�010 vol. 34 no. 1 AUSTRALIAN AND NEW ZEALAND JOURNAL OF PUBLIC HEALTH ��© 2010 The Authors. Journal Compilation © 2010 Public Health Association of Australia

A serosurvey of Coxiella burnetii infection in children

and young adults in South West Queensland

Neil ParkerDarling Downs Public Health Unit, Queensland Health

Jennifer RobsonSullivan Nicolaides Pathology, Queensland

Morton BellDarling Downs Public Health Unit, Queensland Health

Q fever is a worldwide zoonosis

causing a wide spectrum of disease

in adults, and is increasingly

recognised as an issue for children.1,2

The vaccine licensed in Australia is not

recommended for children under 15 years

of age and requires prevaccination screening,

including history, serology and skin testing.3

Notif ications of disease underestimate

incidence, and this underestimate is probably

higher in rural areas where doctors are few and

rural workers travel considerable distances

for medical services.1 The underestimate

may be higher in Q fever as serology is

often negative at first presentation, and rural

people may not return for repeat serology

when their health improves.1 A serosurvey

provides an alternative means of assessing

the need for population based vaccination

programs.

The Australian Federal Government funded

a National Q Fever Management Program

from 2001 to 2006.4 Phase I funded medical

consultations and laboratory tests, and its

main beneficiaries were abattoir workers.

Phase II included graziers. It began several

months after Phase I, and only provided free

vaccines and skin test solution.

MethodsSera collected by a private laboratory for

Submitted: August 2008 Revision requested: January 2009 Accepted: September 2009Correspondence to: Dr Neil Parker, Darling Downs Public Health Unit, PO Box 1775, Toowoomba Queensland 4350. Fax (07) 4639 4772; e-mail: neil_parker@health.qld.gov.au

Abstract

Objective: To describe the

seroepidemiology of Coxiella burnetii,

the causative agent of Q fever, in those

under 25 years of age in South West

Queensland.

Methods: A convenience sample of

residual sera from a diagnostic laboratory

was tested for C. burnetii antibodies by

immunofluorescence at 1:10 dilution.

Prevalence and annual incidence were

calculated from the results.

Results: Twenty-nine of 447 (6.5%, 95%

CI 4.5%-9.2%) samples were positive.

Seropositivity increased from 2.5% in

those <15 (95% CI 1.0%-5.5%) to 11.0%

in those 15-24 years old (95% CI 7.4%-

16.0%). The estimated annual incidence for

the latter age group was 7.7 per 1,000.

Conclusions: Q fever is a relatively

common infection in South West

Queensland, even in those aged <15 years

for whom the vaccine is not recommended.

Implications: Vaccination programs, such

as the federally funded National Q fever

Management Program, are needed in this

and similar high risk rural areas.

Key words: Seroepidemiology, Coxiella

burnetii, Q fever, children, rural, Australia.

Aust NZ J Public Health. 2010; 34:79-82

doi: 10.1111/j.1753-6405.2010.00478.x

clinical purposes were tested using a pre-

immunisation serological screen. The study

area was defined by postcodes >4416 and

<4498, which includes all of the Australian

Bureau of Statistics (ABS) South West

Queensland Statistical Division and has

the highest rate of Q fever in Australia at

107 per 100,000 population in 2006.4 Some

adjacent areas in Central West Queensland

and the Darling Downs fall into this postcode

range.

Sullivan Nicolaides Pathology was the

only private provider for most of the study

area. Sera from ambulatory patients were

tested for IgG antibodies to phase 2 Coxiella

burnetii by the indirect immunofluorescent-

antibody (IFA) assay. Sera from patients

known to be febrile were excluded. Sera

were diluted 1:10 in 3% chick yolk sac

in phosphate buffered saline and spotted

onto slides coated with C.burnetii phase

2 organisms prepared by the Institute of

Veterinary and Medical Research (Adelaide,

South Australia). Results were classified

as positive, negative, equivocal or weakly

positive. In final data analysis equivocal

results were combined with negative results,

and weakly positive results were combined

with positive results.

