Serologic Survey and Risk Factors for Coxiella burnetii Infection among Dairy Cattle Farmers in Korea

Q fever is a widespread zoonotic disease caused by Coxiella burnetii, a bacterial pathogen that frequently infects both domestic (primarily cattle, sheep, and goats) and wild ruminants.1 In animals, Q fever is usually asymptomatic or leads to reproductive disorders (e.g., abortion and stillbirth).2 In humans, acute Q fever is similarly asymptomatic in most cases, or may involve non-specific, mild influenza-like symptoms. However, a small minority of patients may develop chronic Q fever infection.3

C. burnetii is transmitted to humans mainly via the inhalation of contaminated aerosols from infected animals that have shed C. burnetii in birth products, milk, and feces,4 although the pathogen may also be transmitted via other routes, such as direct contact with contaminated materials and ingestion of unpasteurized milk.5 C. burnetii infections mostly occur among high-risk populations that have contact with animals, such as livestock farmers, slaughterhouse workers, and veterinarians.6 However, a large Q fever outbreak (> 4,000 cases) was reported in the Netherlands during 2007–2010.7

In Korea, Q fever was designated as a National Notifiable Infectious Disease in 2006, and approximately 10 cases of sporadic Q fever have been reported annually during 2006–2011.8 C. burnetii infection involves two antigenic states, phases I and II. The levels of anti-phase II antibodies are high in acute disease, whereas elevated anti-phase I antibody levels are observed in chronic disease.6 A serologic study of slaughterhouse workers in Korea identified reactive phase II antibodies among 10.2% of participants.9 However, information regarding the serologic results and risk factors of C. burnetii infection in other high-risk groups, such as dairy farmers and veterinarians, remains limited.

In the current study, dairy cattle farmers, the largest group of dairy farmers in Korea, were asked to participate in a serologic survey to evaluate the seroreactivity to C. burnetii and risk factors for infection.

A total of 1,222 dairy cattle farmers from 784 dairy cattle farms (43.0%) participated in this study (Fig. 2), including 704 (57.6%) farmers in Gyeonggi Province and 518 (42.4%) in other provinces. The participants comprised 773 (63.3%) men and 449 (36.7%) women, with a mean age of 51.4 ± 8.9 years. The average duration of work was 20.6 ± 8.6 years, and the average herd size was 81.6 ± 69.6 dairy cattle.

Fig. 2
Flowchart of serologic study participation.

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Seroreactivity of C. burnetii and serologic profiles

Overall, 134 of 1,222 participants (11.0%) exhibited seroreactivity for C. burnetii. The phase II IgG antibody titer cutoffs ranged from < 1:16 to 1:1,024, and 133 participants (10.9%) had IgG titers of ≥ 1:16. The phase II IgM antibody titer cutoffs ranged from < 1:16 to 1:16; however, only 1 participant (0.1%) had an IgM titer of ≥ 1:16 (Table 1).

Table 1
Distribution of titers against Coxiella burnetii phase II antigen among dairy cattle farmers in Korea

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Univariate analysis of C. burnetii seroreactivity and associated risk factors

Generally, sex, region, and herd size were associated with a higher risk of C. burnetii infection. Men were significantly more frequently seroreactive than women (P = 0.034), and the Gyeonggi Province region had a significantly higher prevalence of seroreactive dairy cattle farmers (P = 0.001), compared to the other provinces. A large herd size (≥ 100 dairy cattle) was significantly more likely to associate with C. burnetii seroreactivity, compared to a small herd size (< 50 dairy cattle) (OR, 1.99; 95% CI, 1.20–3.32) (Table 2). Additionally, the farming of beef or Korean native cattle was significantly associated with a lower C. burnetii infection (P = 0.002). However, the status as a farm owner or family member (P = 0.191), work type (e.g., feeding, milking, cleaning cattle excretion, compost production, calf delivery, and artificial insemination), and the consumption of raw milk were not significantly associated with C. burnetii infection (P = 0.562) (Table 3), although the latter factor was rare (4.3% of participants).

