Tick-borne pathogens in Iran: A meta-analysis

Mehdi Khoobdel, Amir Sajad Jafari, Zakkyeh Telmadarraiy, Mohammad Mehdi Sedaghat, Hasan Bakhshi

1Health Research Center, LifeStyle Institute, Baqiyatallah University of Medical Sciences, Tehran, Iran

2Department of Veterinary Medicine, Faculty of Veterinary Medicine, University of Zabol, Zabol, Iran

3Department of Medical Entomology and Vector Control, School of Public Health, Tehran University of Medical Sciences, Tehran, Iran; Rahyan Novin Danesh (RND) University, Sari, Mazandaran, Iran

4Department of Medical Entomology and Vector Control, School of Public Health, Tehran University of Medical Sciences, Tehran, Iran

5Vector-borne Diseases Research Center, North Khorasan University of Medical Sciences, Bojnurd, Iran

ABSTRACT

Objective: Different studies have been performed on the prevalence of tick-borne pathogens in different areas of Iran; however, as far as our knowledge, there is no regional meta-analysis available for consideration and estimation of tick species infected with different pathogens in Iran.

Methods: In this review, among different databases, a total of 95 publications were included, and the infection of different tick species to different tick-borne pathogens was determined;furthermore, presence of pathogens (with 95% confidence intervals)in tick vectors was calculated separately for each province, using Comprehensive Meta-Analysis version 2 (Biostat, USA).

Results: Totally, among all 95 studies, 5 673 out of 33 521 investigated ticks were positive according to different detection methods. Overall estimated presence of pathogens in tick vectors in Iran was 8.6% (95% CI 7.0%-10.6%, P＜0.001). Of all 46 species of ticks in 10 genera in Iran, 28 species in 9 genera, including Alveonasus, Argas, Boophilus, Dermacentor, Haemaphysalis,Hyalomma, Ixodes, Ornithodoros, and Rhipicephalus were infected with at least 20 pathogens in 10 genera including Aegyptianella,Anaplasma, Babesia, Borrelia, Brucella, Orthonairovirus [Crimean-Congo hemorrhagic fever virus (CCHFV)], Coxiella, Ehrlichia,Rickettsia and Theileria in 26 provinces of Iran. The presence of pathogens in ticks collected in western Iran was more than other regions. Hyalomma anatolicum (20.35%), Rhipicephalus sanguineus(15.00%), and Rhipicephalus bursa (14.08%) were the most prevalent infected ticks for different pathogens. In addition, most literatures were related to CCHFV and Theileria/Babesia spp.

Conclusions: Public health and veterinary professionals should be aware of diagnosing possible diseases or outbreaks in vertebrates.

KEYWORDS: Ticks; Tick-borne diseases; Vector-borne diseases;Iran

Significance

Several studies have shown the presence of tick-borne pathogens in ticks in Iran; however, as far as our knowledge, there is no meta-analysis available for estimation of ticks infected with tickborne pathogens. Our analysis showed that the overall estimated presence of pathogens in tick vectors in Iran was 8.6% (95%CI 7.0%-10.6%, P＜0.001). Furthermore, 28 tick species in 9 genera were found to be infected with at least 20 pathogens in 10 genera.

1. Introduction

Ticks are external obligatory blood-sucking parasites of vertebrates (phylum Arthropoda; class Arachnida) that fall into three families including Ixodidae (hard ticks), Argasidae (soft ticks), and Nuttalliellidae[1]. Ticks are the primary vectors and reservoirs for different pathogens including viruses, bacteria,and protozoa all over the world, which pose significant threats to human and animal health[2,3]. Tick-borne pathogens cause thousands of disease cases in human populations worldwide with the animal cases seeming to be more than humans[4]. Different species of ticks are able to transmit different diseases. And Crimean-Congo hemorrhagic fever (CCHF), Colorado tick fever,Q fever, borreliosis, relapsing fever, theileriosis, babesiosis,anaplasmosis, ehrlichiosis and Rocky Mountain spotted fever are among the most significant tick-borne diseases caused by these pathogens[5]. The spectrum of tick-borne diseases of both medical and veterinary importance has increased in recent years as a result of advances in molecular biology. New microorganisms are being detected in ticks collected in different countries, and the list of potential tick-transmissible pathogens is updating[6]. Problems caused by tick infestations are not limited only to transmission of pathogens. Bite stress, production loss, physical damage, anemia and poisoning are other aspects of tick bites[7]. Furthermore,the importance of animal productions in the economy and food industry around the world is undeniable[8]. Animal health can be altered by the direct and indirect effects caused by the bites of ticks and tick-borne diseases, leading to noteworthy production decrement of meat, milk, eggs, and leathers. In some severe cases,tick-borne pathogens lead to the death of humans and animals.Indirect effects are related to the costs associated to the treatment and control[8]. From past to present, ticks and tick-borne diseases have been recognized as a threat for human and animal health.Ticks are responsible for the majority of vector-borne diseases in Asia, America and Europe[9].

