Introduction
Green spaces inside European towns such as parks, leisure-time areas for hiking and biking, botanic gardens, private properties with gardens, cemeteries, and urban forests, create favourable environmental (e.g., temperature and humidity) conditions for
Ixodes ricinus, the most widespread and important vector of tick-borne pathogens (TBPs) in Central Europe. One of the key factors influencing the survival and maintenance of local tick populations is their access to appropriate and abundant tick bloodmeal hosts [
1]. Within urban green areas, immature stages of this primarily a forest-dwelling tick species, feed mostly on small rodents, ground-feeding passerines, and hedgehogs which are additionally important hosts of adult female ticks. The latter feed also on urban pet populations represented by dogs and cats, including stray animals. The observed ongoing increase in the number of pets in towns, suggests that this group of mammals appears to be increasingly important for the persistence and size of tick populations in these ecologically altered habitats. Some of vertebrate species acting as maintenance hosts for
I. ricinus, may concurrently serve as reservoir hosts of TBPs. Their high or low abundance and species composition in urban habitats, influence the level of infection of local tick populations and is critical for public health importance [
1]. Therefore, investigations regarding the prevalence of TBPs infecting ticks, are necessary to establish or predict the emergence of active endemic foci of tick-borne diseases. It is particularly crucial because, there is still a lack of comprehensive knowledge on the eco-epidemiology of these infections in urban ecosystems and our understanding of how urbanization affects pathogen-host-vector relationships [
2].
Ticks can carry two or more pathogenic microorganisms with a subsequent high likelihood of co-transmission to humans or animals [
3,
4].
Ixodes ricinus ticks infected with various bacteria, i.e., spirochetes of the
Borrelia burgdorferi sensu lato (s.l.) complex, the agent of Lyme borreliosis (LB) or
Anaplasma phagocytophilum, the agent of human anaplasmosis, are regularly found in urban and suburban areas across Europe [
5,
6,
7]. Furthermore,
B. burgdorferi s.l. spirochetes and intraerythrocytic parasites of the protozoan genus
Babesia (Apicomplexa: Piroplasmida), including
Ba. microti, Ba. divergens, Ba. venatorum, and
Ba. duncani (present only in North America), can co-occur and be co-transmitted by ticks of the
Ixodes ricinus species complex [
8]. These protozoan pathogens are responsible for human babesiosis causing a febrile hemolytic anemia, that is generally asymptomatic or self-limiting in healthy humans, however, is a serious health concern in splenectomised, immunocompromised patients [
9]. In North America, the major agent of human babesiosis is
Ba. microti, a parasite associated with small mammals serving as primary reservoir hosts. There are eco-epidemiological evidence indicating that coinfections
B. burgdorferi s.l. and
Ba. microti among ticks and
Peromyscus leucopus mice may contribute to the emergence and expansion of
Ba. microti in the enzootic cycle. Ecological models demonstrated the strongest effects when the prevalence of
B. burgdorferi in mice was high [
10]. Recent research clearly demonstrated that infection of
I. scapularis ticks with
B. burgdorferi s.l. spirochetes increases the likelihood of infection with
Ba. microti as well as
A. phagocytophilum compared with borreliae-free ticks [
11]. Furthermore, according to Zembsch et al. [
12] host-seeking
I. scapularis ticks that are infected with
Ba. microti, are more likely to be coinfected with
B. burgdorferi than expected if the pathogens were transmitted independently. This implies that such positive pathogen-vector-host interactions of both tick-borne agents may favour their emergence and maintenance in local tick populations [
13]. Co-infections may also change clinical symptoms, course, severity of tick-associated disease in humans and animals compared to those induced by a single infection [
14,
15,
16]. Patients co-infected with
B. burgdorferi s.l. and
Ba. microti suffer from significantly more diverse, intense, and persisting disease symptoms compared to those infected with each pathogen separately [
17,
18]. It has been shown that
Ba. microti weakens adaptive immunity and increases the severity of LD [
19]. Furthermore, human coinfection with
Ba. microti and
B. burgdorferi s.l. seems to be serious clinical problem because of the difficulties in diagnosis and treatment, since the antibiotics used to treat borreliae are ineffective against
Ba. microti [
20]. In Europe, most cases of human babesiosis are attributed to
Ba. divergens, usually a cattle parasite or less frequently to
Ba. venatorum for which the roe deer is the main reservoir host. Interestingly, European genotypes of
Ba. microti infecting humans are not as infectious or pathogenic than those in the USA [
21].
The aim of our study was to ascertain the co-occurrence of B. burgdorferi s.l. spirochetes and Babesia piroplasms in I. ricinus ticks collected from vegetation as well as from dogs and cats in urban areas of the city of Poznań, west-central Poland.
