Q fever: single-point source outbreak with high attack rates and massive numbers of undetected infections across an entire region.
BACKGROUND
In early 2009, a dairy-goat annex care farm in South Limburg, the Netherlands, reported 220 Coxiella burnetii-related 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.
Department of Sexual Health, Infectious Diseases, and Environmental Health, South Limburg Public Health Service, Geleen, The Netherlands. volker.hackert@ggdzl.nl
Volker H
VH
Hackert
Wim
W
van der Hoek
Nicole
N
Dukers-Muijrers
Arnout
A
de Bruin
Sascha
S
Al Dahouk
Heinrich
H
Neubauer
Cathrien A
CA
Bruggeman
Christian J P A
CJ
Hoebe
D000328:Adult; D000368:Aged; D000383:Agriculture; D000818:Animals; D000907:Antibodies, Bacterial / Q000097:blood; D016358:Contact Tracing; D016997:Coxiella burnetii / Q000302:isolation & purification; D004196:Disease Outbreaks / Q000662:veterinary*; D005260:Female; D015511:Goat Diseases / Q000453:epidemiology* / Q000382:microbiology; D006041:Goats; D006801:Humans; D015994:Incidence; D008297:Male; D008875:Middle Aged; D009426:Netherlands / Q000453:epidemiology; D011247:Pregnancy; D011778:Q Fever / Q000453:epidemiology* / Q000662:veterinary*; D012756:Sheep; D012757:Sheep Diseases / Q000453:epidemiology / Q000382:microbiology; D015047:Zoonoses / Q000453:epidemiology / Q000382:microbiology
D000907:Antibodies, Bacterial