Iran, covering an area of 1 648 195 km, with a population of 83 million, is located in the Middle East. This country is located in Palearctic and Oriental zoogeographic regions, with different types of climate: mild and quite wet on the coast of the Caspian Sea,continental and arid in the plateau, cold in high mountains, desert and hot on the southern coast and in the southeast, resulting in diversity of tick species[10,11]. Ecology of ticks, their interactions with environment and risk of infection by tick-borne pathogens are directly related to the spatial and temporal variations. As a result, diversity of climate, as well as the vast geographical area,increases the diversity of tick populations which leads to the risk of transmission of different tick-borne pathogens[12]. To date, it has been reported that 46 species of ticks (10 Argasidae and 36 Ixodidae) in 10 genera occur in the country[13].

Tick species can be considered as sentinels to track the circulation of tick-borne pathogens before an outbreak breaks out in humans and animals. Although many studies revealed data about prevalence of different tick-borne pathogens in different areas of Iran, as far as our knowledge, there is no comprehensive data available for consideration and estimation of the damages caused by pathogens transmitted by ticks, on the economy and public health in Iran.For this reason, performing an updated regional review and metaanalysis on the studies conducted on the prevalence of tick-borne pathogens in different provinces of this country is highly necessary.Considering the damages caused by tick-borne diseases on the public health, animal husbandry, and Iran tourism industry, the current study attempted to determine and highlight the presence of pathogens in tick vectors and epidemiological aspects of tick-borne diseases in Iran.

2. Materials and methods

2.1. Searching approach

The present meta-analysis was performed according to the guidelines of preferred reporting items for systematic reviews and meta-analyses statement. In this regional meta-analysis study,nine English and Persian language databases including PubMed,Google Scholar, Science Direct, Scopus, Web of Science, Magiran,Civilica, Iranian Research Institute for Information Science and Technology (IranDoc), and Scientific Information Database(SID) were selected to explore the articles and data with no time limitation (last updated: 7 March, 2021). Duplicate articles,case series, animal-based studies, human-based studies and studies carried out in other countries were excluded. All studies,representing the prevalence of tick-borne pathogens in ticks as hosts/reservoirs were concerned and all PRISMA criteria have been met (Figure 1).

Figure 1. Fowchart of studies selection in terms of tick-borne pathogens in Iran.

Totally, 95 articles and data fit into the criteria. Then, author(s)names, year of publication, province of study, tick vectors,pathogens, the number of examined ticks and the number of positive ticks were extracted from the collected data. The search was conducted using English and Persian language keywords with different patterns (e.g.: Tick, Iran, Anaplasma, Babesia, Theileria,Crimean-Congo hemorrhagic fever virus, CCHFV, Ehrlichia,Agyptinella, Francisella, Brucella, Borrelia, Coxiella, and Rickettsia).Advanced search options and Boolean operators ＇AND＇ and ＇OR＇were also used to find more relevant records.

2.2. Paper selection

PICO process or framework (Population, Intervention,Comparator and Outcome) is a common method for formulating a systematic review queries. However, this format is not suitable for prevalence studies. Quality assessment for the included studies of the present research were setup and developed according to CoCoPop structure [Co (Condition) = infection by pathogens; Co(Context) = provinces of Iran; Pop (Population) = ticks]. Studies and the selected data were independently analyzed and the eligibility was determined by HB and ASJ. Disagreements were resolved by MK.

2.3. Meta-analysis

Initially, the prevalence of each genus of pathogen (with 95% confidence intervals) was calculated separately for each province(at least two studies were needed for calculation of each pathogen in separate provinces). Then, an overall prevalence was calculated for all pathogens in respect to each province. Furthermore, the total prevalence for each pathogen in Iran was estimated. Cochran Q test(P＜0.05 shows statistically significant heterogeneity) and Itest[25% (low), 50% (moderate), and 75% (high) heterogeneity]were used to evaluate heterogeneity among studies. To compute overall size effect (Q＜0.05), random model was used; otherwise (Q＞0.05),fixed model was assessed. For determination of publication bias, Egger’s and Begg＇s tests were applied (P＞0.05 indicates a reasonable publication bias). Also, a funnel plot was used to visualize the publication bias. P＜0.05 was considered statistically significant for statistical analysis of prevalence. All statistical analyses were performed using Comprehensive Meta-Analysis version 2 (Biostat, USA).

