Q fever: a disease with underappreciated significance?

Q fever prevalence and significance are potentially underappreciated in the UK and other countries such as France, Italy and Germany. Although its impact should generally be regarded appropriately as lower than acknowledged major infectious causes of ruminant infertility such as bovine viral diarrhoea virus (BVDv) or bovine herpesvirus-1 (BoHV-1), it may not be recognised as significant at all because of poor disease surveillance. Velasova et al (2017) reviewed the herd-level prevalence of selected endemic infectious diseases of dairy cows in Great Britain and although approaching 80% of surveyed herds were seropositive for Coxiella burnetii, only 1–2% of farmers believed that the disease was present on their farm and 0% perceived that the disease was a problem.

Q fever is a zoonotic disease caused by C. burnetii, a bacterium found worldwide in a wide range of animals. The term ‘Q fever’ (for query fever) was proposed in 1937 by Edward Holbrook Derrick to describe febrile illnesses in abattoir workers in Brisbane, Queensland, Australia, where it remains a zoonosis of significance. Since 2007, Q fever has been emerging as a public health problem in several parts of Europe, with a high profile outbreak in the Netherlands (Hermans et al, 2014), as well as increased recognition as a consequence of more robust screening. Although Q fever is asymptomatic in 60% of human cases, it can lead to acute or chronic infections and cause a flu-like syndrome, hepatitis, pneumonia, endocarditis or abortions. In ruminants, C. burnetii may be the cause of abortions, infertility, metritis or chronic mastitis for the infected herds. However, how well recognised is this risk of economic losses and reduced sustainability of production (Cabrera, 2014)? Diagnosis of infertility, including pregnancy loss in cattle and other ruminants, is complex. The diagnostic rate achieved for abortions is generally low, averaging at about 35% of cases where a foetus and placenta are submitted for an abortion investigation; in cattle, there is frequently a delay between fetal death and expulsion (Cabell, 2007). The need for disease mitigation, including reproductive losses, is recognised as an important part of an approach to sustainable herd health (Statham et al, 2017).

Maurin and Raoult (1999) suggested that Q fever should be considered a public health problem in many countries, including France, the UK, Italy, Germany and Canada, as well as in many countries where Q fever is not recognised because of poor disease surveillance. C. burnetii (Q fever) was made reportable in the UK under the Zoonoses Order in 2021 and this article explores whether it should merit further consideration in herd health than it currently generally receives.

Aetiology

C. burnetii is a small, Gram-negative bacterium and obligate intra-cellular pathogen that causes Q fever. C. burnetii can infect humans, cattle, goats, sheep and many other mammals. C. burnetii displays antigenic variations similar to the smooth-rough variation in the family Enterobacteriaceae. Phase variation is related mainly to mutational variation in the lipopolysaccharide (LPS). Phase I is the natural phase found in infected animals (Maurin and Raoult, 1999).

Pathogenesis

The most common route of infection is through inhalation, and the bacterium can persist for extended periods of time in the environment (Barberio et al, 2017). Alongside this environmental persistence, the bacteria can be spread up to 11 miles on the wind. Climate change may contribute to changing disease patterns as insect vectors or favourable dry dusty conditions become more common (Gale et al, 2008; 2009; Grace et al, 2015). The main risk factor for infection remains livestock density (Hawker et al, 1998; Clark and Soares Magalhães, 2018). Der-rick and his collaborators concluded in 1937 that wild animals were the natural reservoir of Q fever, with domestic animals being a secondary reservoir, and that the disease may be transmitted by ticks or other arthropods (Maurin and Raoult, 1999). In humans, infection usually occurs through inhalation of dust or aerosols containing the pathogen shed by ruminants, therefore there is an increased risk of disease in ruminant vets and farmers, as well as abattoir workers.

Following inhalation of the bacterium, haematogenous spread and then infection of organs occurs. Severity of disease depends on the immunocompetence of the infected animal or human and other factors. In farm animals, bacteria tend to localise in the placenta (Barberio et al, 2017). Domestic ruminants (goats, sheep and cattle) are the major reservoirs and sources of human infection (Maurin and Raoult, 1999); goats are the most susceptible to infection, clinically manifesting with abortion and reproductive problems, followed by sheep and cattle.

Infected animals can shed bacteria in significant numbers at parturition (Figure 1); the placenta can have up to 10⁹ bacteria per gram. Bacteria are also shed in milk, faeces and vaginal mucus; milk being the primary route in cattle (Rodolakis et al, 2007). The timing and degree of shedding can vary significantly and contribute to the environmental contamination.

