Persistent high antibody titres against Coxiella burnetiiafter acute Q fever not explained by continued exposure to the source of infection: a case-control study

BACKGROUND From 2007 to 2010, (the southern part of) the Netherlands experienced a large Q fever epidemic, with more than 4,000 reported symptomatic cases. Approximately 1 - 5% of the acute Q fever patients develop chronic Q fever. A high IgG antibody titre against phase I of Coxiella burnetii during follow-up is considered a marker of chronic Q fever. However, there is uncertainty about the significance and cause of persistence of high IgG phase I antibody titres in patients that do not have any additional manifestations of chronic Q fever. We studied whether continued or repeated exposure to the source of infection could explain elevated IgG phase I antibody levels. METHODS A case-control study was performed to analyze predictors for possible chronic Q fever. Possible chronic Q fever cases (n = 53) are patients with phase I IgG antibody titre ≥1:1,024 at any point in the 9 - 18 months after acute Q fever diagnosis, with a negative PCR test result for C. burnetii DNA and without other disease manifestations. Controls (n = 110) are acute Q fever patients that did not develop chronic Q fever, and who consistently had phase I IgG antibody titre <1:1,024 during the 9 - 18 months follow-up. Binary logistic regression was performed to analyze the effect of living close to an infected farm on the high antibody titres. A longitudinal analysis described the serological profiles of cases and controls. RESULTS Proximity to infected farms and contact with animal placental material were not associated with an increased risk for possible chronic Q fever. Possible chronic Q fever patients have high IgG phase II as well as IgG phase I antibody titres, even after 48 months of follow-up. CONCLUSION We were unable to explain the cause of persistent high IgG phase I titres among possible chronic Q fever patients by being continuously exposed to the source of infection.