Immune-Mediated Polyarthritis in Dogs

Jamie Lovejoy, DVM
By Jamie Lovejoy, DVM on Dec. 21, 2023
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What Is Immune-Mediated Polyarthritis in Dogs?

Immune-mediated polyarthritis (IMPA) is a term that encompasses multiple conditions leading to joint inflammation (arthritis) in dogs due to molecules being deposited in the joints by the immune system.

Normally, the immune system combats external threats like viruses and bacteria by binding and disarming them with protective antibodies, forming molecules called immune complexes. In most cases of IMPA in dogs, immune complexes accumulate abnormally in the joint fluid, attracting cells that fight infection to the wrong places and causing damage to the smooth cartilage that covers the bones of the joint.

Primary Immune-Mediated Polyarthritis in Dogs

Most cases of IMPA in dogs are considered idiopathic, meaning that an obvious cause of the immune reaction cannot be found. This is known as primary IMPA.

Secondary Immune-Mediated Polyarthritis in Dogs

Immune complexes in IMPA can also be created because of infection, inflammation, or cancer in other parts of the body, and this is termed secondary IMPA. It’s important to differentiate the two, if possible, because managing the cause in secondary IMPA is crucial for getting your dog to feel better.

While IMPA is considered a rare disease in dogs, the clinical signs can closely resemble other conditions—suggesting that it might be underdiagnosed and potentially more common than we think.

IMPA in dogs can be a very painful and debilitating condition, and clinical signs should be evaluated promptly—ideally within days of onset.

Dogs struggling to rise because of extreme pain, exhibiting fever, or showing no interest in food may benefit from emergency care.

Symptoms of Immune-Mediated Polyarthritis in Dogs

Signs of immune-mediated polyarthritis in dogs may include:

Causes of Immune-Mediated Polyarthritis in Dogs

Most cases of primary IMPA in dogs are considered idiopathic, with no identified cause of the excessive immune response.

Certain breeds—including Shar-Pei, Akitas, Duck Tolling Retrievers, and Greyhounds—show possible genetic predispositions, although any breed can be affected.

Several potential causes exist for secondary IMPA in dogs, as any overreaction of a dog’s immune system can lead to deposition of immune complexes in the joints.

Inflammation or infection of the nervous system, heart, liver, and GI tract have all been implicated as potential causes. Tick-borne illnesses such as Lyme disease may also predispose dogs to IMPA. Rarer causes include drug reactions and cancerous tumors.

How Veterinarians Diagnose Immune-Mediated Polyarthritis in Dogs

Diagnosing IMPA in dogs requires testing to rule out other diseases with similar symptoms.

Your vet will start with a thorough physical exam of your pup to pinpoint areas of pain and inflammation. Suspicion for the disease will be higher in cases where pain is relatively sudden and affecting multiple joints, especially in predisposed breeds.

Tests may include a complete blood count, blood chemistry, urinalysis, tick-borne disease testing, and radiographs of the chest and abdomen. These tests will help your vet rule out other causes of joint pain and look for disease processes that may be leading to secondary IMPA.

If abdominal radiographs are inconclusive, an abdominal ultrasound may be recommended. Radiographs of the joints themselves can help rule out fractures, ligament injuries, and bone cancer.

To rule out joint infection before treating for IMPA in dogs, an evaluation of the joint fluid is needed. Sampling of this fluid, called arthrocentesis, is uncomfortable and requires anesthesia and sterile preparation of the joint to avoid contamination. Analysis of inflammatory cell ratios and protein levels in the joint can provide indications of IMPA.

Treatment of Immune-Mediated Polyarthritis in Dogs

IMPA in dogs is treatable, with the prognosis depending heavily on the type of IMPA suspected and whether it is primary or secondary. If pain and inflammation can be quickly controlled before significant joint damage occurs, your dog can live a long and happy life.

Treatment for Primary Immune-Mediated Polyarthritis in Dogs

Primary IMPA is treated with medications that suppress the immune system (such as steroids or cyclosporine) to halt the deposit of immune complexes. In cases of severe joint damage, pain medications and joint support medications may also be needed. Some dogs can be weaned off of medications after six to eight months, while others may require lifelong therapy.

Treatment for Secondary Immune-Mediated Polyarthritis in Dogs

For secondary IMPA, the underlying cause of the immune reaction must be identified and treated or the immune reaction is likely to recur. In some cases, controlling the underlying condition is all that’s needed, while others may still require some level of suppression of the immune system. The many possible causes for the same clinical signs make IMPA one of the more challenging autoimmune syndromes to manage.

Recovery and Management of Immune-Mediated Polyarthritis in Dogs

Once immune system suppression has been initiated and the underlying causes of IMPA have been controlled or cured, clinical recovery is often quick, with dogs feeling much better within days to weeks.

In most cases, after your dog has been symptom-free for at least two to four weeks, your vet will try to decrease the doses of the immunosuppressive medications.

This weaning process can take many months, especially if multiple medications were needed to control your dog’s symptoms. Some dogs may exhibit clinical signs at lower medication doses and need to stay on higher doses long-term.

Severe cases of IMPA in dogsmay result in long-term joint damage, requiring additional comfort management, because joint cartilage currently can’t be regrown.

Maintaining your dog at a healthy weight is crucial to reducing stress on damaged joints. Low-impact activities like swimming can keep joints moving and healthy without causing further injury.

Talk to your vet about rehabilitation, massage, and laser therapy for pain control. Joint supplements like Movoflex®, Dasuquin®, omega-3 fatty acids like Welactin®, and Adequan® are generally safe to be given alongside medications that treat Immune-mediated polyarthritis in dogs.

Prevention of Immune-Mediated Polyarthritis in Dogs

Primary IMPA in dogs can be very difficult to predict and prevent.

You can minimize inflammatory conditions by keeping up to date on your dog vaccinations and parasite prevention products.

Regular annual exams and screening blood work can also assist you and your vet in identifying potential systemic disease early, which can help prevent the onset of an overactive immune system.

Immune-Mediated Polyarthritis in Dogs FAQs

How long can a dog live with stage 4 arthritis?

Stage four arthritis—characterized by severe joint damage—can significantly impact your dog’s quality of life, leading some pet parents to consider humane euthanasia.

The lifespan with this disease depends heavily on your ability to control your dog’s pain. Pain management often involves a combination of medications. Talk to your vet to explore medication and other pain-control options.

Is IMPA in dogs genetic?

There is evidence to suggest that certain genes make some dogs more likely to have the excessive immune response leading to IMPA.

Featured Image: Mr Vito/E+ via Getty Images

Jamie Lovejoy, DVM


Jamie Lovejoy, DVM


Dr. Jamie Lovejoy graduated from Tufts Cummings School of Veterinary Medicine in 2012 after an undergraduate degree in Marine Biology. ...

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