Idiopathic immune-mediated polyarthritis is caused by an abnormal immune system response. The joints of one, or typically, multiple joints are identified as abnormal and an inflammatory reaction occurs with increased numbers of white blood cells entering the joint space. The white blood cells, themselves, release various chemical mediators that create further swelling and inflammation. Joint pain and many times fever are noted due to the inflammatory response. An obvious swelling in the joints may or may not be noted. If joint swelling is not present, the diagnosis may be quite difficult to establish. Trauma. A traumatic injury to a joint usually causes an acute joint swelling and lameness. The injury may be a soft tissue injury with joint swelling which results in inflammation or bleeding into the joint. Fractures of the bones involving the joint can also lead to severe joint effusion and pain.
There are no consistent signs of the disease and many of the signs are fairly general and may be seen with many other conditions. Animals with this disease generally feel ill and reluctant to move, which may be interpreted as lethargy rather than joint pain or stiffness. Sometimes the only noted clinical problem is a fever of unknown origin, and a very involved work up might be needed before a diagnosis can be established.
Animals with polyarthritis often experience shifting leg lameness or multiple limb involvement, again causing potential confusion. Additionally, the signs of polyarthritis may be acute (happening very quickly) or chronic (long standing). Oftentimes, animals seem to improve for a short time on their own, but usually their clinical signs return or become even worse. It is not uncommon for an animal that has been intermittently ill for months return to the hospital with polyarthritis.
If polyarthritis is suspected, it is important to establish the diagnosis of idiopathic immune-mediated disease. Since there is no specific diagnostic test that will confirm the idiopathic disease the diagnosis must be based on exclusion of other potential causes of polyarthritis.
Many other diseases that stimulate an inflammatory reaction can actually cause a similar immune response in the joints. The diagnosis of idiopathic immune-mediated disease is only made after these other conditions are ruled out. Diseases that can cause similar symptoms as immune-mediated polyarthritis include:
Chronic degenerative joint disease or osteoarthritis is a very common cause of multiple joint swellings and stiffness. Commonly referred to as arthritis, animals are typically not ill. Osteoarthritis is caused by joint laxity, conformational problems or injury and many times is seen in older animals. The joint swelling that results is caused by an increase in joint fluid due to chronic inflammation, and or an actual increase in bone size due to bony proliferation around the joint.
Infectious joint disease. Several causes of infectious polyarthritis may be confused with the immune-mediated disease. Septic (bacterial) arthritis can be caused by either an external wound, or it can be spread by the blood from another area of the body. Typically only one joint is affected but it can also occur in multiple joints. The most common forms of infectious polyarthritis are the tick-borne diseases that include Lyme disease, ehrlichiosis and Rocky Mountain spotted fever. These diseases may cause polyarthritis alone, or be associated with other systemic symptoms. Other more unusual causes of infectious arthritis include fungal and viral agents.
Systemic lupus erythematosus (SLE) is an autoimmune disease that affects multiple body systems. The joints are common sites of inflammation, although the blood, kidneys, muscles, nerves and skin can also be affected.
Rheumatoid arthritis is an immune-mediated disease that causes a bone-deforming, erosive polyarthritis. Multiple joints are often affected and the disease is chronic and painful.
Polymyositis, or inflammation of the muscles, and meningitis, which is inflammation of the lining of the spinal cord, are separate diseases that have several causes including immune-mediated. The diseases can cause generalized pain, a stiff gait and fever, and may be confused with polyarthritis.
Certain breeds are associated with breed specific polyarthropathy. Greyhounds, usually under two years, may develop an erosive bone deforming polyarthritis of their distal extremities. The shar-pei breed may develop a disease called "shar-pei fever", categorized by fevers and swellings of the hock and carpal joints. Over time, severe kidney disease may develop. Akitas, boxers, Weimaraners, Bernese mountain dogs, German shorthair pointers and beagles have been known to develop a polyarthritis or a polyarthritis/meningitis syndrome that affects young dogs.
Reactive polyarthritis is a term used to describe polyarthritis secondary to a primary disease process. Reactive polyarthritis is caused by prolonged stimulation of the immune system by a number of agents. It is fairly uncommon. Primary causes include: chronic bacterial, fungal or parasitic infections, gastrointestinal disease, tumors, long-term therapy with certain drugs and reactions to vaccination.