Table 2
Associations of general characteristics with Coxiella burnetii seroreactivity among dairy cattle farmers in Korea

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Table 3
Associations of farm-related factors with Coxiella burnetii seroreactivity among dairy cattle farmers in Korea

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The questionnaire also solicited information about the use of personal protective equipment (e.g., glasses, masks, gloves, aprons, and boots) during work (e.g., milking, cleaning cattle excretion, artificial insemination, and calf delivery); ocular or oral contact with milk, feces, and birth products during work; and the habit of showering after work. In the univariate analysis, the ORs of C. burnetii infection were significantly lower among those who always wore protective gloves during milking (0.57; 95% CI, 0.39–0.84) and when cleaning cattle excretion (0.62; 95% CI, 0.40–0.96), compared to those who sometimes or rarely wore protective gloves. Additionally, ocular or oral contact with birth products during calf delivery had a significantly higher OR (1.50; 95% CI, 1.00–2.24), compared to no such contact with these products (Table 4). All results from the univariate analysis of the association of C. burnetii seroreactivity and work hygiene-related factors are available in the supplemental materials (Supplementary Tables 1, 2, 3, 4).

Table 4
Associations of significant (P < 0.05) work hygiene-related factors with Coxiella burnetii seroreactivity among dairy cattle farmers in Korea

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Multivariate analysis between C. burnetii seroreactivity and risk factors

The following factors identified as significant (P < 0.05) in the univariate analysis were included in the multivariate model: sex, region, herd size, farm beef or Korean native cattle, wearing protective gloves during milking and while cleaning cattle excretion, and ocular/oral contact with birth products during calf delivery. After a backward stepwise regression, six variables remained in the model. Of these variables, male sex (OR, 1.79; 95% CI, 1.14–2.82), residence in Gyeonggi Province (OR, 2.10; 95% CI, 1.30–3.40), a large herd size (≥ 100 dairy cattle; OR, 2.02; 95% CI, 1.18–3.45), and ocular/oral contact with birth products during calf delivery (OR, 1.54; 95% CI, 1.02–2.33) were significantly associated with a higher risk of C. burnetii infection. Furthermore, the risk of infection was significantly lower among farmers who always wore protective gloves while cleaning cattle excretion (OR, 0.63; 95% CI, 0.39–0.99), compared to those who sometimes or rarely wore protective gloves (Table 5).

Table 5
Multivariate logistic regression analysis of variables associated with Coxiella burnetii seroreactivity among dairy cattle farmers in Korea

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In this study, we observed an overall seroreactivity rate of 11.0% against C. burnetii and seroprevalence rate (phase II IgG or IgM titer ≥ 1:32) of 8.3% (102/1,222). These patterns applied to dairy cattle farmers nationwide. The C. burnetii seroprevalence was considerably higher among these farmers than among non-symptomatic people who attended routine health screenings in Korea (seroprevalence rate = 1.5%).11 Another Korean study of slaughterhouse workers, a high-risk group for Q fever, reported a seroreactivity rate of 10.2%, similar to that observed among dairy cattle farmers in our study.9 However, among European studies, a higher seroprevalence was reported among dairy cattle farmers in the Netherlands, compared to their Korean counterparts,12 and a study of dairy cattle farmers in Denmark reported a seropositivity rate (defined as an IgG or IgM titer ≥ 1:256) of 3.1%; again, this exceeded our findings (0.7%; 8/1,222).13 Regarding other Asian studies, a seroprevalence of 35.6% was reported among cattle farmers and farm residents in China.14 In summary, we observed reduced seroprevalence and seroreactivity among dairy cattle farmers in Korea than among their counterparts in other countries.