Results were de-identified before analysis,

with age, sex, postcode and Q fever serology

result available to researchers. With no

Article Lifestyle

�0 AUSTRALIAN AND NEW ZEALAND JOURNAL OF PUBLIC HEALTH �010 vol. 34 no. 1© 2010 The Authors. Journal Compilation © 2010 Public Health Association of Australia

known studies in similar populations a sample size could not be

calculated. The plan was to obtain 100 samples from each five year

cohort between five and 25. As significant time elapsed without

obtaining 100 samples in younger age groups, children <5 were

also included.

The study received approval from Toowoomba Health Service

District Human Research Ethics Committee. Chi was estimated

squared values were performed with EpiInfo version 6 Statcalc,

November 1993. Agresti–Coull5 95% confidence intervals (95%

CI) were calculated with the utility for calculating binomial

confidence limits for a proportion (AusVet Animal Health

Services site http://www.ausvet.com.au/epitools/content.

php?page=CIProportion accessed 21/1/2009).

The incidence for the 15-24 age group using the following

formula, where page(15-24)

and page(0-14)

are the average prevalences for

ages 15-24 and 0-14 respectively, and age difference is average age

for the 15-24 group minus the average age for the 0-14 group.6

difference 1/age

14)-age(0

24)-age(15

p-1p-1

- 1 r ⎥⎥⎦

⎤

⎢⎢⎣

⎡=

ResultsA total of 466 sera were collected from 19 January 2001 to

4 November 2002. Nineteen samples were excluded, leaving 447

for analysis. Three people had dual collections, with the same

result both times (two negative, one positive). Three negative

samples had incorrect collection dates recorded. Thirteen samples

(all negative) collected within the study area had postcodes outside

this area.

Of the 447 samples, 25 were positive, four weakly positive, four

equivocal and 414 negative (Table 1).

Trend with ageThe prevalence for the total sample was 6.5% (29 of 447, 95%

CI 4.5%-9.2%). Seropositivity in the originally planned age

group of 5-24 years was 7.1% (28 of 395, 95% CI 4.9%-10.1%).

As expected the rate increased with age, from 2.5% in those <15

to 11.0% in those 15-24 years old (see Table 1 for confidence

intervals). The trend by five-year cohort was highly significant

with a chi squared for linear trend of 12.24 (p<0.0005).

Association with sexIn those >14 years, 15.6% of males and 8.9% of females were

Table 1: Seropositivity for Q fever in South West Queensland by age, sex and health district.

Total samples Positive Weakly Positive Total Positive Rate 95% CI

Age (years) 0 to <5 52 0 1 1 1.9% 0.6%-11.1%

5 to <10 78 2 0 2 2.6% 0.2%-9.4%

10 to <15 107 3 0 3 2.8% 0.6%-8.3%

15 to <20 101 7 2 9 8.9% 4.6%-16.3%

20 to <25 109 13 1 14 12.8% 7.7%-20.5%

Total 447 25 4 29 6.5% 4.5%-9.2%

0 to <15 237 5 1 6 2.5% 1.0%-5.5%

15 to <25 210 20 3 23 11.0% 7.4%-16.0%

Total 447 25 4 29 6.5% 4.5%-9.2%

Sex Female 253 14 3 17 6.7% 4.2%-10.6%

Male 194 11 1 12 6.2% 3.5%-10.6%

Total 447 25 4 29 6.5% 4.5%-9.2%

Sex and age ≥15 years Female 146 10 3 13 8.9% 5.2%-14.8%

Male 64 10 0 10 15.6% 8.5%-26.6%

Total 210 20 3 23 11.0% 7.4%-16.0%

Sex and age <15 years Female 107 4 0 4 3.7% 1.2%-9.5%

Male 130 1 1 2 1.5% 0.1%-5.8%

Total 237 5 1 6 2.5% 1.0%-5.5%

District Roma 300 17 1 18 6.0% 3.8%-9.3%

Charleville 128 7 3 10 7.8% 4.2%-13.9%

Other 19 1 0 1 5.3% 0.9%-26.5%

Total 447 25 4 29 6.5% 4.5%-9.3%

District and age ≥15 years Roma 151 14 0 14 9.3% 5.5%-15.1%

Charleville 57 6 3 9 15.8% 8.3%-27.6%

Other 2 0 0 0 0.0% Total 210 20 3 23 11.0% 7.4%-16.1%

Parker, Robson and Bell Article

�010 vol. 34 no. 1 AUSTRALIAN AND NEW ZEALAND JOURNAL OF PUBLIC HEALTH �1© 2010 The Authors. Journal Compilation © 2010 Public Health Association of Australia

