Introduction

Coxiellosis is a bacterial zoonosis caused by Coxiella burnetii (C. burnetii), a deadly pathogen of worldwide distribution with the exception of Antarctica and New Zealand1. C. burnetii is a Gram negative obligate intracellular bacterium which is able to survive in phagosome of the phagocytes2. Although, C. burnetii was initially classified within the Rickettsiaceae family, 16s RNA gene-based phylogenetic analysis later showed that it is different from the Rickettsia3. Potentially, C. burnetii may infect all species within the animal kingdom including arthropods2 although coxiellosis, as a disease, is mainly seen in humans, cattle, sheep and goats4. While domestic ruminants are the main reservoirs of C. burnetii, cats, dogs, rabbits, and birds have also been reported to be sources of the human infection2. It is currently known that C. burnetii-infected animals excrete the bacterium in body fluids such as milk, urine, feces, birth products or semen5,6.

As C. burnetii is mainly transmitted from animals to humans through infected animal body secretions, livestock farmers7, veterinarians7, and abattoir workers8 are at increased risk of infection with C. burnetii. Besides, the inhalation of desiccated aerosol particles is considered as major route of infection of humans by C. burnetii9. Studies reported have shown that the occurrence of coxiellosis in domestic animals tends to parallel with coxiellosis outbreaks in humans10,11,12. Human infection with C. burnetii may result in different outcomes ranging from asymptomatic disease to an acute febrile illness associated with headache, myalgia, and atypical pneumonia13. Although most C. burnetii infections are self-limiting in humans, some patients may develop a chronic disease associated with life-threatening complications which include endocarditis and hepatitis14,15,16,17. However, because of lack of awareness among healthcare workers and the lack of routine laboratory tests in many Low- and Middle-Income Countries (LMIC) settings, it is presumed that clinical C. burnetii infection (Q fever cases) are largely misdiagnosed and thus under reported18,19.

In domesticated ruminants, clinical coxiellosis is associated with late-term abortions and other related reproductive disorders such as premature births, stillbirths, or birth of weak offspring4. Additionally, affected ruminants may demonstrate metritis and infertility2. Thus, clinical coxiellosis may be considered an important production disease potentially responsible for reproductive losses, reduced milk production and chronic mastitis20.

Recent studies in Africa, the Arabian Peninsula, and Asia reported high seroprevalence values of coxiellosis in dromedary camels1. Dromedary camels play a major socioeconomic role for millions of people who live in the arid zones of Africa, Middle East and Asia. Thus, camel meat and milk are widely consumed in these regions, which can pose a risk of transmission of C. burnetii from camels to humans. For example, consumption of raw camel milk is widespread in the Arab world due to the perception of considering the camel as a uniquely clean livestock species that was indicated in the Qur’an and Islamic scriptures; which could suggest role the camel in the transmission of C. burnetti to humans21.

Within the greater Middle Eastern region, Q-fever has been documented in Saudi Arabia22, Oman12, and Iran23. In the UAE, few seroprevalence studies were done on coxiellosis in cattle24 and small ruminants24. Besides, only one study on coxiellosis in dromedary camel nearly three decades ago25. However, none of these UAE-based studies have evaluated the potential zoonotic transmission of C. burnetii from livestock species to humans. Thus, although the role of ruminants as source of infection of C. burnetii to humans has been reported from different parts of the world, there is a shortage of scientific evidence on the role of dromedary camels in transmitting C. burnetii to humans. Hence, the role of camels as reservoirs of C. burnetii requires investigation. Owing to their continuous exposure to potentially infected material from slaughter livestock including camels, abattoir workers are at high risk of C. burnetii infection. The objectives of this study were to investigate the seroprevalence on the C. burnetii infection in camels and abattoir workers, and to detect C. burnetii DNA in the clotted blood of study subjects in the Al Ain city of Emirate of Abu Dhabi, the UAE.