In Korea, dairy cattle farms are characterized by family-based management and small-scale farming; in the former, men are more likely to participate in dairy cattle farming. The male-to-female ratio of Q fever prevalence was > 2:1 in both Australia and France, where the extensive occupational exposure of men to Q fever is assumed to influence the ratio.3 Similarly, a study of dairy cattle farmers in the Netherlands reported a higher seroprevalence among men than among women.12 We additionally observed regional differences in C. burnetii seroreactivity between Gyeonggi Province and other provinces in Korea. The overall seroprevalence among dairy cattle in Korea was 25.6%, and the highest regional rate of 59.3% was observed among dairy cattle in Gyeonggi Province,11 which corresponded with the high rate of seroreactivity among dairy cattle farmers in this province. Furthermore, a large herd size was associated with C. burnetii seroreactivity in our study, consistent with a study from the Netherlands wherein the seroprevalence of C. burnetii infection among dairy cattle farmers tended to increase with herd size.12

We further identified ocular or oral contact with birth products during calf delivery as a significant correlate of C. burnetii seroreactivity. C. burnetii are present in large populations in the birth products of infected dairy cattle; accordingly, direct contact with these products facilitates the spread of infection.15, 16, 17 Our analysis revealed that wearing a protective mask during calf delivery was not significantly associated with C. burnetii infection. As healthcare personnel have been recommended to use N-95 masks when performing potentially aerosol-generating procedures for patients with suspected or confirmed Q fever,18 we assume that dairy cattle farmers may be using insufficiently protective masks (e.g., cloth masks and face masks). C. burnetii is similarly present in feces, therefore dairy cattle farmers should minimize fecal exposure.4 We found that among work hygiene-related factors, the consistent use of protective gloves while cleaning cattle excretion yielded a significantly lower OR for infection, compared to the occasional or infrequent use of such gloves. Full compliance with the use of gloves during and around calving was found to protect against C. burnetii infection in a cross-sectional study of dairy cattle farmers in the Netherlands.12 In summary, gloves protect the hands from direct contact with feces and might lower the risk of fecal-oral transmission.

The consumption of raw milk has also been identified as a risk factor in the transmission of C. burnetii.19 We note that dairy cattle farmers in Korea rarely consume raw milk and although this consumption was slightly more frequent among seroreactive dairy cattle farmers, it was not significantly associated with C. burnetii infection. Our univariate analysis also revealed a significantly lower number of farm beef or Korean native cattle among seroreactive dairy cattle farmers, although this significance was not maintained in the multivariate analysis. Interestingly, a previous study of the seroprevalence of C. burnetii infection among cattle in Korea observed a lower seroprevalence among Korean native cattle (1.7%) than among dairy cattle (10.5%).20

To the best of our knowledge, this was the first nationwide cross-sectional study of C. burnetii infection among dairy farmers in Korea. However, several factors might affect C. burnetii seroreactivity. First, a disproportionately large percentage of the farmers in the study were from Gyeonggi Province. Second, this study was conducted during the winter season in Gyeonggi Province and the summer season in the other provinces. As a Danish study of dairy cattle observed higher seroprevalence rates during the summer and winter,21 we must consider the potential effects of seasonal differences on seroreactivity among dairy cattle farmers in Korea.

In conclusion, we observed a C. burnetii seroreactivity rate of 11.0% among dairy cattle farmers in Korea. Our multiple logistic regression analysis led us to identify several risk factors (male sex, residence in Gyeonggi Province, larger herd size, ocular/oral contact with birth products during calf delivery) and one protective factor (wearing protective gloves while cleaning cattle excretion). To prevent C. burnetii infection, dairy cattle farmers must take precautions during calf delivery to avoid ocular or oral contact with birth products and should fully comply with the recommended use of protective equipment, including gloves.

Supplementary Table 1

Associations of work hygiene-related factors during milking with Coxiella burnetii seroreactivity among dairy cattle farmers in Korea

Supplementary Table 2

Associations of work hygiene-related factors while cleaning cattle excretion with Coxiella burnetii seroreactivity among dairy cattle farmers in Korea

Supplementary Table 3

Associations of work hygiene-related factors during artificial insemination with Coxiella burnetii seroreactivity among dairy cattle farmers in Korea

Supplementary Table 4

Associations of work hygiene-related factors during calf delivery with Coxiella burnetii seroreactivity among dairy cattle farmers in Korea