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

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.