What is Immune-Mediated Polyarthritis (IMPA)?

Immune-mediated polyarthritis is a disease in which the immune system mounts an inflammatory response within the joints, causing pain, swelling and difficulty walking. In IMPA, the immune system is inappropriately activated to send white blood cells to the joints. The white blood cells release chemicals and enzymes into the fluid that bathes the joints, disrupting the protective function of this fluid. 

A diagnosis of IMPA is made after the veterinarian is certain that there is not an underlying infection or cancer that may have triggered swelling in the joints. Because the white blood cells are attacking the joints inappropriately, IMPA can occur independently or in combination with more severe immune-mediated disease in which other body systems are affected. 

IMPA is seen more commonly in dogs than cats, but can occur in both species. Rheumatoid arthritis is a form of IMPA in people.

What are the clinical signs of IMPA?

IMPA causes sore, swollen joints; patients are often lethargic and reluctant to move. Frequently the patient will walk only when forced, cry with movement or may seem to walk as if stepping on a hot surface. When the joints in the spine are affected, patients may cry when the neck is touched or their collar is pulled. In the early phase of the disease, the patient may only have a low-grade fever or poor appetite and the joint swelling may be subtle. 

How do you diagnose IMPA?

When a veterinarian detects joint inflammation and fever, tests will be done to look for a systemic infection or cancer that may have caused an immune response. This may include the following tests:

• Blood work
• Testing for infectious diseases
• X-rays of the chest and joints
• Imaging of the abdomen with ultrasound
• Joint taps (collection of fluid from the joints)

A patient is sedated or anesthetized to complete a joint tap. During a joint tap, fluid is withdrawn from the joint with a small needle for microscopic evaluation and culture. Very often multiple joints are sampled. Only when other potential inciting causes of joint inflammation are eliminated is the disease called immune-mediated polyarthritis.

How do we treat IMPA?

Patients with IMPA are treated with medications that calm the immune response. These medications include steroid medications (such as prednisone) sometimes in combination with other immune-suppressing medications. Throughout treatment, it is important to remember:

• Careful monitoring is necessary during therapy, and the medications should be handled with care.
• Dosage adjustments are made often, especially early in the disease process, depending on the patient’s response. Close communication and follow up with a veterinarian is critical so the appropriate medication and dosage is used.
• Medication side effects such as excessive thirst and urination generally become less severe as medication dosages are reduced. 

IMPA can relapse if medications are tapered too quickly. Some patients require lifelong medications, often at very low dosages; other patients are able to come off medications altogether.

What is the prognosis for pets with IMPA?

Most patients with IMPA survive and maintain a good quality of life for the long term. Patients that are diagnosed early rarely have irreversible damage to their joints. Although some patients require lifelong medication to ward off a relapse of joint inflammation, medication side effects are generally tolerable and do not affect longevity. IMPA, like other immune-mediated diseases, requires diligent follow up and communication with a veterinarian.

Edited by:
Joseph Cyrus, DVM, DACVIM (SAIM)
April, 2020