IgM and IgA rheumatoid factors in canine polyarthritis.

IgM and IgA rheumatoid factor (RF) were detected by ELISA using a purified dog IgG as antigen in normal controls (N = 84), dogs with unclassified polyarthritis (N = 95), dogs with rheumatoid arthritis (RA) (N = 22), dogs with systemic lupus erythematosus (SLE) (N = 35), dogs with leishmaniasis or heart worm disease (N = 20) and dogs with pyometra (N = 16). Frequency and titre of IgM and IgA RF are low and comparable (P < 0.05) in dogs with unclassified polyarthritis or RA: respectively 24.2% and 27.3% for IgM RF and 21.0% and 18.2% for IgA RF; the mean titre being respectively 0.781 +/- 0.581 and 0.649 +/- 0.365 for IgM RF, and 0.774 +/- 1.331 and 0.740 +/- 1.169 for IgA RF. The frequencies of IgM and IgA RF are a little higher in dogs with SLE (IgM RF: 37.1%, IgA RF: 25.7%) and higher in dogs with leishmaniasis or heart worm disease (45.0% and 30.0%), especially in dogs with pyometra (68.7% and 37.5%). So, although dogs can produce IgM and IgA RF, these auto-antibodies are uncommon in dogs with RA. Furthermore, when RF are present their titre is much lower than in human RA.