Materials and methods

Study setting

The present study was conducted at the Al Bawadi abattoir which is located on the outskirts of Al Ain city within the Emirate of Abu Dhabi, UAE. The Al Bawadi abattoir was established in 2007 and has two sections, namely the commercial and the public section. The former section is designated for slaughtering animals for merchants and organizations while the latter slaughters animals for the general public. When this study was performed the total number of workers at the Al Bawadi abattoir was 110. Study camels included randomly were selected from among the camels brought to the abattoir from different farms located across the UAE along with a minority of camels originating from Oman. According to the information obtained from the Al Bawadi abattoir, about 98% of the camels originate from the UAE while the remaining 2% are sourced from Oman. Inclusion and exclusion criteria were not considered. During the study period, the number of camels slaughtered at the Al Bawadi abattoir on daily basis was low. Hence, all camels brought to the abattoir were subjected to blood sample collection for serum extraction.

Study subjects, design and sample size

The study was a cross-sectional study that was conducted at the Al Bawadi Abattoir between March 2022 to July 2023. The camel sample size was calculated by considering a camel population size of 450,000 in26 the UAE, and an expected prevalence of 50% at 95% confidence interval and 5% margin of error. Furthermore, the sample size calculation assumed that all the study animals originated from the UAE since about 98% of the camels slaughtered at the Al Bawadi abattoir were sourced from the farms located within the UAE. Accordingly, a total of ≥ 384 camels were required for the study while nine additional camels were added to the minimum sample size. In addition, after obtaining consent from the abattoir workers, blood samples were collected from 86 (out of 110 abattoir workers) abattoir workers who agreed to participate in the study.

Data and sample collection

During antemortem examination of the study camels, data on the age, sex and body condition of the animals were collected. The estimation of the ages was made based on the dental eruption and/or wearing according to the protocol reported by Bello et al.27. Accordingly, the ages of camels with partial or all erupted deciduate teeth were estimated to be less one year while the ages of camels with partial or all worn deciduate teeth were estimated to be ≤ 4 years. Furthermore, the ages of camels with partial or all erupted permanent incisors and canine teeth were estimated to be > 4 years and ≤ 7 years, respectively while those with partial or all worn permanent incisors and canine teeth were estimated to be > 7 years and ≤ 15 years, respectively.

Body condition scoring was done based on the protocols previously published by other researchers28,29. Based on the criteria set by these researchers, the amount of fat deposition at specific anatomical sites was evaluated to assess the body condition of camels. Accordingly, if the ribs, ischiatic and coxal tuberosities, and scapula were visible, the flanks were hollow, the recto-genital zones deep and the humps small, the body condition was classified as “thin”. While the body condition of camels with slight visibility of the aforementioned body tissues, slight hollow flanks, slightly deep recto-genital zones and intermediate hump sizes was considered as “medium”. On the other hand, the body condition of camels in which the ribs, the ischiatic and coxal tuberosities, and scapula were not visible, the flanks were not hollow, the recto-genital zones were not deep and the humps were large were grouped as “fat”.

Blood samples were collected from the jugular veins of the study camels into 5 ml plain vacutainers for serum extraction. Besides, 5 mL of blood samples were also collected from cubital veins of the abattoir workers by experience phlebotomist into plain vacutainers tubes for serum extraction. Furthermore, demographic data of the abattoir workers such as age, nationality, occupation, duration of working in the abattoir were collected and analyzed.

Detection of C. burnetii antibody in camels

The detection of C. burnetii antibody in camel sera was done using the ID Screen® C. burnetii Indirect Multispecies ELISA Kit (ID VET Innovative Diagnostic, ID Screen®, Montpellier, France) following the manufacturer’s instructions. Briefly, the samples and the controls were diluted to 1:50 by mixing 245 µl of dilution buffer and 5 µl either positive control, negative control or serum sample into the designated wells of pre-coated 96 microplate. Thereafter, the plate was covered and incubated at 21 °C for 45 min. Then, the contents of the wells were discarded and the plate was washed with 300 µl wash solution three times. Thereafter, the plate was dried, 100 µl conjugate 1x was added into each well, covered and incubated at 21 °C for 30 min. This was followed by discarding the contents of the plate wells, washing and drying the plate in the same way described above. Thereafter, 100 µl of the substrate solution was added into each well and the plate was incubated at 21 °C for 15 min in the dark. Lastly, 100 µl of the stop solution was added into each well and then the optical densities (OD) values were determined at 450 nm using a BioTek spectrophometer plate reader (BioTek Instrument Inc, Highland Park, US). Then, the assay result for each plate was evaluated for its validity based on the OD450 values of the positive and the negative controls in line with the manufacturer’s recommendation. The assay result of each plate was considered to be valid when the mean OD450 value of the positive control (ODPC) was greater than 0.350 (ODPC > 0.350) and if the ratio of the mean OD450 value of the positive and negative controls (ODPC/ODNC) was greater than 3.0. After validity was confirmed, the result of each plate was interpreted individually. Briefly, the sample percentage (S/P%) was calculated as S/P% = (ODsample -ODNC) / (ODPC - ODNC) × 100. Then, samples with S/P% ≤ 40% were considered as negative; samples with S/P% between 40% and 50% including those with 50% were considered doubtful while samples with S/P% > 50% were considered positive. In this study, however, all the inconclusive samples were considered as negatives.

