Serological diagnosis and follow-up of asymptomatic and acute Q fever infections

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During an outbreak of Q fever at a farmer's market in Soest (North Rhine-Westphalia, Germany) in 2003, we examined 263 serum samples of presumably infected persons for Q fever antibodies. One hundred and seventy-one of these patients were tested positive for acute Q fever infection. Furthermore, 29 persons of certain risk groups like pregnant women ( n = 11 ) or patients with valvular heart disease ( n = 18 ) were examined. Among these, in four pregnant women and two heart patients an acute but asymptomatic infection could be diagnosed. With 30 patients we performed a serological follow-up for 8–60 weeks. In our study, phase 2 (PH2)-IgM antibodies as a marker for acute infection were present in all 30 patients 3–4 weeks after onset of clinical signs and disappeared 3–4 months later. Six weeks to three months after clinical manifestation, all patients developed PH1-IgG antibodies in low levels with no clinical signs of chronic Q fever. Three patients, including one pregnant woman showed high-level titres and were treated for chronic Q fever. Eleven patients with low PH1-IgG antibodies and all three patients with high titres developed IgA antibodies from 10 weeks after clinical manifestation; therefore PH1-IgA cannot be used as the only serological marker for chronic Q fever. Chronic infections were indicated only by a continuous increase of PH1 antibodies and a high persistence of PH2-IgG. We therefore conclude that the exclusion of chronic Q fever infection by a single serological examination cannot be done. At least three consecutive tests should be performed, that is 3, 6, and 9 months after initial Q fever infection.