycycline is often recommended. If this completely resolves all clinical signs within 24-48 hours, your dog likely has a tick-borne disease (in which case, doxycycline is usually recommended for 3-4 weeks). If there is a modest improvement in clinical signs or no improvement, then immunosuppressive agents are considered. Arthrocentesis (process to remove excess fluid from a joint)is often performed at this time as well.

Immunosuppressive Medications

Cyclosporine and leflunomide are immunosuppressive options (medications that aim to reduce your dog’s immune response). Combination therapy (using multiple medications) is used if monotherapy (using just one therapy) fails. Mycophenolate mofetil and azathioprine are common additional drugs.

Supportive Therapy

Supportive therapies are also important to treat underlying causes and manage IMPA signs and include pain medications like gabapentin, joint supplements, anti-nausea medications, and IV fluids if needed. Some veterinarians may also suggest physical therapy to help with joint pain and mobility. Your dog will need periodic monitoring for their IMPA signs and to be sure medications are well tolerated. The overall treatment goal is to administer the lowest effective dosage of the fewest drugs and help your dog have a good quality of life.

Monitoring

It's a complex disease, but careful monitoring and adjustments help manage it. Monitoring involves joint taps, blood tests, and observing your dog for changes in signs and potential drug side effects. Your dog's therapies may be adjusted depending on their clinical signs and test results.

Prognosis

The overall prognosis for IMPA cases is fair to good, depending on the cause. Non-erosive IMPA with no known cause may go away and never come back or may return if medications are changed. Most dogs with erosive IMPA (rheumatoid arthritis) generally have a poorer prognosis, experience worse joint damage and mobility difficulties over time despite medications, and some will need lifelong treatment and monitoring. Eventually, surgical treatment to stabilize joints may be recommended. Some dogs may have a good quality of life with this disease and still be able to do the things they enjoy.

Talk with your veterinarian about your dog’s specific condition and what you might expect to help your dog be as healthy and happy as possible.

Janet Stomberg, BS, MS, Veterinary Partner Content Coordinator, contributed to this article.