Rickettsial disease IFA-IgG titres in auto-immune diseases: What do they imply?

Abstract

BACKGROUND: Rickettsial infections are known to present mimicking autoimmune disorders. The gold standard diagnostic test for rickettsial diseases is based on the detection of IgM and or IgG antibodies against these infections by immuno-fluorescent technique (IFA). While confirmation of rickettsial diseases warrant demonstration of rising or declining antibody titres between acute and convalescent samples, high titres of either IFA-IgM or IFA-IgG in acute phase serum in patients with a compatible clinical illness may help in the presumptive diagnosis and introduction of anti-rickettsial antibiotics. During the IFA test, patient sera containing anti rickettsial antibodies are made to react with rickettsial antigens that are grown in cell culture media. However, presence of nuclear material in these cell cultures may react with anti-nuclear antibodies that are produced in autoimmune disorders and cause a false positive immunofluorescent signal. METHODS & MATERIALS: In order to evaluate the reactivity of rickettsial disease IFA-IgG test [IFA-IgG-OT (Orientia tsutsugamushi) and IFA-IgG-SFG (spotted fever group)] among patients with autoimmune diseases, an analytical cross-sectional study was carried out using sera of 38 patients with confirmed auto-immune diseases. RESULTS: The 38 patients included 15 systemic lupus erythematosus (SLE), 5 autoimmune-thyroiditis, 13 idiopathic-thrombocytopenia (ITP), 4 autoimmune-haemolytic-anaemia (AIHA), 1 polymyositis, 1 polyglandular syndrome and 1 Anti-phospholipid syndrome. The IFA-IgG reactivity of ≥ 1:128 was noted in 14/38 (37%); IFA-IgG-SFG in 7, IFA-IgG-OT in 3 and for both in 4. Of the 14; titre of 1:128 in 2, 1:256 in 4, 1:512 in 5, >1: 1024 in 3 and 8/14 (57%) were SLE, 3/14 (21.4%%) were ITP, 2/14 (14.3%) were AIHA, 1/14 (7.1%) were polymyositis and none were thyroiditis. 8/14 had received anti-rickettsial antibiotics during the early stages of illness based on the clinical presentation and high IFA-IgG titres. CONCLUSION: There was a significant reactivity of Rickettsial disease IFA-IgG assay in auto-immune diseases. Further studies are needed in order to ascertain whether this is due to recent rickettsial infections, false positive cross reactivity of autoimmune antibodies with rickettsial antigens or with cell culture nuclear antigens. We did not carry out IFA-IgM due to non-availability and non-affordability.