Your 4-year-old dog has difficulty getting up, limps on one leg then another depending on the day, has a fever for no apparent reason, and seems lethargic. The veterinarian rules out an infection, a fracture, or dysplasia and suggests immune-mediated polyarthritis. This diagnosis is surprising because the dog is not old and has not had an accident. And yet, several of its joints are simultaneously inflamed: its immune system is mistakenly targeting its own joints.

Source: Perez et al., 2024 

What is immune-mediated polyarthritis (IMPA)?

Immune-mediated polyarthritis results from an abnormal immune response in which immune complexes accumulate within the synovial membrane of several joints.

These deposits trigger an influx of white blood cells and the release of enzymes, causing swelling and pain.

This mechanism radically differs from osteoarthritis:

• Osteoarthritis is a mechanical and progressive wearing down of cartilage.

• IMPA is an inflammatory immune attack where the body's defenses targets the synovial tissues surrounding the joint. Over time, cartilage and bone may also become damaged in some forms.

Erosive and non-erosive forms: a very different prognosis

Radiographic diagnosis distinguishes two main forms:

• Non-erosive IMPA (over 99% of cases): Inflammation is present but joint surfaces are not destroyed. The prognosis is generally favorable.

• Erosive IMPA (rare): Cartilage and subchondral bone are progressively destroyed, as in canine rheumatoid arthritis. The prognosis is more guarded and may require arthrodesis (Shaughnessy et al., JAVMA, 2016).

Classification: primary or reactive?

The origin of the disease directly influences the therapeutic approach:

Sources: Johnson & Mackin, JAAHA, 2012; Merck Veterinary Manual, 2025

Important to understand

In reactive forms, immunosuppression alone can be detrimental if the underlying infection or cause is not treated. For this reason, identifying the underlying cause is an essential part of the diagnostic work-up.

Is IMPA contagious ?

No.

Immune-mediated polyarthritis is not an infectious disease and cannot be transmitted to other dogs or people. Even when triggered by an infection elsewhere in the body, the joint inflammation itself results from an abnormal immune response rather than the infectious agent.

Clinical signs: what should alert you

IMPA typically affects dogs around 4–5 years of age.

Signs that should alert you:

• A stiff gait, described as "walking on eggshells."

• A shifting lameness that changes from one limb to another day by day.

• Swollen, warm, and painful joints (often symmetrical).

• Unexplained fever and apathy.

• Reluctance to walk, climb stairs or jump.

Diagnosis and treatment

There is no single test that confirms IMPA. Diagnosis relies on a combination of clinical signs, exclusion of infectious or other inflammatory diseases, and analysis of joint fluid obtained by arthrocentesis.

A complete workup is usually required, including blood tests, urinalysis, radiographs and screening for vector-borne diseases. Joint fluid analysis typically reveals sterile neutrophilic inflammation in at least two joints (Johnson & Mackin, JAAHA, 2012).

Treatment mainly relies on immunosuppression (prednisone as a first-line treatment) for type I. For reactive forms, the priority is to treat the underlying cause before initiating immunosuppression.

Treatment mainly relies on immunosuppression (prednisone as a first-line treatment) for type I. For reactive forms, the priority is to identify and treat the underlying cause before initiating immunosuppression.

Treatment is usually tapered very gradually over several months. Reducing or stopping immunosuppressive medication too quickly increases the risk of relapse, which is why regular veterinary follow-up is essential.

Prognosis and predisposed breeds

Although 92% of dogs respond to initial treatment, relapses are common (48%). Certain breeds have specific predispositions:

• Akita: Juvenile form with often guarded prognosis.

• Shar Pei: Associated with Shar Pei fever and long-term renal risks (amyloidosis). • Beagle, Boxer, Weimaraner, Bernese: Documented predisposition.

• Doberman: Increased sensitivity to drug-induced forms (sulfonamides).

What you can do now

• Act quickly: Don't downplay unusual lameness or a stiff gait.

• Inform your veterinarian: Specify all recent medications and vaccination history.

• Ensure follow-up: Rigorous long-term follow-up is crucial for managing relapses.

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Sources

1.  Perez J.C. et al. (2024). Canine immune-mediated polyarthritis: A review of 84 cases in the UK between 2011 and 2021. Veterinary Medicine and Science. PMC10825372
2. Johnson K.C. & Mackin A. (2012). Canine immune-mediated polyarthritis (Part 1 & 2). JAAHA, 48: 12–17 and 71–82.
3.  Shaughnessy M.L. et al. (2016). Clinical features and pathologic joint changes in dogs with erosive immune-mediated polyarthritis. PMC5332147
4.  Merck Veterinary Manual (2025). Immune-Mediated Polyarthritis in Dogs and Cats. merckvetmanual.com
5. Cervone M. et al. (2025). Clinical presentation, diagnostic findings, and outcome in dogs with IMPA in France. ScienceDirect.
6. Bennett D. (1987). Immune-based non-erosive inflammatory joint disease of the dog. J Small Anim Pract, 28: 909–928.