Lifestyle Coxiella burnetii infection in children and young adults

positive. This difference is not statistically significant (p=0.15,

95% CI 8.5%-26.6% vs 5.2%-14.8%). In younger children there

were too few positive tests for meaningful comparison.

Comparisons between districts South-west Queensland ABS Statistical Division (SD) had two

health service districts, Roma (300 samples) with six Statistical

Local Areas (SLAs) and Charleville (128 samples) with the four

most westerly SLAs. Charleville district had a higher rate (7.8% vs

6.0%) that was more pronounced in those 15-24 (15.8% vs 9.3%),

but the difference was not statistically significant. There were 19

samples from the study’s postcode range outside this SD.

There were 9,793 people under 25 in the SD in 2001/02, so the

428 samples from this area represented 4.4% of the population.

IncidenceThe calculated average annual incidence for those aged 15-24

was 7.7 per 1,000. Average incidence should be used with caution

as outbreaks are a feature of Q fever.1,6

DiscussionLimitations of this study and serosurveys

Laboratory tests

The most important uncertainties are a test’s sensitivity and

specificity for past Q fever infection, and these vary with the cut-off

titre used.7 Specificity is reduced by cross reactions with antibodies

induced by other bacteria such as Legionella and Bartonella species.1

Sensitivity is reduced when higher titres are used. This study employed

a titre of 1:10, which is routinely used as a prevaccination screen.

This titre prioritises sensitivity over specificity, as severe side-effects

may occur following vaccination of immune people.3 Serosurveys

have used titres ranging from 1:87 to 1:64.8 Different cut-offs may

be justified by different background rates of seropositivity,7 but make

comparisons between serosurveys difficult.

Sensitivity decreases over time. The IFA test remains positive

for 10 years or more, so titre decline is unlikely to be an important

factor in this study.9

The sample population

The survey used a convenience rather than a random sample.

To reduce bias towards inclusion of persons infected with Q fever,

those with a known febrile illness were excluded. A bias against

including positives is possible as children from remote properties

are more likely to have Q fever, but have less access to laboratory

testing. More samples were collected from females than males

(ratio 1.3:1), which could lead to an underestimate.

The influence of vaccination

Vaccination may increase seropositivity, but an IgG response

is usually seen only in those who already have low positive titres

(IFA 1:10 or 1:20).10 The observed seroprevalence in Roma district

was not higher than Charleville, despite the fact Roma had a large

Q fever vaccination campaign in 2000, while one in Charleville

was cancelled because of a vaccine shortage.

While the vaccination rate in the study area is unknown, the

federally funded vaccination program occurred predominantly after

the survey.3,4 In Queensland Phase I began in November 2001, and

Phase II in July 2002. Phase I was predominately for meatworkers, of

whom there were few in South West Queensland. Phase II extended

the program to farmers, but our last sample from a person >14 years

was taken on 28 March 2002, before Phase II began.

Infection and disease

About 50% of Q fever infections are asymptomatic, but this rate

is higher in children.1 Serosurveys cannot provide an estimate of

disease burden unless a reliable estimate of the age-specific rates

of symptomatic disease in infected persons is known. These rates

may vary from place to place, and also with the type of contact

with animals, as different activities result in different ranges of

inhaled numbers of organisms.

