High incidence of asymptomatic phase I IgG seroconversion after acute Q fever episode: implications for chronic Q fever diagnosis.

BACKGROUND The aim of this study was to describe the natural history of acute Q fever, including its clinical and serological evolution and progression to chronic Q fever. METHODS Observational cohort study (January 2011-September 2020) performed at Valme University Hospital (Seville, Spain). Inclusion criteria: 1) Patients over the age of eighteen; 2) Acute Q fever diagnosis, defined as suggestive symptoms in the presence of phase II IgG titer > 1:256; 3) At least 6 months follow-up after the acute Q fever episode. The incidence of seroconversion to a chronic Q fever serological pattern, defined as phase I IgG titers ≥ 1:1024 six months after acute Q fever diagnosis, was assessed. RESULTS During the study period, 117 patients were included. Thirty-four (29%) patients showed phase I IgG titers ≥ 1:1024 six months after acute Q fever diagnosis. All patients with classic serologic criteria for chronic Q fever diagnosis remained asymptomatic despite no specific treatment, with a median (Q1-Q3) follow-up of 26.5 (14-44) months in this subgroup. No cases of Q fever endocarditis neither other persistent focalized infection forms were observed during the study period. CONCLUSIONS A significant proportion of acute Q fever patients develops classic serologic criteria for chronic Q fever diagnosis in the absence of additional data of chronic Q fever. Consequently, phase I IgG cutoff titer > 1:800 should not be used as a criterion to consider such a diagnosis. The incidence of persistent focalized infection forms after acute Q fever is extremely low and does not justify the use of prophylaxis strategies.