Coxiellosis or Q fever is a notifiable zoonotic disease caused by the bacterium Coxiella burnetii. Ruminants have been identified as the main reservoir for human infection (1). In animals, coxiellosis can pose a significant risk to herd health and loss of production through decreased fertility, abortions, and anorexia (2). In humans, Q fever most commonly presents as a self-limiting flu-like illness but may cause pneumonia and hepatitis in rare cases and has been associated with spontaneous abortion in pregnant women (3). Infection in humans is frequently subclinical, leading to a consensus in both medical and veterinary literature that Q fever is likely underdiagnosed and underreported (4, 5).

In Q-fever outbreaks, human transmission occurs through inhalation of aerosol dust particles (6). Q fever can also be an occupationally acquired disease transmitted to farmers, their families, staff, and veterinarians, through inhalation of contaminated particles, ingestion or direct contact with birth fluids or placenta. The organism is also shed in milk, urine and faeces. Factors such as the proximity and intensity of exposure to infected animals or their products, personal hygiene practices, and individual susceptibility further influence transmission risk (7).

To accurately assess the potential human health risk, it is essential to understand the prevalence of Coxiella burnetii in the cattle population. Previous studies have indicated that Q fever is endemic in the Irish dairy herd, with a reported herd-level prevalence of 19.5% from 1489 randomly selected bulk milk tank samples — slightly above the estimated European average of 15.1% (8). However, this data is now a decade old. Since then, average herd size in Ireland has increased significantly, and changes in herd management, biosecurity practices, and environmental factors may have influenced disease dynamics. Updated prevalence estimates are therefore needed to better assess current animal and public health risks. Bulk tank milk (BTM) sampling combined with enzyme-linked immunosorbent assay (ELISA) has been shown to be a cost-effective method for estimating national herd-level prevalence (9).

The objective of this study was to estimate the current herd-level prevalence of C. burnetii in the Irish dairy herd and to identify associated herd-level risk factors using bulk tank milk ELISA results. These findings will support veterinary and public health decision-making, particularly in the context of occupational and environmental exposure.

This study utilised BMT samples collected as part of an Irish dairy cooperative herd health programme. The study sample comprises all participants in an Irish dairy cooperative Herd Health Programme in 2022. By their nature, dairy cooperatives are regionalised. Herd owners subscribe to the Herd Health Programme annually for a subscription fee. The programme is open to all Irish milk producers, regardless of their co-operative affiliation.

The samples used for BMT testing are taken concurrently with routine milk constituent payment testing. BMT samples were collected aseptically from each herd and stored at 4°C until testing, which occurred within 72 hours. BMT samples were tested using the ID Screen Q Fever Indirect Multi-species (Innovative Diagnostics, Grabels), an indirect ELISA designed to detect antibodies against Coxiella burnetii in milk samples. Testing was performed according to the manufacturer’s protocol.

Of the 2,691 herds tested, 1,465 (54.4%; 95% CI: 52.5%–56.3%) tested positive on bulk tank milk. After adjusting for test sensitivity (86%) and specificity (99%) using the Rogan-Gladen estimator, the true herd-level prevalence was estimated at 62.9% (95% CI: 60.7%–65.1%).

There was no significant association found between testing positive for Coxiella burnetii on BMT sampling and stomach worm burden (P = 0.58). As such, this variable was not included in the multivariate analysis.

The variables which were deemed significant in the above univariate analysis were then carried through to a multivariable analysis for backward stepwise regression. Only one variable, neospora, was removed during this process due to lack of statistical significance (P = 0.88). The results of the multivariable analysis are included in Table 2.

In the multivariable logistic regression model, several significant associations were identified. Herds which were vaccinated for IBR had significantly (P = 0.01) higher odds of testing positive for Coxiella burnetii on bulk tank sampling than herds which were unvaccinated testing positive for IBR antibodies. There was no significant difference between herds which were vaccinated for IBR and those which were unvaccinated negative. Herds which were vaccinated for leptospirosis had significantly (P = 0.018) higher odds of testing positive for Coxiella burnetii antibodies in bulk tank samples than herds who were unvaccinated negative for leptospirosis. There was no significant difference between herds who were vaccinated for leptospirosis and those who were unvaccinated testing positive. Herds which were vaccinated for salmonella had significantly (P = 0.048) lower odds of testing positive for Coxiella burnetii antibodies in bulk milk sampling than those herds who were unvaccinated negative. There was no significant difference noted between herds who were vaccinated for salmonella and those who were unvaccinated testing positive for salmonella antibodies.

Herds which were positive for liver fluke had significantly (P = 0.0015) lower odds of testing positive for Coxiella burnetii antibodies in bulk tank samples.

Increasing herd size had significantly (P < 0.001) increased the odds of testing positive for Coxiella burnetii on bulk tank sampling. Increasing herd average EBI (P < 0.001) and proportion of the herd in first lactation (P = 0.0020) significantly decreased the odds of testing positive for Coxiella burnetii on bulk tank sampling.

The large study sample population of 2,691 dairy herds included in this study represent nearly 15% of the total number of dairy herds in Ireland as per the Irish Cattle Breeding Federation. The sample population was relatively well matched with the national average in terms of, herd size, replacement rate and genetic merit (10).

In this study the apparent prevalence for Coxiella burnetii was 54.4% rising to 62.9% after adjusting for ELISA sensitivity and specificity (13). Earlier Irish bulk-tank ELISA studies reported a true prevalence of 19.5% in 1,489 randomly selected dairy herds (8) and an apparent prevalence of 37.9% in 290 self-selected dairy herds (9).