Australian serosurveys and estimates of annual risk

All the published Australian Q fever serosurveys we found were

in occupational risk groups with one exception; a 1953 serosurvey

in Northern Territory Aboriginal people.11 In 1998, Casolin listed

eight published seroprevalence studies, mainly in abattoirs.12 One

listed paper briefly mentions that only one in 577 opportunistically

tested sera from New South Wales was positive.13 No demographic

details were given. Casolin’s study, and four published since then

are listed in Table 2. These are all pre-vaccination studies, so are in

high risk populations excluding children <15. The seroprevalence

in these studies varies between 4.2% and 21.2%, comparable to

the rates in this study.

The annual notified incidence of Q fever in Australia peaked at

0.049 per 1,000 in 1993.17 Estimates of Q fever disease in Australian

Table 2: Comparison of skin test positive and blood test positive results in previous Australian serosurveys.

Author Year Type of Total Positive Positive Skin test positive Seropositive in those blood Test tested serology skin test in those seropositive in skin test positive

n % n % n % n %

Casolin12 1999 IF 829 44 5.3% 51 6.2% 17 38.6% 17 33.3%

CF 829 35 4.2% 51 6.2% 14 40.0% 15 29.4%

Hutson14 2000 EIA / CF 979 208 21.2% 345 35.2% 165 79.3% 165 47.8%

Taylor15 2001 CF 265 25 9.4% 42 15.8% 18 72.0% 18 42.9%

Mak16 2003 IF 47 3 6.4% 30 63.8% 2 66.7% 2 6.7%Greig21 2005 ns 8464 907 10.7% 1320 15.6% 644 71.0% 644 48.8%

Note: ns = not stated All the above surveys were in occupationally at-risk groups, and did not include children under 15 years of age.

�� AUSTRALIAN AND NEW ZEALAND JOURNAL OF PUBLIC HEALTH �010 vol. 34 no. 1© 2010 The Authors. Journal Compilation © 2010 Public Health Association of Australia

abattoir workers vary from 3.318 to 10 per 1,000 per year, the latter

being the average rate in one Queensland abattoir between 1968 and

1977, excluding one ‘epidemic’ year.19 An economic evaluation of

Q fever vaccination used an estimate 30 infections per 1,000 per

year.20 A similar rate (45.0 per 1,000) was found in Victorian abattoir

workers.21 The seropositivity in this study was 11.7% (1076 of 9196),

similar to our finding of 11.0% in those aged 15-24.

Overseas studies in children

There are numerous published serosurveys from other countries,

some of which include children.1 Eight of 1,200 hospitalised

children in Greece had sera positive for Q fever.22 A Spanish

survey found 12% of those <15 years were positive.23 Comparisons

between surveys are difficult for the reasons outlined above.

Conclusion and recommendationsThe National Q fever Management Program began with abattoir

workers (Phase I) and only after seven months extended to graziers

(Phase II). This study shows that unselected young people in

South West Queensland have seropositive rates approaching those

of abattoir workers, so calling into question the prioritisation of

abattoir workers over rural workers.

High seropositive rates in young people in South West

Queensland demonstrate the need to focus vaccination strategies

on this population. As the risk in these younger people approaches

the risk in abattoir workers, the life-time risk for the population as

a whole will be greater. We therefore recommend that all children

in South West Queensland be vaccinated at age 15. Rural areas

with a similar risk could be easily determined through an analysis

of routine notification data by SLA. Further research is needed

before recommendations for younger children are formulated.2

In the meantime, clinicians should make individual decisions for

children <15 who engage in high-risk farm activities, particularly

butchering and birthing.

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in Queensland. Med J Aust. 2004;180(11):596-7.3. National Health and Medical Research Council. Q fever. In: The Australian

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4. Gidding HF, Wallace C, Lawrence GL, McIntyre PB. Australia’s national Q fever vaccination program. Vaccine. 2009;27(14):2037-41.

5. Brown LD, Cai TT, DasGupta A. Interval estimation for a binomial proportion. Statistical Science. 2001;16(2):101-33.

6. Beutels P, Bonanni P, Tormans G, Canale F, Cuneo Crovari P. An economic evaluation of universal pertussis vaccination in Italy. Vaccine. 1999;17(19): 2400-9.

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Parker, Robson and Bell Article