Objective: The aim of this study was to assess the clinical significance of serological profiles suggestive of chronic Q fever after acute infection. Methods: A prospective follow-up study consisting of two separate cohorts was conducted to monitor the serological evolution of Q fever. The first cohort comprised subjects with acute Q fever diagnosed in 2004-2007 and the second enrolled subjects whose infection occurred in 2009. The indirect immunofluorescence assay was used for serological monitoring, with serum PCR testing added for subjects whose serological profiles revealed high titers of anti-phase I IgG ≥ 800, titers suggestive of chronic Q fever. Results: In the first cohort of 92 persons, seventeen (18%) subjects had serological profiles suggestive of chronic Q fever (titers of anti-phase I IgG: 1280-5120, median: 1280) after a median follow-up period of 606.5 days. After a further follow-up (median period: 592 days) exclusively for those seventeen subjects, serological resolution with fourfold decrease of titers of anti-phase I IgG was noted in five of them. In the second cohort, only one (4%) of the twenty-eight subjects had high levels of anti-phase I IgG 180 days after acute infection. All the eighteen subjects with high levels of anti-phase I IgG were asymptomatic and had negative serum PCR testing. The different prevalence of subjects with high titers of anti-phase I IgG in the two cohorts was associated with duration of follow-up period (P < .01). Conclusions: Subjects with high titers of anti-phase I IgG ≥ 800 was not uncommon and might not be detected until more than six months after acute Q fever infection. Asymptomatic subjects with high levels of anti-phase I IgG alone should not be treated as chronic Q fever and might not need continued serological monitoring in the absence of predisposing factors to chronic Q fever.
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