Immune-mediated Polyarthritis: An Overview of Pathology and Treatment from a Technician’s Perspective

Lynda Vermillion, CVT, VTS (SAIM) | Veterinary Emergency Center of Manchester | Published: Issue 2 2024


Editor's Pick

Immune-mediated polyarthritis (IMPA) can be described as “chronic synovial inflammation in two or more joints, failure to isolate an organism from the joint fluid, and a positive response to immunosuppressive therapy.” It occurs when immune complexes are stimulated to enter the synovial fluid (a hypersensitivity reaction) and this draws neutrophils and sometimes T-cells. 

IMPA is a painful condition of dogs and cats that can present with obvious clinical signs such as swollen joints and altered gait, or with very subtle and nondescript signs, such as fever of unknown origin, hyporexia, and lethargy. Obtaining a definitive diagnosis can be challenging for the clinician, as can managing the patient at home for the client. The technician/nurse can assist in achieving these goals by taking a complete history at the time of presentation, preparing items for the clinician to obtain and process diagnostic samples, and helping the client set up a treatment regimen for administering medications at home. Ideas to manage the painful patient in the home environment and encouragement in following the clinician’s prescribed treatment plan can help the patient navigate this painful condition while medications take effect. To be an effective advocate for the patient, the veterinary technician/nurse must have a basic understanding of the disease process.

Immune-mediated polyarthritis may also present with vague signs such as fever, lethargy, and anorexia

There are many forms of arthritis ranging in cause from degenerative joint disease, infectious disease, to immune-mediated with sub classifications, such as erosive and non-erosive, breed-specific, and chronic progressive (cats). This article will focus on an overview of primary non-erosive and secondary non-erosive IMPA, with primary being idiopathic and secondary discussing those types in which a cause outside of the joints has been isolated. We’ll cover the technician/nurse role in assisting the clinician in the diagnostic process and how assistance can be provided in helping the client manage the patient and follow-up visits to monitor treatment effectiveness.


  • Immune-mediated polyarthritis (IMPA) can have an obvious presentation with swollen joint(s), lameness, and a history of environmental exposure to a secondary cause.
  • It may also present with vague signs such as fever, lethargy, and anorexia.
  • Primary IMPA is a diagnosis of exclusion.
  • Secondary IMPA should be considered and treated in areas of vector-borne disease prevalence.
  • Diagnostic sample preparation and handling are vital to distinguishing primary versus secondary IMPA.
  • The technician/nurse plays an important role through the diagnostic and treatment process, especially since treatment may be lifelong or when relapses occur.


  • Breed: There are some breed-specific forms of arthritis that affect Shar Peis, Akitas, Weimaraners, Newfoundlands, Rottweilers, Labrador Retrievers, Staffordshire Terriers, and young Greyhounds. There is no breed predilection among cats. Any feline breed can be subject to this disease.
  • Sex: Dogs: no predilection
  • Cats: Young male cats (feline chronic progressive polyarthritis), otherwise no predilection.
  • Age: No predilection (except as previously stated).

Primary IMPA

Primary, or idiopathic, IMPA can be non-erosive or erosive, in which there is active degradation of the joint. In the erosive form, degradation and malformation can be due to loss of self-tolerance or as a result of production of rheumatoid factor (RF). A process occurs in which inflammatory mediators and enzymes are released and result in an osteolytic condition forming cysts in the subchondral bone. Pannus and joint deformation result.

Secondary IMPA

Secondary IMPA can also be referred to as reactive and may be erosive or non-erosive as well. It has been further classified in subtypes labeled type II, which is most often associated with inflammatory or infectious disease as a causative agent, type III which is described as enteropathic in origin involving the GI tract and/or liver, and type IV which is paraneoplastic in which distant neoplasia sites and myeloproliferative disease are associated.

In these diagnoses, the pathologic process involves translocation of the causative agent via the bloodstream to the joint.

