Markers of exposure to spotted fever rickettsiae in patients with chronic illness, including fatigue, in two Australian populations

Abstract

Background: Some investigators believe that a proportion of chronically unwell patients, many with fatigue, have an underlying rickettsial disease. Aim: To investigate the prevalence of markers of rickettsial infection in patients with chronic illnesses. Design: Observational study. Methods: A 526 patient cohort with chronic illnesses from Melbourne, Australia and 400 control patients from Newcastle, Australia were assessed using serology, culture and PCR for the detection of rickettsiae. Rickettsial serology was performed on another cohort of 581 chronically unwell patients (and 34 non-fatigued patients from the same practice) from Adelaide, Australia. Results: Of the Melbourne patient cohort, 14/526 (3%) were real-time PCR positive for rickettsial DNA compared to none of the 400 control patients (P<0.001). Of these 14 patients, Rickettsia honei strain 'marmionii' was detected in 5 and isolated from 2. Rickettsaemia was seasonal, with more in winter (8/145; P<0.03) and less in spring (0/143; P<0.03). Positive rickettsial serology titres of ≥1:256 were seen in 206 (39%) patients. Of the Adelaide patient cohort, 238/581 (41%) had positive rickettsial antibodies titres. Of the 34 control sera, 5 (15%) were serologically positive (P<0.002). Both Melbourne and Adelaide patient cohorts had significantly higher seropositivity than the Newcastle control cohort (3/399; P<0.0001). Conclusions: In patients with chronic illness, rickettsial DNA in peripheral blood and/or rickettsial seropositivity may represent exposure to rickettsiae or underlying rickettsial diseases. It is not known whether the presence of rickettsiae is causally related to the patients' chronic illnesses, or reactivation of a latent rickettsial infection. © The Author 2008. Published by Oxford University Press on behalf of the Association of Physicians. All rights reserved.