Detection of C. burnetii antibody in abattoir workers

The wells of pre-coated microtiter ELISA plates were set for the assay as per the recommendations of the kit manufacturer (Abbexa LTD Cambridge, UK; website: www.abbexa.com). Accordingly, 2 positive controls, 2 negative controls, test sample, and control i.e. zero or blank well were labeled. Then, 50 µl of negative control and 50 µl positive control were added to their designated wells while 50 µl of sample diluent was added into the zero (blank) well. And 50 µl of diluted serum samples were added into the sample wells. Thereafter, the plates were covered and incubated at 37 °C for 30 min, and then removed, washed five times with 1x wash buffer and dried. Following this step, 50 µl of the kit detection reagent was added into each well except the blank wells and then further incubated at 37 °C for 30 min. The plates were removed, washed five times with 1x wash buffer and then dried. Thereafter, 50 µl of TMB substrate A and 50 µl TMB substrate B were added into each well, mixed well and then the plates were incubated at 37 °C for 15 min. Then, 50 µl of stopping solution was added into each well and the plates were immediately read at 450 nm using the BioTek spectrophotometer plate reader. Then, the validity of the assay was checked based on the recommendation of the kit manufacturer. The test was considered to be valued when the mean OD value of the positive control was ≥ 1.0 and when the OD value of the negative control is ≤ 0.20. The interpretation of the result was based on the cut-off value. The cut-off value = mean OD value of negative control + 0.15. Then, samples with OD values < cut-off value were considered as negative while samples with OD values ≥ cut-off were considered as positive.

Detection of C. burnetii DNA in clotted blood by qPCR

C. burnetii DNA samples were extracted from clotted blood samples of 86 abattoir workers and 366 slaughtered camels. Taco™ DNA/RNA extraction kit (GeneReach Biotechnology, Taiwan) and Taco™ Nucleic Acid Automatic Extraction System (GeneReach Biotechnology, Taiwan) DNA extraction kits were used as per the manufacture instructions. Insertion sequence 1111 (IS1111) gene was as a target gene by qPCR as described earlier30. The validation of the performance of IS1111 based qPCR was done earlier by Klee et al.30. According to the result of the validation done by these authors, the sensitivity of IS1111-based qPCR is 95% and the minimal number of genome equivalents that could be detected was 6.5. In the present study, the minimal number of genome equivalents was set at 10 per reaction.

The primers used were Cox-F: GTCTTAAGGTGGGCTGCGTG (219–238) and Cox-R: CCCCGAATCTCATTGATCAGC (493–513). The primers for TaqMan probe were Cox-TM: FAM-AGCGAACCATTGGTATCGGACGTTTAMRATATGG (259–287)30. In brief, the PCR master mix (QuantiFast Probe PCR kit {Qiagen}) was prepared in a total volume of 10 µl which consisted of 0.65 molecular-grade water, 7.5 µl of QuantiFast Probe PCR master mix (2 ×), 0.75 µl of each primer (10 pmol/µl) and 0.35 µL of each probe (10 pmol/µl). The DNA template used was 5 µl making a total volume of 15 µl. The amplification was conducted in BioRad CFX 96 (BioRad) with cycling conditions consisting of 95 ◦C for 3 min initial heating, and 40 cycles at 95 ◦C for 0.3 s and 60 ◦C for 30 s.