Prevalence

The prevalence of Q fever recorded varies across Europe and inevitably depends partially on the level of surveillance performed. Valla (2014) described a prevalence of 40.1% in Italy and Valergakis et al (2012) carried out polymerase chain reaction (PCR) testing of bulk milk across 155 herds in 2009–10 in south west England and found a 70% prevalence. PCR provided evidence of C. burnetii DNA equivalent to more than 10² bacteria/ml in 108 of 155 bulk milk samples, equating to an overall herd prevalence of 69.7%. They noted that the results of their study ‘fall well with-in the 38–94% range of herd prevalence reported in Europe and North America using bulk tank milk or serum samples’ by others including Capuano et al (2001) and Agger et al (2010).

As in Figure 2, UK prevalence was high and approaching 80% (Velasova et al, 2017). Velasova et al (2017) found that ‘the prevalence of Salmonella spp. and C. burnetii in a population of dairy herds in GB was high. However, no farmer reported problems due to these pathogens, indicating that they are mostly subclinical or unrecognised’ (Figure 3).

Risk of disease transmission is also increased in areas with high cattle density and in favourable environmental and climatic conditions (ie type of landscape, temperature, rainfall, wind) in relation to the regional variations observed. The study further suggested that the importance of a pathogen or disease and willingness to act depend not just on prevalence but also on attributable economic effects. As a result, without routine screening, infected herds will remain undetected and so potentially uncontrolled. The behavioural drivers for farmers to change management can be complex. Garforth et al (2004) concluded that economic drivers are not necessarily paramount for all farmers, but environmental, family, lifestyle and stewardship motives are equally (and sometimes even more) important for many (Statham, 2012).

Valla (2014) surveyed bulk tank milk samples from 344 Italian dairy herds between October 2011 and July 2013 using PCR. From 246 of the 344 farms, data about the incidence of metritis/clinical endometritis and abortion were also recorded. In total, 138 of 344 farms (40.1%) were positive. Out of the 246 farms included in the second part of the study, 106 farms (43.1%) were positive.

A Q fever survey of PCR was carried out on bulk milk from dairy farms located in south west and north east England by RAFT Solutions (2021, data on file), targeting herds with a range of infertility concerns, including higher than expected rates of endometritis or pregnancy loss. Although these survey findings were not controlled and data cannot be robustly extrapolated to national level, 31/50 (62%) positive bulk tank milk were recorded overall by PCR, with 14/28 (50%) positive results from south west England and 18/25 (72%) positive results from north east England. Historic infection was completely absent in some herds but at >60% prevalence overall, which is comparative to the literature described above. C. burnetii prevalence of 97.6% was found by enzyme-linked immunosorbent assay (ELISA) and PCR tests of bulk tank milk in dairy cattle farms in Hungary (Dobos et al, 2020).

Clinical signs

In cattle, clinical signs associated with Q fever infection include:

• Abortion, stillbirth, premature and/or weak calf births (Cabell, 2007; Wouda and Dercksen, 2007; Barberio et al, 2017)
• Retained fetal membranes (Valla, 2014)
• Metritis and endometritis (Valla, 2014)
• Infertility (Valla, 2014; Dobos et al, 2020)
• Increased calving to conception intervals
• Increased returns to service
• Poor pregnancy rate (Valla, 2014; Dobos et al, 2020).

Therefore, this is a disease with negative reproductive impacts for cattle. Clearly these impacts are not 100% specific presentations of Q fever and it is important to carefully consider other potential causes such as other infectious diseases, semen quality or nutrition and investigate appropriately. However, a number of studies across several countries are consistent with a significant association of Q fever infection with the clinical signs described above.

Valla (2014) reported the wide spread of C. burnetii infection in the Italian national herd as described above and indicated the potential association with a higher incidence of metritis and/or clinical endometritis. In total, 40.1% of 344 herds surveyed were PCR positive on bulk milk. Positive herds were found to have 2.5 times the odds ratio of having a high prevalence (>15–17%) of endometritis than negative herds (P<0.001). Endometritis was scored as described by Sheldon et al (2008).

In a study by Dobos et al (2020), a higher (Phase I) seropositivity rate (50.0%) was found in cows with pregnancy loss than in pregnant animals (38.5%). The high prevalence of C. burnetii in dairy farms is therefore potentially a risk factor related to pregnancy loss (Figures 4 and 5).