There are other causes of non-erosive IMPA such as systemic lupus erythematosus (SLE) and temporary signs related to vaccine administration or drug administration. Some breed-specific erosive and non-erosive causes exist as well.


IMPA may also present with vague signs such as anorexia.

The first diagnostic step is to obtain an accurate history and observe the patient upon presentation. When the patient is in the lobby, are they willing to stand to greet you as you approach? Do they appear to be lethargic? What is their mentation?

Observe the gait of those walking to the exam room. If the client is carrying the patient, once in the exam room ask the client to place the patient on the floor and observe the patient. Do they appear uncomfortable while standing? Are they reluctant to move? Is there evidence of pain in more than one joint (shifting weight)? When were the signs first noticed? They can be as subtle as decreased energy that progressed to lethargy or “picky eating” that progressed to anorexia.

While taking vitals, obtain a history that includes any recent travel (this author prefers over past two to three months), all supplements and medications, including recently completed antibiotic treatments, diet (remaining non-judgmental in demeanor to encourage accuracy), current parasite prevention methods and when last administered, and vaccination history with dates last received.

While obtaining vital signs, observe the coat and skin for signs of external parasites to indicate potential parasite prophylaxis failure. Note any swollen joints or areas of pain observed. The use of bite restraint may be necessary in cases where the patient is obviously quite painful to keep the client, technician, and clinician safe.

Once the client interview and patient exam have been conducted by the clinician, diagnostic tests such as a baseline complete blood count (CBC), serum chemistry (SC), point of care (POC) infectious disease tests (heartworm, lyme, ehrlichia, anaplasma), urinalysis, and radiographs may be ordered.

Depending upon the degree of suspicion for IMPA and past treatment history (if initiated at rDVM), a joint tap may be ordered.

The technician/nurse is often responsible for obtaining preliminary diagnostic samples and performing in-house tests. If the clinician pursues arthrocentesis (joint taps), the technician may assist with or perform sample collection and preparation for transfer to the appropriate reference lab.

To prepare for a joint tap, the author would set out on a mayo stand: 

  • A minimum of six to eight 3mL syringes with 22g X 1” needles
  • One box of frosted edged slides, with multiple slides lined up and ready to prep samples
  • Slide holders
  • EDTA blood collection tube 
  • No-additive blood collection tube (or a culturette) in which a small amount of joint fluid would be placed
  • Sterile gloves for the clinician (one to two pairs)
  • Sterile drape(s)
  • Clippers
  • 2% chlorhexidine scrub sponges
  • Alcohol sponges
  • Pencil to label slides and permanent marker to label tubes
  • Surgical monitor for the sedated/anesthetized patient
  • Two assistants should be present. One to aid in positioning or stabilizing the site of the tap and prepare slides/tubes and one to monitor the patient.

Joints typically tapped are the carpus, tarsus, elbow and stifle. Once the patient is sedated/anesthetized and placed in lateral recumbency, the insertion sites (similar to an incision site in a surgical procedure) are aseptically prepared by clipping and scrubbing a 3 to 5-inch area.

This author would aseptically prep all the sites initially, placing a chlorhexidine scrub sponge around the sites not being tapped. A final scrub of those sites would be applied just prior to needle insertion by the clinician. The typical prep sites were the dorsal aspect of the carpus, the dorsolateral aspect of the tarsus, the caudolateral aspect of the elbow, and the medial aspect of the stifle.

After joint fluid was removed (usually in aliquots of 0.5mL or less) the author would place one drop on several slides (usually five to 10 slides), spreading the viscous fluid as thinly as possible using a squash-prep technique, then labeling the slide naming the joint from which the sample was obtained prior to setting to one side to dry. Once dried, slides were placed in slide containers for transfer to the reference lab. Some of the remaining fluid would go into the culturette to evaluate for infection and some into the EDTA tube for further cellular evaluation and counts if needed. Blood-contaminated samples can be used for culture, but clean samples should be used for cytology and cell counts.


