This disease is more common in dogs than in cats, however, there are several types of IMPA that affect cats. IMPA can be erosive, that is damaging to bone and causing bone to deteriorate, or non-erosive. IMPA may occur by itself or in conjunction with other autoimmune disorders that affect other organs and systems. Feline progressive polyarthritis is a type of IMPA that is erosive and is similar to rheumatoid arthritis in people. Systemic lupus erythematosus is the most common type of IMPA in cats and is non-erosive but may affect other organs and skin. All types of IMPA are characterized by inflammation and pain in joints and affect the cat's ability to move freely. If IMPA symptoms occur in your cat a veterinarian should be consulted, as early treatment is effective and prevents damage to joints.

Immune-mediated polyarthritis (IMPA) occurs in cats when an inappropriate immune system response occurs, resulting in the cat's immune system attacking two or more articulated (movable) joints. This causes inflammation and interferes with joint function. The immune system disorder can occur independently/idiopathically (without an outside agent or known cause) or may be due to a reaction to a pathogen or infection. This results in antibodies reacting in the joint fluid that lubricates the joint and causes an immune system reaction resulting in swelling and affecting the joint cartilage. 

IMPA symptoms can overlap with symptoms of other immune system disorders also present and may appear off and on. Common symptom of IMPA to be alert for are:

Non-infectious, non-erosive IMPA such as systemic lupus erythematosus, in which material from cells become antigenic and cause antibodies to form and attack the body’s own joints, may also have symptoms related to the disease's effect on other organs and the skin.

Non-infectious, erosive IMPA such as feline chronic progressive polyarthritis (FCPP) is associated with more severe symptoms which become progressively worse.

Symptoms of other infectious disease may also be present with the occurrence of infectious erosive and non-erosive polyarthritis.

Causes and associations may vary with the type of IMPA. The cause of immune system attack on joint tissue may be due to antibodies produced by infection or a malfunction in the body's autoimmune response. There may be a genetic component making some animals more susceptible to immune system malfunction or inappropriate immune system reaction to infection.

Causes of malfunctioning immune system response in joints may include:

FCPP has been linked to viral infections such as feline leukemia, feline immunodeficiency virus and feline foamy virus, although whether the relationship is causal or whether the link is due to other factors has not been determined. FCPP has also been observed to occur predominantly in male cats.

A comprehensive physical exam and medical history of your cat will be performed by your veterinarian. Before finding a diagnosis of IMPA, other causes of joint swelling will need to be ruled out including cancer and underlying infection. Blood and urine tests will be ordered including a chemical profile, blood count urinalysis and electrolyte panel to rule out other disease and identify contributing factors to immune system malfunction. In addition, x-rays may be ordered that will show abnormalities in the bone, in cases of erosive IMPA, and may show joint effusion (fluid buildup) and soft tissue swelling in cases of non-erosive IMPA.

Your veterinarian will draw fluid (synovial fluid) from the joint(s) and analyze neutrophils (white blood cells) for indication of polyarthritis and/or bacterial infection. A tissue sample (biopsy) of joint tissue may also be taken and analyzed for signs of disease.

Treatment will vary depending on the cause and type of IMPA. IMPA caused by abnormal immune response to an infection will be treated with an antibiotic. Treatment for IMPA that occurs independent of infection will be treated with a variety of medications used to suppress immune system functioning. Supportive care including anti-inflammatories and painkillers will be prescribed to treat symptoms if IMPA. Anti-inflammatory medication such as steroids may need to be continued life long. Additional treatment your veterinarian may recommend include physiotherapy to restore joint functioning or splints and bandages to provide additional support to the joint. If your pet is overweight, a weight loss diet may be recommended. If infection of the joint tissue has occurred along with IMPA, surgery to remove infected tissue may be necessary.

IMPA can be a condition that may require life-long treatment and monitoring, as it is likely to recur. However, early treatment is associated with a favorable prognosis. If caught early there should be no permanent damage to joints. 

Your cat will need regular follow-up visits with your veterinarian as IMPA is prone to relapse and continued monitoring and medication may be required.

Usually, remission of symptoms occurs within a few weeks with therapy. Supportive therapy such a physiotherapy or supportive bandaging, if recommended, may need to be continued by the pet owner as advised by your veterinarian.