Defining the clinical characteristics of Q fever endocarditis: A case-control study in China

Q fever is a worldwide zoonosis caused by Coxiella burnetii. Atypical presentations of Q fever can cause diagnostic difficulty or be misdiagnosed. Here we compared the clinical and diagnostic features of Q fever endocarditis and endocarditis caused by other bacteria to identify features of Q fever endocarditis that might facilitate early diagnosis.

This was a retrospective case-control study of eight cases of Q fever endocarditis diagnosed between 2000 and 2018 at Peking Union Medical College Hospital in China and 24 age- and gender-matched patients diagnosed with bacterial endocarditis over the same period. Clinical and laboratory data were collected and compared between groups.

The median time interval between symptoms and diagnosis was significantly longer in the case group than the control group (8.0 months (IQR 7.0-16.0) vs. 4.0 months (IQR 1.0-7.0); p = 0.002). Patients in case group had significantly lower white blood cell counts (5.8 ± 2.4 × 109/L vs. 10.0 ± 3.4 × 109/L; p = 0.003), percentage of neutrophil (62.4 ± 14.7% vs. 79.1 ± 9.2%; p = 0.014), high-sensitivity C-creative protein levels (21.1 mg/L (IQR 18.5-32.8) vs. 45.3 mg/L (IQR 32.9-54.3); p = 0.038), and platelet counts (133 ± 73 vs. 229 ± 65; p = 0.001) but higher levels of rheumatoid factor (104.3 U/L (IQR 99.0-132.8) vs. 10.2 U/L (IQR 6.9-32.5); p = 0.011) than controls. Elevated creatinine (50.0% vs. 12.5%; p = 0.047) and liver enzymes (50.0% vs. 0%; p = 0.002) were more common in cases than controls. Q fever endocarditis was less frequently diagnosed than controls before cardiac surgery (62.5% vs. 100%; p = 0.011), with negative blood cultures in all cases.

The diagnosis of Q fever endocarditis can easily be delayed compared to other causes of infectious endocarditis. Patients with chronic fever and new valve dysfunction require careful assessment, especially when presenting with negative blood cultures and high rheumatoid factor levels. Clinical and laboratory evaluation of these patients should include routine serological testing for C. burnetii.