Serosanguineous fluid from the joint of a cat.

Treatment of IMPA largely revolves around the designation of primary versus secondary and the likelihood of infection. Indications of infection, especially if there is a high suspicion of rickettsial or Lyme disease infection, usually result in a course of doxycycline — even if PCR results have not yet confirmed presence of tick-borne disease. If signs resolve, the patient is monitored for further signs at least until the PCR results are in. If there is no improvement in clinical signs, concurrent steroid therapy may be prescribed by the clinician to aid in the resolution of those signs. In the author’s experience, this is typically followed by a slow taper of the steroid to try to avoid a relapse of clinical signs.

Treatment of primary IMPA in which there is low suspicion for secondary cause would be treated initially with a corticosteroid (prednisone or prednisolone), and in refractory cases, a second immunosuppressive would be added. Once there was complete resolution of clinical signs for two to four weeks, a second immunosuppressive would be added (if not already in place) for an additional two weeks and a gradual taper of the steroid would occur. Gradual steroid tapers would continue every two to four weeks based on patient response. Once the steroid was discontinued or had reached the lowest effective level in which clinical signs remained resolved, a taper of the additional immunosuppressive medication would begin following the same pattern until the lowest effective dose was achieved. If resolution of clinical signs remained, the medication would be discontinued and the patient carefully monitored for recurrence of lameness, reluctance to walk, etc. in which case the prescribed medications would be restarted at that last effective dose.

Nursing Care

In this author’s experience, IMPA patients were rarely admitted for overnight hospitalization. Nursing care revolved around keeping the patient as comfortable as possible during and after arthrocentesis by providing a thick layer of soft bedding on which the patient comfortably recovered. Pain medication was administered and often an appetite stimulant would be prescribed for the first few days.

As an advocate for the patient, the technician/nurse can keep the clinician informed regarding the level of pain the patient may be enduring so proper analgesia may be prescribed and administered. Anorexia can make it difficult for the client to administer oral medications, so advocating for an appetite stimulant is another area in which the technician/nurse can help the client avoid the frustration (and possible injury) of trying to “pill” the patient.

Discharge Instructions/Client Communication

Client communication is very important in the initial stages of treatment. Clients can become easily overwhelmed dealing with the side effects of steroid treatment — the PU/PD, urinary accidents in the home while the client is away, and potential for “counter surfing” or getting into trash due to increased hunger. They may need to be reminded that the goal is to taper the drugs to the lowest effective dose as soon as possible, but too soon means potentially starting over, meaning it will take longer to establish a more stable state in which, hopefully, the patient will be able to discontinue the steroid.

Reminding the client to finish all antibiotics prescribed will also help to prevent relapse in those cases where an infectious cause has been identified. As a final reminder, vaccines should probably be separated to avoid having an “over-excited” immune response that may result in relapse.


About the Author

Lynda Vermillion, CVT, VTS (SAIM)

Lynda worked for several years as a veterinary assistant at a small animal hospital in Virginia in the late 1980s. After attending a veterinary conference in Baltimore, she learned about the technician career path and was accepted into Northern Virginia Community College’s Vet Tech program where she graduated magna cum laude. After a move to Wisconsin and experience in a mixed animal practice, Lynda and her family moved to New Hampshire where she broadened her experience base as a technician supervisor and a hospital manager. In 2010 she decided to pursue a specialty in small animal internal medicine and received her VTS (SAIM) in 2016. Lynda currently works at the Veterinary Emergency Center of Manchester as an Education Mentor. She is a strong advocate for technician and assistant education and has served on the NHVTA board as the CE Committee Chair. Her work in internal medicine has shown her that one of the most important jobs a technician/nurse has is to support the client while they are treating their pet. Understanding the “why” behind the treatment prescribed can go a long way towards encouraging client (and therefore, patient) compliance, helping to achieve a positive outcome.

Images courtesy of Shutterstock, Adobe Stock, & Canva

Become a member to receive articles in our quarterly publication!