Dobos et al (2020) investigated an association between evidence of C. burnetii infection and pregnancy loss of dairy cows at the early stage of gestation, measured by biomarker PSPB 29–35 days post-insemination and then by subsequent ultrasound examination. Three high-producing dairy farms were studied which had previously been found antibody ELISA- and PCR-positive for C. burnetii by bulk tank milk testing. C. burnetii antibody was detected in 52% of the 321 cows tested by ELISA. Pregnancy loss was detected in 18% of the cows between days 29–35 and days 60–70 of gestation. The study found a higher within herd seropositivity rate of 80.5% in the cows and 94.4% in the first-bred cows that had lost their pregnancy at an early stage.

In goat herds, clinical signs associated with infection include (Brom et al, 2015):

• Increased abortion, stillbirth or weak newborn kids
• Increased retained fetal membranes
• Milk drop.

As with cattle it is important to rule out other infectious or management causes of losses.

In humans, although Q fever is asymptomatic in 60% of cases, it can lead to acute or chronic infections and cause a flu-like syndrome, hepatitis, pneumonia, endocarditis or abortions. In Australia, Q fever has long been recognised as an important zoonosis and it is the most commonly notified zoonotic disease (Eastwood et al, 2018). In the Netherlands, an increase in the number of human cases was observed in 2007, 2008 and 2009. A link was established between some human cases and farms of small ruminants where abortions resulting from Q fever were detected (Commandeur et al, 2014; Clark and Soares Magalhães, 2018).

Diagnosis

Diagnosis at a practical herd level may be challenging, as reproductive signs such as abortion, endometritis and pregnancy loss are multifactorial and presence of infection may not be causative of disease. Clinical signs are common to other infectious diseases, such as BVDv, leptospirosis and infectious bovine rhinotracheitis, in addition to nutritional deficiencies or semen quality effects, and so differential diagnoses should be carefully considered before concluding that Q fever is the causative agent for reproductive performance issues in dairy or beef herds (Cabell, 2007; Statham, 2011a; 2011b; Statham et al, 2019).

It should also be considered that infected animals can be asymptomatic but still be shedding the bacteria; animals can also shed from differing routes and the amount of shedding can vary significantly over time. In cattle particularly, the presentation is often subclinical. However, Q fever should be considered when other important causes of reproductive losses are ruled out and clinical signs are consistent with those described above. Diagnostic tests at a herd level can have an important role, although they are not perfect. A combination of PCR and serology testing can represent a helpful approach in practice, as discussed below, but should be interpreted with caution.

Serology

ELISA tests are available to measure antibody levels, which can be a useful indicator of exposure to the bacteria in unvaccinated animals. Antibodies are mostly detected after 2–3 weeks from the onset of the disease and so serological tests should be performed on both acute- and convalescent-phase sera, if clinical suspicions of C. burnetii infection are apparent. The agglutinating antibodies gradually decreased in vaccinated cattle, but the geometric mean titre (GMT) remained approximately 4 times higher than that for the non-vaccinated group for at least 20 months after vaccination in a study by Behymer et al (1975). Serology potentially might allow the differentiation of acute and chronic Q fever infection. Plummer et al (2018) described how most commercially available antibody tests for livestock detect the titre to phase I and phase II antibodies. In humans, it is thought that phase II antibodies indicate an acute infection when found in higher titres than phase I antibodies; however, phase-specific antibody testing in livestock remains poorly characterised at present.

PCR

PCR testing can be run on a variety of biological (eg bulk milk or cervical fluid) and environmental samples to demonstrate presence of the bacteria. Owing to its zoonotic nature, fresh samples can be a risk to the laboratory staff. When investigating abortion in cows, PCR can be performed preferably on cervical swabs to manage contamination risk, ideally on a minimum of two aborted dams, or alternatively, PCR on placenta or the aborted fetus (liver, spleen, stomach contents).

Sampling should be performed within 8 days following the abortion and samples should be refrigerated and sent to the lab as soon as possible, with care to prevent environmental sample contamination.

‘QTest’

Recent work has validated the QTest for use on bulk tank milk samples from dairy herds (Treilles et al, 2021). QTest uses a ‘FTA card’ which uses cellulose paper with lyophilised chemicals to capture and bind DNA from the samples as a transport solution. The DNA is then extracted in the laboratory, before amplification for a real-time PCR which is fast sensitive and semi-quantitative. DNA is stable for up to 28 days on the FTA cards and detection was higher using FTA cards (91.4%) than raw milk (77.6%).

