Case series summary: Cats with non-erosive immune-mediated polyarthritis (IMPA) were identified from seven referral hospitals between 2009 and 2020 for a multicentre retrospective case series. Data were obtained from hospital records and referring veterinarians were contacted for follow-up. Twenty cases were identified: 12 castrated males (60%), one entire male (5%) and seven spayed females (35%). Common clinical signs included lameness (n = 20/20) and pyrexia (n = 10/18). Three cats presented with and two cats developed ligament laxity during treatment. Thirteen cats (65%) were diagnosed with non-associative IMPA and seven (35%) with associative IMPA. Comorbidities identified included chronic enteropathy (n = x/7), feline immunodeficiency virus (n = x/7) feline herpesvirus (n = x/7), bronchopneumonia (n = x/7) and discospondylitis (n = x/7). Sampling of the tarsal joints most frequently identified an increased proportion of neutrophils, consistent with IMPA. Eighteen cats (90%) received immunosuppressants. Eleven cats were started on prednisolone; eight had a poor response resulting in the addition of a second agent, euthanasia or acceptance of the persisting signs. One cat received ciclosporin and required an alternative second agent owing to adverse effects. Five cats were started on prednisolone and ciclosporin; three had a poor response and required an alternative second agent. One cat received prednisolone and chlorambucil and had a good response. Two cats (10%) received meloxicam and had a good response, although the clinical signs recurred when medication was tapered. A good outcome was achieved in 14/20 cats (70%) with IMPA. In the cats with a poor outcome 4/6 were euthanased and 2/6 had chronic lameness.
Relevance and novel information: Prognosis for feline IMPA can be good. Multimodal immunosuppression was often required. IMPA should be considered in lame cats, with or without pyrexia, when there is no evidence of trauma or infection. The tarsal joints should be included in the multiple joints chosen for sampling. Ligament laxity can occur in non-erosive feline IMPA.