4. Discussion
In this study, we described prevalence of B burgdorferi s.l. and Babesia spp. found in mono- and double infections among I. ricinus ticks occurring in urban areas of the city of Poznań. We focused on two tick groups: (i) a group of host-seeking collected from vegetation, and (ii) a group of feeding ticks removed from pet animals, dogs and cats.
According to a review by Hansford et al. [
31], the mean
Borrelia prevalence in questing ticks in urban green areas across Europe was 17.3% (range: 3.1% to 38.1%). In our study, 8.7% of ticks (range: 0.0% to 40.6%) yielded borreliae. Comparable or higher mean infection rates were found in
urban I. ricinus populations of several European cities from neighboring countries. For example, in Slovakia, the prevalence of
B. burgdorferi s.l. ranged from 6.8% to 15.3% in parks of Bratislava,
[32,33], and in agglomerations of Košice and Bardejov reached 10.2%
[34]. In the Czech Republic, 12.1% of ticks collected in parks in Brno and 13.2% in Ostrava city, yielded spirochetes
[35,36]. Furthermore, in the city parks of Vilnius, Lativa, and in recreational areas of Hanover, Germany, 25% of ticks yielded B. burgdorferi s.l. [
37,
38]. Comprehensive studies on host-seeking ticks infected with B. burgdorferi s.l. in strictly urban agglomerations in Poland, are relatively rare. In city forests and parks of Warsaw, the mean prevalence of Borrelia spirochaetes was 10.9% [
6], and in the Tri-City agglomeration area of Gdańsk, Gdynia, and Sopot, reached 12.4% [
39]. To date, the highest levels of Borrelia infections have been described in green areas of the city Białystok (25.7%) and Olsztyn (27.4%) [
40,
41]. In our study, the highest prevalence of 40.3% (28/69) was recorded in ticks of the Citadel Park, the largest municipal park strictly in the center of the city. Although, the number of tested ticks was limited, this high infection level, may result from the lack of roe deer, that is considered to eliminate
B. burgdorferi s.l. in feeding ticks [
42].
The overall prevalence of B. burgdorferi s.l. found in ticks from pet animals (4.7%), was almost two-fold lower than in host-seeking ticks (8.7%). Comparable prevalences were described in the Netherlands [
43,
44] and Austria (4.8% and 5.2%, respectively) [
45]. Higher infection rates in I. ricinus mainly from dogs, were recorded in Lativa (10.7%), Germany (11.6%), Finland (11.8%), Norway (14%), and Denmark (15%) [
46,
47,
48,
49,
50]. So far, the highest prevalences in I. ricinus from dogs have been found in two city agglomerations: Wrocław (21.7%), in the southwest [
51] and Olsztyn (34.4%), in the northeast of Poland [
52].
Borrelia afzelii and B. garinii proved to be the most prevalent species among infected ticks both from vegetation and pets. Our results are in agreement with the frequency of main Borrelia species infecting host-seeking I. ricinus ticks in central Europe [
53] and with a report by Skotarczak [
54], documenting that B. afzelli and B. garinii are the most common species in dogs. Dogs can develop infection of B. burgdorferi s.l. and exhibit the presence of antibodies, but unlike humans, they rarely get sick [
55].
The distribution of Borrelia species in our study, may be explained by species composition of wild hosts. The predominant B. afzelii, together with B. spielmanii, are maintained in enzootic cycles associated with small rodents and medium-sized mammals, including hedgehogs [56,57,58]. We found B. spielmanii only in three feeding females, what confirms the rarity and highly focal distribution of this spirochete. The mentioned groups of mammals were observed in our study locations, with predominance of small rodents. In the present research, B. afzelii prevailed in feeding ticks (3.7%). This spirochete was also the most prevalent (3.6%) followed by B. garinii (1.7%), B. valaisiana (1.4%) and B. spielmanii (1.4%) in ticks from dogs in Lativa [49]. A reverse pattern, with predominance of B. garinii over B. afzelii (28.1% vs. 3.2%), was observed in ticks from dogs in north-eastern Poland [52]. Avian associated B. garinii and B. valaisiana [59] were present in both infected tick groups, with almost a fourfold higher prevalence of B. garinii in ticks from vegetation in comparison to feeding ticks (42.2% vs. 11.2%). The lizard-associated B. lusitaniae [60] prevailed in host-seeking ticks compared to pet-derived ticks (10% vs. 1.7%). The finding of this spirochete only at the Rusałka lake, suggests its highly focal distribution.
In this study, the method of amplification and sequencing of the V4 region in 16S rRNA was not specific enough to clearly identify the obtained B. burgdorferi s.l. sequences to species as displayed on the phylogenetic tree. The 16S rRNA region had different sequences of V4 for some species identified by flaB gene, therefore the complex analysis, including other marker genes is necessary to correctly assign 16S rDNA sequences to the species.