Reportable disease and Zoonoses Order

C. burnetii (Q fever) was added to the Zoonoses Order in February 2021 in England, and in April 2021 in Wales and Scotland. The legislation for each country was updated accordingly (Animal and Plant Health Agency (APHA), 2022). The reportable disease requirement for Q fever requires that APHA are notified of positive PCR results as soon as possible, with the C. burnetii Immediate Report Form completed as fully as possible and returned to APHA (by email, with the email title indicating private and confidential or official sensitive). This responsibility will generally fall on laboratory managers as the results will be generated there.

What has to be reported

• All positive PCR results (even if more than one positive from the same farm). This includes Q Test
• Modified Ziehl–Neelsen (MZN) smears on placental samples and suspicious of Q fever, should be sent for further analysis to APHA Penrith VIC (sending fresh placenta with cotyledon) to confirm by PCR
• Serology – if only serology is performed, only report to APHA if there are concerns regarding C. burnetii on the farm – confirmatory PCR testing will then need to be carried out.

Treatment and prevention

Treatment

There is limited efficacy of use of antibiotics (oxytetracycline) in small ruminants and cattle (Ordronneau, 2012; Astobiza et al, 2013). However, with growing awareness of antimicrobial resistance as part of ‘One Health’, it is increasingly inappropriate to rely on blanket anti-microbial therapy for control in livestock health and welfare, unless other avenues are first fully considered (Taurel et al, 2012).

Prevention

Prevention measures consist of strict biosecurity and environmental hygiene. Infected animals shed large quantities of bacteria into the environment through faeces, vaginal mucus, urine, milk and especially parturition products. C. burnetii survives very well in the environment and contaminates aerosols and dust. Any control measure leading to a decrease in either the prevalence of shedders or in the environmental bacterial load will help limit both the spread of the infection in ruminants and the zoonotic risk.

In infected cattle herds, control measures generally consist of environmental measures such as destruction of placentas or disinfection of birth locations, antibiotic treatment such as oxytetracycline injections during the last month of gestation, where absolutely necessary and, importantly, vaccination.

Vaccination

In a study by Courcoul et al (2011), the long-term effectiveness of three different vaccination strategies in a recently infected dairy cattle herd was studied through a modelling approach. A vaccine protocol needs to be implemented over at least 3 years to maximally reduce shedding and therefore environmental contamination and disease. Since infection was seldom eradicated in the first years of vaccination, an early cessation of vaccination could prove ineffective in the long term. Ideally, primary vaccination should be completed before first service. The primary course of two injections was given 3 weeks apart.

LÓpez Helguera et al (2013) showed that vaccinating against Q fever with an inactivated phase-1 vaccine (COXEVAC®) improved reproductive performance in C. burnetii-infected dairy herds (Figure 6).

Conclusions

C. burnetii infection causing Q fever is commonly found in the UK, Europe and internationally, with recent surveys showing seroprevalence ranging circa 60–80% in bulk milk in dairy herds in the UK. Q fever has potentially underappreciated significance. It is a cause of reproductive disease in cattle and small ruminants and as such potentially has a negative impact on the sustainability of livestock food production. It is also a zoonosis and was added to the Zoonoses Order as a reportable disease in 2021 in England, Wales and Scotland. However, it has perhaps remained a disease with low levels of recognition in the UK. Although the impact of C. burnetii should be considered with appropriate diagnostics and alongside other potential pathogens, the gap between recorded prevalence and perception as a herd problem appears to be wide. It may therefore merit further veterinary attention than it currently receives.

KEY POINTS

• Q fever is a zoonotic disease caused by Coxiella burnetii, a bacterium found worldwide in a wide range of animals.
• In ruminants, the infection may cause abortions, infertility, metritis or chronic mastitis, which can lead to economic losses for the infected herds and impacts sustainability. Other differential diagnoses should be carefully considered.
• Although Q fever is asymptomatic in 60% of human cases, it can lead to acute or chronic infections and cause flu-like syndrome, hepatitis, pneumonia, endocarditis or abortions.
• C. burnetii (Q fever) was made reportable in UK under the Zoonoses Order in 2021. Diagnosis may be achieved through serology, PCR on infected tissues and transport-stabilised Qtest cards.
• UK prevalence was high and approaching 80%.
• Control relies on good calving hygiene and vaccination strategies.