Abstract
Q fever (caused by Coxiella burnetii) is thought to have an almost world-wide distribution, but few countries have conducted national serosurveys. We measured Q fever seroprevalence using residual sera from diagnostic laboratories across Australia. Individuals aged 1-79 years in 2012-2013 were sampled to be proportional to the population distribution by region, distance from metropolitan areas and gender. A 1/50 serum dilution was tested for the Phase II IgG antibody against C. burnetii by indirect immunofluorescence. We calculated crude seroprevalence estimates by age group and gender, as well as age standardised national and metropolitan/non-metropolitan seroprevalence estimates. Of 2785 sera, 99 tested positive. Age standardised seroprevalence was 5.6% (95% confidence interval (CI 4.5%-6.8%), and similar in metropolitan (5.5%; 95% CI 4.1%-6.9%) and non-metropolitan regions (6.0%; 95%CI 4.0%-8.0%). More males were seropositive (6.9%; 95% CI 5.2%-8.6%) than females (4.2%; 95% CI 2.9%-5.5%) with peak seroprevalence at 50-59 years (9.2%; 95% CI 5.2%-13.3%). Q fever seroprevalence for Australia was higher than expected (especially in metropolitan regions) and higher than estimates from the Netherlands (2.4%; pre-outbreak) and US (3.1%), but lower than for Northern Ireland (12.8%). Robust country-specific seroprevalence estimates, with detailed exposure data, are required to better understand who is at risk and the need for preventive measures.
Highlights
Q fever is a zoonotic disease caused by the highly infectious bacterium Coxiella burnetii, which has an almost world-wide distribution
A recent study among Australian blood donors estimated that 29%–39% of people with symptomatic Q fever in the past had not been diagnosed with the disease [4]
A total sample of 1800 would produce a confidence interval (CI) of ⩽±1.1% for an estimate of Q fever seroprevalence for Australia in the expected range of 1%–5% and to detect a minimum of 3.6% difference between seroprevalence in nonmetropolitan and metropolitan regions [17]
Summary
Q fever is a zoonotic disease caused by the highly infectious bacterium Coxiella burnetii, which has an almost world-wide distribution. C. burnetii infects both wild and domestic animals and their ticks, and humans are exposed by inhalation of infected droplets or dust. In Australia, Q fever has been a notifiable disease in humans since 1977 [2], and in the past 5 years (2013–2018) there have been on average 517 cases reported annually (notification rate 2.1/100 000) [3]. There is a consensus that Q fever notifications underestimate infection rates, due to the asymptomatic nature of many acute infections, as well as underestimating disease rates, because the signs and symptoms are non-specific and diagnosis relies on clinicians suspecting Q fever, and ordering appropriate tests. A recent study among Australian blood donors estimated that 29%–39% of people with symptomatic Q fever in the past had not been diagnosed with the disease [4]
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