3. Results

Among all databases screened, 3 328 records were identified through database searching; then, a total of 95 publications were selected and included in this review. Among these 95 publications,33 521 ticks were surveyed and 5 673 were positive according to different detection methods in all provinces of Iran. Of all 46 species of ticks (in 10 genera) which occur in Iran[13], 28 species(in 9 genera) including Alveonasus (1 species: Al. canestrinii), Argas(2 species: Ar. persicus, Ar. reflexus), Boophilus (Boophilus spp.),Dermacentor (2 species: D. marginatus, D. niveus), Haemaphysalis(4 species: Ha. concinna, Ha. inermis, Ha. punctata, Ha. sulcata),Hyalomma (10 species: H. aegyptium, H. anatolicum, H. asiaticum,H. detritum, H. dromedarii, H. excavatum, H. marginatum, H.rufipes, H. schulzei, H. scupense, H. sp.), Ixodes (1 species: I. ricinus),Ornithodoros (3 species: O. erraticus, O. lahorensis, O. tholozani), and Rhipicephalus (5 species: R. annulatus, R. appendiculatus, R. bursa,R. sanguineus, R. turanicus, R. spp.) were found to be infected with at least 20 pathogens (in 10 genera) including Aegyptianella (1 species: Ae. pullorum), Anaplasma (4 species: An. ovis, An. bovis, An.phagocytophilum, An. marginale, An. spp.), Babesia (3 species: Ba.ovis, Ba. bigemina, Ba. occultans, Ba. spp.), Borrelia (3 species: Bo.microti, Bo. anserina, Bo. persica, Bo. sp.), Brucella (Brucella sp.),Orthonairovirus (1 virus: CCHFV), Coxiella (1 species: Cx. burnetii),Ehrlichia (2 species: Eh. canis, Eh. ovina, Eh. spp.), Rickettsia (1 species: Ri. hoogstraalii, Ri. sp.), Theileria (4 species: Th. annulata,Th. lestoquardi, Th. ovis, Th. equi, Th. spp.), as well as unspecified An. centrale/An. bovis (Table 1). In this review, D. marginatus, D.niveus, H. detritum and H. scupense were considered as separate species.

Table 1. Summary of tick-borne pathogens in tick species in different provinces of Iran.

Table 1. Continued

Table 1. Continued.

Table 1. Continued.

Table 1. Continued.

Table 1. Continued.

Table 1. Continued.

Among the provinces where ticks were found to be infected with different genera of pathogens (including CCHFV), Lorestan (7 genera), Ardabil (6 genera), Golestan (5 genera), and Sistan and Baluchestan (5 genera) provinces had the most number of ticks infected with different genera of pathogens (Table 2).

Table 2. Different genera of pathogens (as well as CCHFV) detected in tick vectors in different provinces of Iran.

Among 31 provinces of Iran, 26 provinces were surveyed in terms of detection of infection of different pathogens in ticks; meanwhile,the status of tick infection with different pathogens remained unclear in Alborz, Bushehr, Chaharmahal and Bakhtiari, Markazi,and Zanjan provinces. The provinces in which the most studies have been carried out are Sistan and Baluchestan (12 studies),Lorestan (9 studies), Razavi Khorasan (8 studies), Mazandaran(8 studies), Kerman (7 studies), and Ardabil (7 studies). On the other hand, Hormozgan, Ilam, Isfahan, Khuzestan, Kohgiluyeh and Boyer-Ahmad, and Qom were among the least studied provinces(only one study in each province). More than 60 literatures were related to CCHFV and Theileria/Babesia spp., while Aegyptianella,Brucella and Rickettsia were limited to less than 10 publications(Table 1). According to a random effect model, the total prevalence of tick-borne pathogens in Iran was calculated as 8.6% (95% CI 7.0%-10.6%, P＜0.001). The highest and lowest prevalence rate occurred in Kurdistan (20.5%; 95% CI 14.0%-29.1%, P＜0.001),and Khorasan, Razavi (2.4%; 95% CI 0.8%-6.7%, P=0.008),respectively. In addition, Anaplasma sp. was the pathogen with the highest statistically significant prevalence (23.5%; 95% CI 15.1%-34.7%, P＜0.001), while the lowest infection rate belonged to Babesia sp. (4.0%; 95% CI 1.9%-8.1%, P＜0.001) (Table 3).

