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Immune Mediated Polyarthritis in Dogs

Immune-mediated polyarthritis is a disorder of the immune system leading to inflammation in multiple joints called polyarthritis. Polyarthritis refers to multiple inflamed, swollen and painful joints. This pain will often cause pets to be reluctant to walk or may cause limping on one or more legs.

This condition in dogs is similar to the rheumatoid arthritis in humans. It is often associated with systemic signs of illness such as pyrexia, lethargy and hyporexia. Most affected dogs are between three and seven years of age at the time of diagnosis, although cases of IMPA have been reported in dogs ranging from six months to 12 years of age. Any breed can be affected and there is no apparent sex predilection

Immune-mediated polyarthritis is different to degenerative joint disease (i.e. wear and tear arthritis or osteoarthritis) which occurs much more commonly in dogs after injuries or due to old age.

Historically, canine IMPA has often been classified into 4 types:

  • Idiopathic (type I)
  • Secondary to a non-joint infection (type II),
  • Secondary to gastrointestinal disease (type III)
  • and secondary to neoplasia (type IV)

Causes of Immune Mediated Polyarthritis in Dogs

There are several causes if Immune Mediated Polyarthritis in dogs. Some include;

  • Idiopathic (unknown)
  • Immunologic mechanism likely: abnormal immune response to the system
  • May occur secondary to a hypersensitivity reaction to sulfas, cephalosporins, lincomycin, erythromycin, and penicillins, involving the deposition of drug antibody complexes in the blood vessels of the synovium (lining of the joint)
  • Chronic cases:
  • Antigenic stimulation along with concurrent meningitis (brain swelling)
  • Gastrointestinal disease
  • Periodontitis: infection of the tissues that support the teeth
  • Neoplasia: uncontrolled growth of tissue
  • Urinary tract infection
  • Bacterial endocarditis: bacterial infection of the heart lining
  • Heartworm disease
  • Pyometra: infection and accumulation of pus in the uterus
  • Chronic otitis media (infection of the middle ear) or external fungal infections
  • Chronic Actinomyces or Salmonella infections: bacterial infections, with fever, abscesses.

Symptoms of Immune Mediated Polyarthritis in Dogs

Initial symptoms can be frustratingly non-specific, with signs like general malaise and reduced physical activity. In many dogs with IMPA, fever is present, but in some, it is not. With time, signs that help to localize the problem to the joints become more apparent. These include difficulty walking or lameness, joint swelling, and joint pain.

Many dogs will have other signs of illness not related to their joints, including vomiting and decreased appetite. If joint swelling and pain are present on initial exam, it is easier to know to look to the joints for more information, but in some pets, the joint signs are not as evident, and without some index of suspicion for the disorder, the diagnosis can be delayed.

Diagnosing Immune Mediated Polyarthritis in Dogs

A diagnosis of polyarthritis is made by taking joint fluid samples from multiple joints and submitting them for laboratory examination. The next step in the investigation of polyarthritis is to look for any possible underlying trigger factors for this disorder. This step is likely to include blood and urine tests, X-rays, abdominal ultrasound scanning or CT scans.


Polyarthritis is usually treated with the use of medications to suppress the over-active immune system. This includes the use of steroids and other similar medications. These medications can have side effects which will need to be discussed with you and monitored carefully as the treatment goes on, through examinations and blood tests. Most dogs with immune-mediated polyarthritis are treated for several months with immune-suppressive medications, but the doses of the drugs are gradually reduced over that time.


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