Q Fever: Single-Point Source Outbreak With High Attack Rates and Massive Numbers of Undetected Infections Across an Entire Region

• Published in: Clinical infectious diseases 2012-12, Vol.55 (12), p.1591-1599
• Main Authors: Hackert, Volker H., van der Hoek, Wim, Dukers-Muijrers, Nicole, de Bruin, Arnout, Dahouk, Sascha Al, Neubauer, Heinrich, Bruggeman, Cathrien A., Hoebe, Christian J. P. A.
• Format: Article
• Language: English
• Subjects: Adult; Aged; Agricultural population; Agriculture; Animals; Antibodies, Bacterial - blood; ARTICLES AND COMMENTARIES; Bacterial diseases; Biological and medical sciences; Contact Tracing; Coxiella burnetii; Coxiella burnetii - isolation & purification; Disease Outbreaks - veterinary; Disease transmission; Epidemics; Female; General practice; Goat Diseases - epidemiology; Goat Diseases - microbiology; Goats; Health risk assessment; Human bacterial diseases; Humans; Immunoglobulins; Incidence; Infections; Infectious diseases; Male; Medical sciences; Middle Aged; Netherlands - epidemiology; Pregnancy; Public health; Q fever; Q Fever - epidemiology; Q Fever - veterinary; Retirement communities; Rickettsial diseases; Seroepidemiologic studies; Sheep; Sheep Diseases - epidemiology; Sheep Diseases - microbiology; Tropical bacterial diseases; Zoonoses - epidemiology; Zoonoses - microbiology
• ISSN: 1058-4838; 1537-6591
• DOI: 10.1093/cid/cis734

Background. In early 2009, a dairy-goat annex care farm in South Limburg, the Netherlands, reported 220 Coxiella burnetii-vehted abortions in 450 pregnant goats. These preceded human cases and occurred in a region that was Q-fever free before 2009, providing a unique quasi-experimental setting for investigating regional transmission patterns associated with a Q-fever point source. Methods. Index-farm residents/employees, visitors, and their household contacts were traced and screened for C. burnetii. Distribution of community cases was analysed using a geographic information system. True incidence, including undetected infections, was estimated regionwide by seroprevalence in a pre-versus postoutbreak sample, and near-farm by immunoglobulin M seroprevalence in a municipal population sample. Environmental bacterial load was repeatedly measured in surface and aerosol samples. Results. Serological attack rate was 92% (24/26) in index-farm residents/employees, 56% (28/50) in visitors, and 50% (7/14) in household contacts, and the clinical attack rate (ie, the proportion of persons seropositive for acute infection who also had clinical illness) was ≥80%. Notified symptomatic community cases (n = 253) were scattered downwind from the index farm, following a significant exposure-response gradient. Observed incidence ranged from 6.3% (0–1 km) to 0.1% (4–5 km), and remained high beyond. True incidence of infections was estimated at 2.9% regionwide, extrapolating to 8941 infections; estimated near-farm incidence was 12%. Coxiella burnetii load was high on-farm (2009), and lower off-farm (2009-2010). Conclusions. Linking a single dairy-goat farm to a human Q-fever cluster, we show widespread transmission, massive numbers of undetected infections, and high attack rates on-and off-farm, even beyond a 5-km high-risk zone. Our investigation may serve as an essential case study for risk assessment in public health and related fields such as bioterrorism response and preparedness.