High Incidence of Asymptomatic Phase I IgG Seroconversion After an 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 aged ≥18 years; (2) acute Q fever diagnosis, defined as suggestive symptoms in the presence of phase II immunoglobulin G (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 6 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 6 months after acute Q fever diagnosis. All patients with classic serological criteria for chronic Q fever diagnosis remained asymptomatic despite no specific treatment, with a median (quartile 1-quartile 3 [Q1-Q3]) follow-up of 26.5 (14-44) months in this subgroup. No cases of Q fever endocarditis nor other persistent focalized infection forms were observed during the study period.

Conclusions: A significant proportion of acute Q fever patients develop classic serological criteria for chronic Q fever diagnosis in the absence of additional data of chronic Q fever. Consequently, phase I IgG cutoff titers >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.