Hatchette, T. et al. Goat-associated Q fever: A new disease in Newfoundland. EID 2001; 7: 413419.
1. What was the main purpose/hypothesis of the study?
An epidemiologic investigation and serologic survey were conducted to determine the
extent of the outbreak in animals and humans, the clinical illness, and risk factors for Q
Fever.
SUMMARY: To determine risk factors for Q Fever infection during an outbreak among
persons at goat farms.
2. What was the study design? What are its strengths and weaknesses?
DESIGN:
Cross-sectional study. During a brief period in time, a survey was conducted of persons
in the goat farm cooperative; interviews were conducted to obtain information about
exposures and sera were collected to test for Coxiella burnetti antibodies. Persons
infected and noninfected with C.b. were identified and their exposures compared.
STRENGTHS:
Prevalence
Cost efficient
Can be done in a short time, no follow-up
Can examine the influence of many exposures
WEAKNESSES:
Since exposures and outcomes are determined at the same time, may be unable to
establish a temporal relationship between exposure and outcome
Confounding is possible
Recall bias
3. What was the study population? Was it representative?
STUDY POPULATION:
HUMANS:
Goat farm [outbreak] cohort – farmers, workers and contacts [family members of the
farmers or workers and other persons who may have had contact with 8 goat farms in a
cooperative]. Interviewed using a detailed questionnaire and sera obtained to determine
antibodies to C. b. Three months later, follow-up sera obtained to assess seroconversion.
Community cohort – 154 volunteers from adjacent communities interviewed and sera
obtained.
Blood donor cohort – 387 random blood donors, primarily from urban areas had sera
obtained.
Family physicians in the area – submitted serum from patients with symptoms
compatible with Q Fever.
GOATS:
Eight farms in the Cooperative had 174 goats, with 10-38 goats per herd. Sera obtained
from 147 [84%] goats.
Sera obtained from 16 goats from other farms in the province.
REPRESENTATIVE:
Goat farm [outbreak] cohort – unclear whether this is the total population at risk. A total
of 179 provided serum; 146 [82%] completed questionnaires and their data were used to
identify risk factors for infection.
4. What exposures or risk factors were measured? Were there any biases or limitations in their
measurement?
EXPOSURES/RISK FACTORS:
A standardized questionnaire was administered to participants, who submitted serum, by
direct interview and included:
Demographic data
Detailed history of exposure to goats
Clinical history and symptoms
A total of 179 provided serum; 146 [82%] completed questionnaires.
BIASES/LIMITATIONS:
Assume interviewers were unaware of status [had illness] of interviewee but not
indicated.
Time between interview and onset of illness varied which may have resulted in
differences in recall of exposures.
5. What was the principal outcome of interest (infection or disease) and how was it measured?
Identify advantages and disadvantages with this measure.
OUTCOME:
Infected and noninfected persons
MEASURED;
Antibody titers [IgG] to C.b. phase I [chronic infection] and phase II [acute infection]
antigens were determined by IFA.
An antibody titer of >1:8 to phase I antigen was considered indicative of prior exposure
to C.b.
An antibody titer of 1:64 or a fourfold rise in titer between two separate serum samples to
phase II antigen was considered indicative of acute infection with C.b.
ADVANTAGES:
Antibody tests were able to distinguish acute from chronic infection in persons.
DISADVANTAGES:
Sensitivity and specificity of tests not provided.
Duration of antibodies not provided.
If more than one method is used to classify persons, a table showing which samples
were considered positive by which method is useful information; also documentation of
whether an single antibody titer of 1:64 is indicative of acute infection should be
provided.
6. What were the main findings? Do you agree or disagree? Support your position.
MAIN FINDINGS:
GOATS:
Clinical illness:
A total of 30 abortions occurred at 6 of the 8 farms; farms had abortion rates of 16-22%.
Serology:
82/147 [55.8%] were seropositive; prevalence by farm ranged from 10-100%; 63/147
[42.8%] had antibodies against phase I antigen [chronic infection] and 30/147 [20.4%] had
antibodies against phase II antigen [acute infection].
NUMBER OF SEROPOSITIVE GOATS DOES NOT AGREE – 82 AND 93 [63+30] [p.416]
Agent:
Agent was identified by EM and light microscopy, and demonstrated by PCR.
HUMANS:
Clinical illness:
The number of cases of Q Fever was not provided.
Serology:
Goat farm [outbreak] cohort – 80/179 [44.7%] had antibodies against phase II antigen; 66
had antibodies against phase II antigen that fulfilled the criteria for acute infection. Farm
seroprevalence ranged from 0-87.5%,
Community cohort – 35/154 [22.7%] were seropositive; 2 had antibodies against phase II
antigen [acute infection].
Blood donor cohort – 32/387 [8.3%] were seropositive [range from 6 sites 3.2-12.3%]; 5
had antibodies against phase II antigen [acute infection].
Cross-sectional study:
Of 146 subjects who gave serum and completed questionnaires, it appears that 60 were
infected and 86 were noninfected [Tables 2 & 3].
Infected persons were older than noninfected persons [38.5 vs 33.5 yrs; p=0.054].
Univariate analysis: 13 factors were significantly associated with infection [Table 3].
Multivariate analysis: 3 factors were significantly associated with infection – contact with
placenta [p<0.001], smoking history [p=0.001], eating pasteurized goat cheese [p=0.022].
Petting goats was of borderline significance [p=0.055].
AGREE/DISAGREE:
Goat farm [outbreak] cohort – not indicated as to whether this is the total population at
risk.
Unclear how eating pasteurized goat cheese could be related to risk of infection since
pasteurization should kill the agent. Suggests either that pasteurization was inadequate
or cheese was contaminated after pasteurization. Need to further examine the processing
and source of milk and cheese.
7. Was there any potential confounding in the data analyses? Was it considered in the data
analyses? Explain.
CONFOUNDING:
Infected and noninfected groups were comparable by sex [Table 2].
Possible confounding by age since the infected group was older than the noninfected
group [Table 2].
Of the 146 participants, 9 [6.2%] were farmers, 58 [39.7%] were workers and 79 [54.1%]
were contacts [p.415]. Hence, there could have been some confounding by occupation.
Information on race/ethnicity and SES not provided.
CONSIDERED:
Univariate analysis revealed that being a farmer or worker was a risk factor for infection,
whereas, being a contact was protective [Table 3].
Unclear whether age was included in the multivariate analysis.
8. Were there shortcomings/limitations to the study? If so, were they of sufficient magnitude to
invalidate the results?
SHORTCOMINGS/LIMITATIONS:
Goat farm [outbreak] cohort – not indicated as to whether this is the total population at
risk.
Unclear whether age was included in the multivariate analysis.
SUFFICIENT MAGNITUDE TO INVALIDATE:
No. In general, findings are consistent with what would be expected.
9. Write a one-sentence summary of the article that could potentially be used in the context of
community health promotion campaigns.
The authors suggest a number of control measures for infection, they also comment that
some of these measures are difficult to carry out on a dairy farm, and that data suggest
that human infection can be prevented by vaccination [p.418]. Hence, a one-sentence
summary might be:
Persons at risk from occupational exposure [farmers and workers] should be offered
vaccination with formalin-inactivated phase I C.b.
10. Based on this work, what would be the next study you would want to do if you had the
necessary resources? Why?
STUDY:
Conduct a randomized control trial to evaluate vaccination against Q Fever.
WHY:
Since Q Fever is an occupational hazard for goat farmers and workers, an intervention
study is indicated to assess whether vaccination results in reduction or elimination of
infection/disease in this group.