Low antibodies titer and serological cross-reaction between Coxiella burnetii and Legionella pneumophila challenge the diagnosis of mediastinitis, an emerging Q fever clinical entity

Background Coxiella burnetii is an intracellular and fastidious bacterium responsible of acute and persistent Q fever infection. Endocarditis and vascular infections are the most common serious complications of acute Q fever. Case report We report the case of a 63-year-old man that presented a mediastinitis associated with a prosthetic vascular infection. Serological cross-reaction was observed between Coxiella burnetii, the agent of Q fever, and Legionella pneumophila with higher antibodies titer for L. pneumophila (IgG = 1:512) than for C. burnetii (phase I IgG = 1:400). We performed western blot with cross-adsorption that supports the diagnosis of C. burnetii infection. Two weeks later, a positive qPCR and culture for C. burnetii on swab taken from the mediastinal cutaneous fistula confirmed the definitive microbiological diagnosis of Q fever mediastinitis. Conclusion Cross-reactivity between C. burnetii and Legionella spp. has long been known and should be considered in patients with persistent infections. It is important to establish the definite diagnosis because the antibiotic treatment regimens and duration are significantly different. To the best of our knowledge, we reported here the first case of mediastinitis associated to C. burnetii and we diagnosed this persistent infection despite low anti- C. burnetii phase I IgG levels.