Table 3. Meta-analysis result of different genera of pathogens (including CCHFV), detected in each province as well as in the country.

Table 3. Continued.

Statistical analysis revealed that the highest prevalence of Anaplasma sp., Babesia sp., Borrelia sp., CCHFV, Coxiella sp., and Theileria sp. occurred in East-Azerbaijan (36.5%; 95% CI 15%-63.9%, P=0.335), West-Azerbaijan (8.8%; 95% CI 6.1%-12.5%,P＜0.001), Kurdistan (8.5%; 95% CI 1.2%-41.6%, P=0.022),South-Khorasan (14.3%; 95% CI 3.7%-42.0%, P=0.017), Kerman(9.9%; 95% CI 5.8%-16.4%, P＜0.001), and Mazandaran (21.0%;95% CI 1.5%-82.4%, P=0.009), respectively. Brucella sp., Ehrlichia sp., Rickettsia sp., and Aegyptianella sp. did not meet the criteria for entering province-specific meta-analysis (less than 2 publications in each province). A forest plot was used to show the prevalence of tick-borne pathogens across the country (Supplementary Figure 1).In addition, funnel plot revealed an asymmetry in the funnel which might indicate that some studies were missed on the right side of the plot (Figure 2). In line with funnel plot, the results of Egger’s test (P＜0.001) showed a publication bias among studies. Based on the funnel plot, most of the studies with low prevalence of tickborne pathogens were included in this meta-analysis (Figure 2).

Figure 2. Funnel plot of standard error by logit event rate.

4. Discussion

As far as we know, the present meta-analysis is the first largescale study that examined the prevalence of tick-borne pathogens in tick vectors in Iran. Overall estimated prevalence of tick-borne pathogens in Iran was 8.6% (95% CI 7.0%-10.6%, P＜0.001).The greatest infection rates among tick vectors were dedicated to Rickettsia sp. (P＞0.05), and Anaplasma sp., respectively.Anaplasma species are Gram-negative obligate intraerythrocytic bacteria (Rickettsiales; Anaplasmataceae) which are of great veterinary concern. An. marginale, the most probable causative agent of bovine anaplasmosis, has been reported worldwide. This pathogen mainly affects ruminants resulting in mild to severe febrile hemolytic anemia that leads to significant economic losses[109]. Other species are as follows An. ovis and An. mesaeterum(in sheep and goat), An. phagocytophilum (in horse, dogs and cats),An. platys (in dogs) and An. centrale in cattle[110,111]. Although medically important pathogens such as Borrelia sp., Coxiella sp.,and CCHFV were less prevalent in ticks according to the pooled results of literature review, it should be noted that to determine the epidemiological status of a pathogen, all factors affecting pathogen’s life cycle must be taken into consideration. For example, CCHF is endemic in Iran and its neighboring countries and a significant number of human cases are reported each year.In a recent review on distribution of ticks and their infection to CCHFV, the main vectors of CCHF, H. marginatum and H.anatolicum, have been reported in more than 38.7% of provinces of Iran[112]. In our review, among all pathogens, CCHFV positive ticks were reported in 19 provinces. The point may be that in Iran,the main way of CCHFV transmission is not tick bite. CCHFV infection in human mostly occurs due to direct contact with infected livestock (blood, tissues, secretions), which have been infected by ticks[113,114].

Q fever is a zoonosis caused by the bacterium Cx. burnetii. Human infection mainly occurs through inhalation of contaminated animal products, direct contact with infected animals and consumption of unpasteurized milk or other dairy products contaminated with this pathogen. Ticks play a key role in transmitting bacteria between animals, and are considered as reservoirs of Cx. burnetii bacteria and guarantee the long-term presence of this microorganism in nature[84]. Borrelia spp. is the causative agent of Lyme disease and relapsing fever which are zoonotic vector-borne diseases transmitted primarily by ticks[115]. In a descriptive and retrospective study during 1997-2006, Masoumi et al. reported that the disease is detected in humans in 18 provinces of the 31 provinces in Iran[116].Other reports also revealed that Borrelia spp. is present in ticks and other vertebrates[35,117]. According to reports of Cx. burnetii and Borrelia spp. in ticks, humans, and animals in Iran, Q-fever, Lyme disease and relapsing fever can be considered as emerging diseases in the country[118-120].