Statistical analysis

Statistical Package for Social Sciences (SPSS) version 28.0 (IBM Corp 2021, Armonk, NY), R software version 4.1.2, and GraphPad Prism 9.4.1 (681) version (GraphPad Software, 225 Franklin Street. Fl. 26, Boston, MA 02110) were used for data analysis. The data variables were summarized using frequencies and percentages for categorical variables and median and interquartile range (IQR) for skewed numeric variables. Chi-square/ Fisher’s exact test, Mann-Whitney U test, one-way ANOVA, and binary logistic regression were used for the evaluation of associations or variations. In human study, for ease of interpretation, nationality was categorized into two – Africans (abattoir workers from Egypt, Sudan, Morocco, and Ethiopia); and South Asians (abattoir workers from Pakistan, Bangladesh, Nepal, Sri Lanka, and Afghanistan). The overall and category-specific prevalence of C. burnetii was estimated and presented as percentages along with the corresponding 95% confidence intervals. The independent variables examined were age, occupation, and nationality for abattoir workers while sex, age and body condition were considered as independent variables for the camels. In human study, the variable years of experience was not included in the model as it exhibited high multicollinearity with age, hence, only age was used in the model.

Results.

Seroprevalence of C. burnetii infection in camels slaughtered at Al-Bawadi abattoir

The seroprevalence of C. burnetii infection in camels slaughtered at Al-Bawadi abattoir was 52.9% (95% C I = 46.0–60.6%). Table 1 shows the result of bivariable and multivariable binary logistic regression analyses of the effect of host factors on the seroprevalence of C. burentii infection in camels slaughtered at the Al-Bawadi abattoir. As it is presented in Table 1, sex, age nor body condition were not (p < 0.05) associated with the seroprevalence of C. burentii infection in camels.

Table 1 Association of the characteristics of camels on the Seroprevalence of C. burentii infection.

The intensity of anti- C. brunetti antibody response in the 208 seropositive camels was classified into low (50% ≥S/P% ≤ 100%), medium (100%> S/P%≤ 200%) and high (S/P%> 200%) based on the intensity of OD values. The intensity of the OD values among the three groups differed significantly (one-way ANOVA; F(2, 205) = 699.4; p < 0.0001; Fig. 1).

Fig. 1
figure 1

Mean percentage optical density (OD450) ant-C. burnetii antibody using ELISA in dromedary camels slaughtered at the Al-Bawadi abattoir. The OD450 values of 208 positive camels were grouped into low, medium and strong based on the intensity of OD450 values. The intensity of the OD values among the three groups differed significantly (one-way ANOVA; F(2, 205) = 699.4; p < 0.0001).

Seroprevalence of C. burnetii infection in abattoir workers

Eighty-six abattoir workers, comprising butchers (71; 83%), laborers (13; 15%), and supervisors (2; 2%) participated in the study (Table 2). The median age of the workers was 36 years (IQR = 30, 40) with those between the age of 30–39 years making up the majority (47%). Eight years was the median length of work experience of the participants (IQR = 6, 14). In terms of nationality, the workers were grouped a south Asians (79%) and Africans (21%).

Table 2 Demographic characteristics of the abattoir workers (N = 86) at the al Bawadi abattoir.

The seroprevalence of C. burnetii in abattoir workers was 24.4% (95% CI = 16.1–35.1). It is noteworthy that the highest seroprevalence (26.3%) was observed in workers within the age group of 20–29 years, followed by those in the age group of 30–39 years (25.0%) and finally those age 40 years and above (22.2%) (Fig. 2). Although not statistically significant, C. burnetii-seropositive workers had a lower median age (32 versus 36 years), while the butchers had a higher C. burnetii-seroprevalence than other workers (25% versus 20%) (Table 3).

Fig. 2
figure 2

Seroprevalence (and 95% CI) of C. burnetii infection in abattoir workers by age categories (N = 86). The bars present the prevalence of C. burnetii infection while the error bars represent the 95% confidence intervals of the prevalence.

Table 3 Seroprevalence and factors associated with C. burnetii infection in abattoir workers (n = 86).

Adjusting for age and occupation, the abattoir workers from Africa had significantly higher odds of C. burnetii antibodies than those from south Asia (AOR = 3.70, 95% CI = 1.05–13.60, P = 0.042). On the other hand, age and occupation were not associated with C. burnetii seropositivity in abattoir workers (Table 4).