Coxiella burnetii is enzootic worldwide, yet estimations of prevalence among both human and bovine populations vary widely in the literature. Although Coxiella burnetii is a notifiable disease in 29 EU countries, surveillance is passive meaning most data is case based (6).

The EU average for Coxiella burnetii antibody prevalence among all cattle is reported to be 15.1% (6). Ryan et al. (9) had reported a significantly higher risk of Coxiella burnetii among dairy cattle than beef cattle which could account, at least in part, for the higher prevalence noted in this study. Pandit et al. (15) reported a prevalence of 69.3% using ELISA BMT testing in Western France. Wood et al. (16) used immunofluorescent assay testing (IFA) to estimate a seroprevalence of 5.2% among extensively managed beef cattle in Australia. Evidently, prevalence estimations differ widely depending on geographical location and farm management system.

Increasing herd size was found to be associated with significantly (p < 0.0001) higher odds of BMT Coxiella burnetii antibody positivity, echoing prior Irish studies (8, 9). This is particularly pertinent for the Irish context as the Irish dairy industry has undergone a period of significant growth and expansion since the abolition of EU milk quotas in 2015. During this period, the Irish national dairy herd has increased by 25%, demonstrating an increase each year. Furthermore, in the same period the Irish suckler herd (breeding beef cows) population has declined by over 18%. With previous research indicating that dairy cows are significantly more likely to test positive for Coxiella burnetii, this highlights the potential increasing prevalence of Q fever among Irish dairy cattle due to demographic shifts.

Herds that were unvaccinated and tested positive for IBR had significantly lower odds of Q fever positivity compared to vaccinated herds. Conversely, herds which were unvaccinated and tested negative for leptospirosis or salmonella, had significantly lower odds of Q fever positivity compared to vaccinated herds. Given the observational nature of the data, it is not possible to determine causality between vaccination status and Q fever risk. These opposing patterns likely reflect differences in herd management—such as greater animal movement for replacement stock or contract rearing—rather than direct vaccine effects. The cross-sectional design of this study cannot infer causality and that apparent associations may be due to confounding factors such as broader management strategies, biosecurity or co-infections.

Many of the large human outbreaks of Q fever have been traced to small ruminant farms such as the Netherlands 2007–2010 outbreak (17). Ireland has a thriving dairy industry, but minimal dairy goat or dairy sheep farms. The differing ruminant population patterns between countries may confer differing Q fever risks. Whole Genome Sequencing is an interesting development in the field of Q fever dynamics and zoonotic disease overall. Genotyping research from the United States identified a single genotype, ST20, in bovine milk samples and different genotype, ST8, in caprine milk (18), bringing the possibility of subspecies adaptations into question. Given the most recent large human outbreaks have been linked to a small ruminant source, perhaps the genotype associated with bovine Q fever has more limited transmissibility to humans but to explore this further genotyping studies are required in human cases. Evidently this issue is not limited to Q fever, a One Health pathogen genotype database would enable rapid and accurate source detection enabling faster implementation of outbreak control measures as well as prevention measures.

The high prevalence of Q fever in dairy herds warrants further investigation. While clinical cases in individual cattle are usually treated by vets, many animals remain asymptomatic, making BMT testing valuable for detecting hidden infections. Shedding of bacteria can occur without obvious symptoms, posing ongoing zoonotic risks. For example, Ladbury et al. (19) linked a large human outbreak in the Netherlands to an infected goat farm that showed no animal health issues. A Teagasc survey found 92% of Irish farmers were unaware of the zoonotic risk from seemingly healthy animals, highlighting the need for farmer education (20).

The occupational risk of Q fever is well documented with transmission via birthing fluids, urine and faeces. Given the high dairy herd prevalence, human Q fever may be underdiagnosed and underreported, particularly among farmers and vets. Critically, vulnerable groups, like pregnant women or those with vascular conditions, should avoid animals during calving season (21). Hygiene measures and personal protective equipment during calving are critical.

The large sample population included in this study is a significant strength. The high prevalence detected in this study may prompt physicians to include Q fever as a differential diagnosis when patients are presenting with non-specific symptoms, potentially revealing a greater burden of disease than currently recognised. The high prevalence reported in this study also needs to be highlighted to farmers, farm workers and their families so that hygiene and mitigation practices can protect those at risk.

A possible limitation of this study is the regional scope of the testing data, because of this, the results may not fully reflect national-level prevalence or risk factors. Bulk milk tank testing detects herd exposure rather than active shedding so it may overestimate infection risk. In addition, sampling bias may exist as herds included in this study are paid participants in a herd health programme whose management practices may differ from those who chose not to enrol in such a programme. Furthermore, herds that have enrolled will have received specialist veterinary advice to maximise herd health and productivity so that perhaps vaccination or dosing regimens may differ for herds enrolled in the programme.

Future research should include a spatiotemporal analysis of human Q fever notifications in Ireland and compare with the bovine prevalence and distribution. Inclusion of small ruminant data may further quantify the risk posed by Q fever in Ireland. The contrasting vaccination patterns between herds warrant further investigation to elucidate their impact on C. burnetii infection dynamics. Finally, genotypic analyses of isolates from animal and human sources would enhance understanding of transmission pathways and inform One Health strategies.

In conclusion, this study revealed a high herd-level prevalence of Coxiella burnetii, with larger herd size and certain vaccination practices emerging as significant risk factors. Bulk milk tank testing proved an efficient, cost-effective method. Future work should integrate human Q fever case notifications, include small ruminant populations, and apply genotypic analyses to clarify transmission pathways. Strengthened collaboration across veterinary and human health sectors, supported by ongoing research, is essential to guide targeted interventions and reduce zoonotic Q fever risk.