The most infected provinces in terms of tick-borne pathogens were Kurdistan (20.5%), Ardabil (18.6%), South Khorasan(17.6%), Lorestan (17.2%), East Azerbaijan (13.3%) and Fars(11.5%), respectively. Geographically, these provinces (except South Khorasan) are located in the western parts of Iran. Therefore,it can be concluded that although tick-borne pathogens have been reported from different regions of Iran, the western part of the country is more infected than other regions. This high prevalence can be justified due to high livestock population, common border with neighboring countries and traditional livestock holding methods with low hygiene.

In this analysis, 26 out of 31 provinces were surveyed regarding tick-borne pathogen detection in ticks; meanwhile, the status of infection of ticks to different pathogens remained unclear in five provinces: Alborz, Bushehr, Chaharmahal and Bakhtiari, Markazi,and Zanjan. Due to the importance of ticks and their impact on human and animal health, it is highly advisable to conduct studies concerning tick-borne diseases to clarify the status of these provinces. Vector surveillance seems to be vital for observing the presence or occurrence of emerging and reemerging tick borne diseases in Iran and provides a preliminary warning for predicting probable epidemics.

In our analysis, H. anatolicum (20.35%), R. sanguineus (15.00%),and R. bursa (14.08%), were the most prevalent infected ticks in Iran. Genera of Hyalomma species have received much attention due to the role in the transmission of Theileria spp., Babesia spp., Rickettsia spp., and CCHFV. R. sanguineus (brown dog tick,kennel tick) is found worldwide with an interest toward warmer climates (tropics and sub-tropics)[121]. Dogs are specific host for R. sanguineus, however, it can be found on domestic ruminants and other vertebrates. Several pathogens such as Ba. canis, Cx.burnetii, Eh. canis, Ri. conorii, Ri. rickettsii, Theileria sp., Anaplasma sp., and CCHFV have been isolated from R. sanguineus[122-124].R. bursa is common among livestock transmitting the protozoans Ba. bigemina, Ba. caballi, Th. equi and Ba. bovis[125]. Following these highly infected vectors, much lower prevalence levels were detected in R. appendiculatus, H. schulzei, H. rufipes, H. aegyptium and Boophilus sp. These vectors should not be underestimated, as future investigations may reveal a high tendency of these species to transmit pathogens.

Controlling strategies against ticks and tick-borne diseases for prevention of significant losses due to both economic and public health problems are also seem to be important and helpful. Many attempts have been carried out for the control of ticks and tickborne diseases[126]. Some other additional methods have been suggested: (1) livestock sheds should be checked regularly in terms of tick infestation; (2) different species of livestock should be held separately to avoid interspecies tick infection; (3)quarantine of newly purchased animals decreases the chance of tick transmission to existing animals; (4) periodic application of acaricide and chemotherapy according to regional and national guidelines is sometimes suggested; (5) clearance of vegetation cut off the connection between different stages of tick’s life and disrupts their life cycle is also suggested; (6) some novel methods including application of vaccines against tick-borne pathogens,biological control, and genetically resistant livestock breeds are in the spotlight[127].

This investigation had some limitations: In the old classification of Iran provinces, some provinces are currently divided in two or more provinces, resulting in the less accuracy of the old literature,as they cover a larger area. In addition, access to the full text of some dissertations required a visit to the relevant center, which was very difficult due to the COVID-19 pandemic. In such cases,we missed some dissertations. Furthermore, the scientific name of some of tick species had changed since the publication of the associated papers, so we had to search with the old names as well.In conclusion, the occurrence of at least 20 different pathogens(in 10 genera) in 28 species (in 9 genera) of ticks in 26 provinces of Iran, sheds light on the current status of the country in terms of tick-borne pathogens. Rate of infection to different pathogens in different regions, especially western parts of Iran, is a warning for public and animal health. Further investigations and persistent surveillance of vectors as well as vertebrate hosts will expand the chance of controlling tick-borne pathogens. In most parts of the meta-analysis concerning total pathogens of Iran, the results showed high heterogeneity (I＞ 75%). Similarly, meta-analysis of separate provinces revealed high heterogeneity. This is not unexpected due to the variations associated with the different detection methods, sample size, geographical traits, location, time of the study, and population of interest. While the significance of a meta-analysis in regarding to the prevalence of tick-borne pathogens is undeniable, it is suggested that meta-analysis should not be an adequate alternative for large-scaled epidemiological studies due to heterogeneous approaches, regions and times of different studies.

Conflict of interest statement

The authors declare that there is no conflict of interest.

Authors’ contributions

HB, MK, and ASJ planned for the study. HB, ASJ, MK, and MMS performed the literature search and data extraction. MK and ZT critically evaluated the manuscript. ASJ performed the metaanalysis. The final manuscript approved by all the authors.