Table 4 Univariable and multivariable binary logistic regression for factors associated with C. burnetii seropositivity in abattoir workers.

The intensity of C. burnetti antibody response in the 21 seropositive abattoir workers was classified into low (OD450 rages from 0.339 to 1.00), medium (OD450 > 1.00 and ≤ 2.00) and high (OD450 > 2.00) based on the OD450 values. The difference in the intensity of response among the three groups was significant (one-way ANOVA; F(2, 18) = 79.99; P < 0.0001; Fig. 3).

Fig. 3
figure 3

Mean optical density (OD) in abattoir workers following ELISA for anti-C. burnetii antibodies at Al-Bawadi abattoir. The OD450 values of the positive abattoir workers were grouped into low (n = 7), medium (n = 9) and high (n = 5) based on the intensity. The difference in the intensity of response among the three groups was significant (one-way ANOVA; F(2, 18) = 79.99).

Detection of C. burnetii DNA in the clotted blood of camels and abattoir workers

C. burnetii DNA could not be detected from the clotted blood samples collected from 86 abattoir workers and 366 camels. Figure 4 shows the amplification curves of internal controls while all samples were negative for DNA of C. burnetii.

Fig. 4
figure 4

Results of q-PCR of sera samples camels and abattoir workers. The DNA samples were extracted from clotted blood of 86 abattoir workers and 366 camels, and then amplified using qPCR. However, none of the samples from both camels (A) and abattoir workers (B) were positive for the DNA of C. burnetii.

Discussion

The present study was an observational study that evaluated the seroprevalence of C. burnetii infection and attempted the detection of C. burnetii DNA in clotted blood derived from camels and abattoir workers in the UAE. The study on the abattoir workers was the first report documenting apparent C. burnetii infection in humans in the UAE along with the implied zoonotic transmission of the pathogen from domestic animals to humans. Apart from a coxiellosis study performed earlier by Afzal et al.25, this was the only study focusing on dromedary camels in the UAE. Although this study did not include human subjects from the general population for comparison, the recorded 24.4% seroprevalence in abattoir workers could be considered high8. It is plausible that this could be the result of a significant risk of exposure to infected animals as the seroprevalence in slaughtered camels recorded in the present study was relatively high (52.9%).

Subsequent to the study reported by Afzal et al.25, serological evidence of coxiellosis in animals in the UAE was reported by Chaber et al.32 who detected C. burnetii antibodies in semi-free-ranging ungulates in the Emirate of Dubai. These authors reported a 6% seropositivity of C. burnetii infection by testing 333 semi-free-ranging ungulates. More recently, Barigye et al.24 have reported a seroprevalence of 36.5% in cattle by testing 759 bovine sera collected from the Al Ain region of the Abu Dhabi Emirate. In addition, El Tigani-Asil et al.33 detected C. burnetii antibody in three camels tested in the Al Dhafra region of the Abu Dhabi Emirate. Furthermore, El Tigani-Asil et al.33 sequenced the DNA of C. burnetii from the three camels and observed that the UAE isolates exhibited phylogenetic similarity with the strains from Europe, Africa and Asia.

However, in this study, the DNA of C. burnetii could not be detected in the clotted of both seronegative and seropositive camels and abattoir workers. The failure of detection of C. burnetii DNA in this study could partly be due to the use clotted blood for DNA extraction. Clotted blood may not represent ideal sample for the extraction of C. burnetii DNA. Moreover, the other reason could be due to use the primers of IS1111 repetitive region for the diagnosis of C. burnetii, as IS1111 repetitive element is missing in some strains of C. burnetii34. Previous studies have used conventional PCR and qPCR to detect the DNA of C. burnetii in the sera of both humans and animals35,36,37,38,39,40. Fenollar et al.35, assessed the effectiveness of nested PCR in detecting of C. burnetii DNA extracted from the sera. According to the findings of these authors, as the IgG titer against C. burnetii increases, the sensitivity of the nested PCR in detecting the DNA of C. burnetii decreases. As the result, the authors suggested that using nested PCR could help establish an early diagnosis of chronic C. burnetii infection. Marmion et al.36, used qPCR assay with Taqman probe targeting insertion sequence (IS)1111a and IS30a to test the sera of 546 individuals for the presence of C. burnetii DNA. They identified 34 individuals as qPCR positive, with 22 having active acute Q fever and six having active Q fever endocarditis. Hence, these authors indicated that IS1111a repetitive element-based PCR is the best method for the detection of C. burnetii in patients with active infection.

