IMPA stands for immune-mediated polyarthritis, a condition where a dog’s immune system mistakenly attacks the lining of multiple joints, causing inflammation, pain, and stiffness. It’s one of the more common immune-mediated diseases in dogs and can range from a treatable episode to a chronic condition requiring long-term management.
How IMPA Affects the Joints
In a healthy dog, the immune system targets foreign invaders like bacteria and viruses. In IMPA, immune cells flood the joints with inflammatory white blood cells, particularly neutrophils, even though there’s no infection present. This creates swelling and pain in two or more joints at the same time, which is what distinguishes polyarthritis from a simple injury or single-joint problem.
The joints most commonly affected are the carpus (wrist), hock (ankle), and stifle (knee). The hock joint tends to show the most reliable signs of disease, making it a key target when veterinarians are investigating.
IMPA comes in two forms. The vast majority of cases are non-erosive, meaning the inflammation causes pain and swelling but doesn’t permanently destroy the joint surfaces. Non-erosive IMPA typically affects large-breed dogs. Erosive IMPA, which accounts for roughly 16% of cases, damages cartilage and bone within the joint, similar to rheumatoid arthritis in humans. Erosive IMPA tends to strike middle-aged, small-breed dogs and carries a significantly worse prognosis, often requiring lifelong high-dose medication or even surgical joint fusion.
Signs to Watch For
The hallmark signs of IMPA are stiffness, reluctance to walk, and lameness that seems to shift between legs or affect multiple legs at once. Some dogs have visible joint swelling, but many don’t, which can make the condition tricky to spot. Owners often describe their dog as simply “slowing down” or being reluctant to stand up after resting.
Beyond joint symptoms, more than half of dogs with IMPA are feverish at the time of diagnosis, with many running temperatures at or above 40°C (104°F). Lethargy and loss of appetite are common. A smaller number of dogs develop vomiting, diarrhea, or skin problems. Because these systemic signs can look like so many other illnesses, IMPA is sometimes missed on the first vet visit, especially in dogs that aren’t obviously limping.
What Causes It
IMPA is broadly divided into two categories based on cause. Primary (or idiopathic) IMPA has no identifiable trigger. The immune system simply misfires, and no underlying disease can be found. This is the most common form.
Secondary IMPA is triggered by something else in the body that stimulates the immune system inappropriately. Known triggers include chronic infections, certain medications, tick-borne diseases, and cancer. When secondary IMPA is identified, treating the underlying cause can sometimes resolve the joint inflammation entirely, which is why veterinarians work to rule out these triggers before settling on a diagnosis of idiopathic IMPA.
Breeds at Higher Risk
Several breeds appear to be over-represented in IMPA cases. Recent studies have identified the Whippet, Cocker Spaniel, Miniature Schnauzer, Cairn Terrier, and Hungarian Vizsla as having higher-than-expected rates. German Shepherds were historically considered predisposed as well. Crossbreeds also show up frequently in case studies, so IMPA is not limited to purebred dogs.
Some breeds develop IMPA as part of a broader syndrome. For example, certain dogs present with both polyarthritis and steroid-responsive meningitis, a combination that affects the joints and the membranes surrounding the spinal cord and brain simultaneously.
How Veterinarians Diagnose IMPA
Diagnosis requires sampling the fluid inside the joints, a procedure called arthrocentesis. A veterinarian inserts a small needle into at least two joints (typically under sedation) and examines the fluid under a microscope. In a healthy joint, the fluid contains very few inflammatory cells. In IMPA, the fluid is flooded with neutrophils, specifically exceeding 12% of the total cell count. This neutrophilic inflammation in two or more limb joints is the definitive diagnostic criterion.
The hock joint produces the most reliable results, with one study finding that 97% of IMPA diagnoses could be confirmed from hock joint samples alone. Veterinarians will also run blood work, urine tests, X-rays, and sometimes infectious disease panels to distinguish primary IMPA from secondary causes and to check for the rarer erosive form, which shows visible bone damage on imaging.
Treatment and What to Expect
The cornerstone of IMPA treatment is immunosuppressive therapy, most commonly corticosteroids like prednisone. A typical starting protocol involves prednisone at a dose given twice daily, then gradually tapered by about 25% every two to three weeks as symptoms improve. The goal is to find the lowest effective dose and eventually stop the medication altogether if possible.
For dogs that don’t respond well to steroids alone, or that need unacceptably high doses to stay comfortable, veterinarians may add a second immunosuppressive drug. These additional medications work through different pathways to calm the immune response and allow the steroid dose to be reduced, which matters because long-term steroid use comes with real side effects.
Side Effects of Long-Term Steroids
Dogs on prolonged corticosteroid therapy commonly experience increased thirst, increased urination, and a noticeably bigger appetite that can lead to weight gain. Up to 30% of dogs on long-term steroids develop urinary tract infections, which can be silent (no obvious symptoms) and require periodic urine cultures to catch. Over time, some dogs develop thinning skin, hair loss, muscle weakness, poor wound healing, and a pot-bellied appearance. In severe cases, prolonged steroid use can trigger signs resembling Cushing’s disease.
Dogs on long-term immunosuppressive treatment typically need quarterly veterinary checkups along with blood work and urine cultures every six months. These visits help catch side effects early and confirm the medication is still working.
Relapse and Long-Term Outlook
The prognosis for non-erosive IMPA is generally favorable, but relapse is a real possibility. In one study tracking dogs after initial treatment, about 35% relapsed within the first 12 months and 41% within 24 months. On the more encouraging side, roughly 69% of dogs were eventually able to stop immunosuppressive treatment entirely, meaning their disease went into lasting remission. The remaining 31% needed ongoing medication to keep symptoms controlled.
Erosive IMPA carries a tougher outlook. Dogs with this form respond less well to standard treatment and often require lifelong, high-dose immunosuppressive therapy. Joint damage from the erosive form is permanent, and some dogs eventually need surgical procedures to fuse damaged joints and relieve pain.
For most dogs with the non-erosive form, IMPA is a manageable condition. The initial treatment phase can take weeks to months of adjusting medications, but many dogs return to comfortable, active lives once the right balance is found. Staying on top of follow-up appointments and watching for signs of relapse, such as returning stiffness, reluctance to walk, or fever, gives you the best chance of catching any flare-ups early.