What Is Immune-Mediated Polyarthritis?
Immune-mediated polyarthritis (IMPA) is inflammation of multiple joints caused by the immune system rather than a routine bacterial joint infection. Immune complexes settle in joint linings, producing pain, stiffness, fever, and a shifting lameness. Dogs are the primary patients; cats can develop immune joint disease but are less often labeled classic IMPA.
IMPA may be idiopathic (no underlying trigger found) or secondary to infection elsewhere, drugs, vaccines, cancer, or other immune disease. It sits in the same family of immune mischief as immune-mediated hemolytic anemia and immune-mediated thrombocytopenia, though those target red cells or platelets instead of joints. Neuromuscular look-alikes such as myasthenia gravis cause weakness without the hot, fluid-filled joints of IMPA.
Why it matters
Untreated IMPA leaves dogs painful and feverish, and delaying joint taps can miss a diagnosis that usually responds well to immunosuppression once infection is ruled out.
Signs Owners Notice
- Stiff, stilted gait or “walking on eggshells,” often worse after rest
- Shifting leg lameness — different limbs seem sore on different days
- Reluctance to jump, climb stairs, or play
- Fever, lethargy, reduced appetite
- Warm, swollen joints (carpi/wrists and hocks are common)
Some dogs are simply “painful all over” without an obvious single bad knee.
Diagnosis and Treatment
Veterinarians combine exam, bloodwork, infectious disease testing when indicated, radiographs, and arthrocentesis (joint taps). Synovial fluid in IMPA typically shows sterile inflammation (many neutrophils without bacteria). Culture and cytology help exclude septic arthritis — a critical fork in the road before steroids.
Treatment often includes:
- Immunosuppressive therapy — prednisone/prednisolone as a backbone; additional agents if needed
- Pain control — while inflammation is brought down
- Treat or remove triggers — when a drug, tick disease, or other cause is identified
- Tapered monitoring — rechecks of comfort, fever, and sometimes repeat joint fluid
- Physical support — soft bedding, controlled activity, weight management
Never start high-dose steroids for a limp at home — septic joints need a completely different plan.
Practical Tips for US Pet Owners
- Video the gait on hard floors; shifting lameness is easier to show than describe.
- Ask whether joint taps and infectious screening are planned before immunosuppression.
- Track fever and willingness to rise each morning during taper.
- Locate referral-friendly practices through veterinary clinic listings if IMPA is complex or relapsing.
- Discuss tick prevention — some vector diseases mimic or trigger immune joint disease.
When to see a vet
Prompt evaluation for fever with multi-limb stiffness, shifting lameness, or swollen joints. Urgent care if your dog cannot rise, is extremely painful, or deteriorates quickly.
Key Takeaways
- IMPA is sterile, immune-driven inflammation of multiple joints — mainly a dog diagnosis.
- Joint fluid analysis plus ruling out infection guide safe immunosuppression.
- Most dogs improve on a structured steroid plan; triggers and relapses need follow-up.
FAQ
Is IMPA contagious?
No. It is immune-mediated, not a kennel cough-style infection.
Is it the same as Lyme arthritis?
Tick-borne disease can cause or mimic joint inflammation. Your vet’s infectious workup decides whether IMPA is primary or secondary.
Will my dog need steroids forever?
Many dogs taper to low or zero doses; some need long-term low-level control. Relapse plans should be written before you leave the clinic.