Ughetto et al.37 reported that qPCR is a sensitive and a specific rapid tool for the diagnosis of Q fever during the early infection. Furthermore, Schneeberger et al.38 reported the percentage of qPCR positivity declines as the C. burnetii infection progresses from acute phase to convalescent phase. Additionally, Bae et al.39 observed that conventional PCR is more suitable than serology for detecting C. burnetii infection within the first 14 days of illness. While, similar to the result of the present study, Dhaka et al.40 reported high seroprevalence (89.5%) of C. burnetii infection in farm workers using ELISA although they did not detect the DNA of C. burnetii in the seropositive workers using PCR.

Researchers from various countries reported variable seroprevalence values in different camel-rearing countries41,42,43,44,45,46,47. A seroprevalence of 71.2% was reported from Algeria by testing 184 camels42 while two studies conducted on coxiellosis in camels in Egypt reported seroprevalence values of 40.7% (215/528)44 and 22.0% (69/315)45. Additionally, studies conducted in southern Ethiopia on 458 camels41 and in eastern Ethiopia on 150 camels46 reported seroprevalence of 90% and 55.7%, respectively. Furthermore, studies conducted in Kenya43 on 344 camels and in Pakistan47 on 920 camels reported seroprevalence of 18.6% and 31.3%, respectively. Additionally, Pirouz et al.48 reported a seroprevalence of 28.26% by testing 167 one-humped camels in Iran.

Thus, when compared with the results of the previous camel studies, the seroprevalence value of 52.9% recorded in the present study is higher than the seroprevalence values reported from Kenya43,49, Pakistan47 and Iran48 but lower than those reported from Algeria42 and southern Ethiopia41. On the other hand, seroprevalence of C. burnetii infections recorded in the present study is comparable with camel studies from eastern Ethiopia46, Saudi Arabia50,51 and Jordan52.

Recently, Devaux et al.1 published a review on C. burentii infection in dromedary camels from the studies conducted in north Africa, and the Near and Middle East region. The review also reported the detection of C. burnetii infection in Hyalomma dromedari and Hyalomma impeltatum ticks collected from camels. As camels play significant role in socioeconomics of millions of people in the arid regions of the world as a valued source of milk and meat, they could potentially be a significant source of C. burnetii infection in humans in the UAE and region. There is speculation about the possibility of persistence infection of C. burnetii in the adipocytes of camel hump thus making camels as continuous source of infection1. Thus, as described by Devaux et al.1, the observation of high seroprevalence values of C. burnetii infection in camels in the arid regions could be a potential threat to the other livestock and humans requiring for the screening of camel herds and implementation of control measures.

In the present study, the seroprevalence of C. burnetii infection in abattoir workers was 24.4%. Although a few UAE-based studies have previously reported on the seroprevalence of C. burnetii infection in domestic and wild animals24,32,33, no study was conducted on the seroprevalence of C. burnetii infection in humans in the country. Nonetheless, a few studies were conducted on the seroprevalence of C. burnetii in humans in east and north Africa, and Middle East12,53,54. Scrimgeour et al. (2000) reported two confirmed cases of Q fever in Oman while Almogren et al.53. detected C. burnetii IgG antibodies in 35.2% of the 51 patients tested in Saudi Arabia. In Israel, 30% of the 125 patients were confirmed to be acute cases of Q fever using definitive diagnostic methods54. Besides, Ibrahim et al.46. reported a seroprevalence of 27.0% C. burnetii infection by testing 188 humans in eastern Ethiopia.

On top of research reports, a sizable number of review articles has reported the results of meta-analysis of research work on seroprevalence of C. burnetii infection1,55,56,57,58. Vanderburg et al.55. reviewed the epidemiology of C. burnetii infection in Africa in animals and humans in one health approach. These authors reviewed 24 articles and re-analyzed the seroprevalence of C. burnetii infection in humans. According to these authors, the average seroprevalence of C. burnetii infection in Africa is less 8%; but higher seroprevalence values (10–32%) were observed in children in Egypt.

In addition to the review published by Vanderburg et al.55, other authors58 published a similar review article on the epidemiology of C. burnetii infection in Africa from 88 studies conducted in animals and humans. According to the result of the meta-analysis these studies, the average seroprevalence of C. burnetii infection in humans was 16%. Besides, Mobarez et al. performed meta-analysis of seroprevalence of C. burnetii infection on 28 studies conducted in humans in Iran. According to the report of these authors, the seroprevalence of IgG phase I and phase II antibodies in humans in Iran were 19.8% and 32.9%, respectively. Furthermore, Devaux et al.1 published a review that documented the presence of human cases of Q fever in North America, South America, Europe, Asia, Africa, and Australia/Oceania; but not in Antarctica and New Zealand. Furthermore, Ahmadinehad et al.57 reviewed 112 articles published on Q fever from the Eastern Mediterranean region and reported an average seroprevalence of 25.5% in humans. As it can be noted from these reports, the seroprevalence of C. burnetii infection recorded in humans by the present study was higher than those reported from African countries with the exception of those reported from the North African region. However, the seroprevalence value of C. burnetii infection from the present study is comparable with the mean seroprevalence values reported from the Mediterranean region. The differences the seroprevalences of C. burnetii in human reported by different studies are influenced by the level of exposure of the study participants to the potentially infected animals and their products. As the present study included highly exposed groups and hence high level of seropositivity is expected in this study human subjects.

The seroprevalence values recorded in both camels and abattoir workers in the present study could be considered comparable to the seroprevalence values previously from other countries. Though there is a shortage of epidemiological studies on C. burnetii infection both in humans and animals worldwide, research done so far has shown that C. burnetii infection may be associated with significant economic and public health impacts as documented by the World Organization for Animal Health2. In ruminants, a significant number of C. burnetii infection is subclinical, but the clinical form (Q fever) causes abortions and reproductive disorders such as metritis and infertility4. Similarly, the effect of C. burnetii infection in camels is expected to be similar with its clinical effects in ruminants. However, additional studies are required on the impact of C. burnetii infection in camels. In humans, C. burnetii infection causes either acute, chronic or asymptomatic infection59. The acute infection is characterized by clinical manifestation ranging from a self-limiting flu-like symptoms to serious disease conditions such as pneumonia or granulomatous hepatitis while the chronic form is characterized by endocarditis, vascular or valvular infection, hepatitis, pneumonia or chronic fatigue2.

Since C. burnetii infection is zoonotic, its control efforts should involve coordination of human health and veterinary authorities in integrated multidisciplinary approach. Control measures in ruminants and camels could include testing and culling of shedding animals, control of animal movement, preventive vaccination, farm management, biosecurity and tick control. C. burnetii is classified as a group B biological agent and it is occupational zoonosis that is extremely hazardous to humans60. Hence, personal protective equipment should be used when handling infected animals, humans and specimens or contaminated materials. Laboratory specimens should be handled inside a biological safety cabinet level III. Selective vaccination should be given to occupationally exposed groups such as abattoir workers, veterinarians and laboratory workers.

Study limitations

The study is not without limitations. The first limitation was the that the study was conducted at one abattoir and low number of study subjects. Secondly, failure to identify the origin of each study camel. Although 98% of the study camels were originated from the UAE, the camels could not further be identified at the level of emirates and regions. This information would help to trace back and identify the districts with high seroprevalences. The third weakness of the study was failure to collect appropriate samples for the detection of the DNA of C. burnetii. The chance of extraction of DNA from the buffy coat, tissues (liver, spleen), aborted materials, environmental samples airborne dusts and vectors such as ticks. Lastly, the use of only IS1111 repetitive region primers for qPCR to amplify C. burnetii DNA could be considered as this repetitive region is not found in all strains of C. burnetii.

Conclusion

In conclusion, the present seroprevalence values recorded in both camels and abattoir workers were among the high seroprevalence values reported so far from the other countries. However, additional extensive studies are required to estimate the seroprevalence of C. burnetii infection in livestock and human occupational contacts in the UAE. It is also recommended to isolate and characterize the strains of C. burnetii circulating in the UAE. Parallel to conducting further research on C. burnetii infection in livestock and humans, implementation the existing control tools in both livestock and human occupational contacts is highly recommended.