We detected
Babesia DNA in 4.4% of host-seeking ticks, which were recorded in four out of five locations (range: 0.8% to 9.8%). The meta-analysis by Onyiche et al. [
61] estimated the overall prevalence of babesiae in questing
I. ricinus in Europe at 2.1%. Our results agree with infection rates found in the city Białystok (3.7%) [
41] and within the Tri-City agglomeration (4.5%) [
62]. Lower prevalences (range: 0.4% to 0.5%) were reported in ticks tested in Bavarian public parks [
63] or in urban Bielański Forest (0.8%), in Warsaw [
64].
Babesia DNA was identified in 6.0% of feeding ticks, with a higher prevalence in ticks from dogs (6.8%) compared to those from cats (3.9%). Stensvold et al. [
48] documented a prevalence of 8.0% in dog-derived ticks in Denmark. Lower prevalences of
Babesia spp. in pet-derived ticks (usually from dogs) were 0.8% in the Netherlands [
44], 1.0% in Finland [
50], 1.4% (62/4316) in the United Kingdom [
65], 2.5% in Germany [
47], and 4.7% in Latvia [
49]. The highest infection rates in ticks feeding upon dogs (66.8%) and cats (15.4%) were reported in southern Poland [
66].
In Europe,
I. ricinus is involved in the transmission cycles of
Ba. divergens, Ba. venatorum, Ba. microti, and
Ba. capreoli, of which the first three are considered as human pathogens [
8]. In our study, three species including
Ba. microti, Ba. venatorum and
Ba. canis were identified in both groups of ticks. The finding of the first two species, might be explain the availability of ticks to suitable reservoir hosts of both parasites. In Europe,
Ba. microti infects small rodents [
67]. The reservoir host for
Ba. venatorum, is the roe deer, and this species was observed in at least four out of five study sites. However, we did not find
Ba. divergens, the main etiological agent of human babesiosis in European patients [
21]. Since cattle are regarded as the reservoir hosts for
Ba. divergens, their absence in urban areas of Poznań, may explain that we failed to find the pathogen. In our study,
Ba. canis and
Ba. microti were the most prevalent species in feeding (2.6% and 1.4%, respectively) and host-seeking ticks (2.8% and 2.2%, respectively), whereas
Ba. venatorum was the rarest piroplasm. Although, the role of
I. ricinus in the transmission of
Ba. canis remains still unclear, this parasite has already been found in questing ticks in northern Poland (range: 0.2% to 5.8%) [
40,
62,
68], in Slovakia and the Czech Republic (range; 0.2% to 1.4%) [
69,
70]. In our previous study, we found that 10.2% of ticks in forest ecosystems in west-central Poland were positive for
Ba. canis DNA, the highest prevalence ever reported for this tick [
22]. European
Ba. canis strains belong to the type A or type B, which induce different clinical symptoms of canine babesiosis [
71]. In this previous and the current study, all
Ba. canis obtained sequences represented the milder type A. In Europe,
Ba. canis infections in
I. ricinus from dogs were previously documented only in Latvia and Russia, with prevalences of 1.0% and 3.8%, respectively [
49,
72], and a prevalence of 3.2% in our study is comparable with those data. In Europe, most
Babesia infections in
I. ricinus collected from companion animals are caused by
Ba. microti and
Ba. venatorum. The overall prevalence of 2.2% of
Ba. microti in feeding ticks in our study was lower compared to a 5.7% infection rate in dog-derived ticks in Warsaw which yielded
Ba. microti Gray strain pathogenic to humans (GenBank: AY693840) [
73]. In Europe, the highest prevalences of
Ba. microti in this group of ticks, were reported in southeastern (42.6%) and southern (24.3%) Poland [
66,
74]. In the case of
Ba. venatorum, the highest prevalence of 1.6% was recorded in dog-derived ticks in Lativa [
49]. According to Onyiche et al. [
61],
Ba. microti is considered to be the most common
Babesia species in questing
I. ricinus in the eastern Europe. In our study,
Ba. microti (1.4%) was the second most prevalent species following
Ba. canis. Much higher infection rates of
Ba. microti in ticks from urban areas were 2.6% in northern [
39], 6.5% in central [
75], and 26.4% in southern Poland [
76].
Babesia venatorum that infected only 0.4% of host-seeking ticks in our study as well as in Bavarian public parks [
63], reached also low infection rates of 0.8% in central [
64] and 0.9% in northern Poland [
68].
In Europe, more than 60 cases of human babesiosis have been caused mainly by
Ba. divergens, with five cases attributed to
Ba. venatorum and eleven autochthonous cases to
Ba. microti, most of which were identified in Poland [
21,
77]. Two European genotypes of
Ba. microti can infect humans, but are less infectious or pathogenic to humans as those in the United States. The first
Ba. microti genotype (strain) "Jena/Germany" (EF413181) is closely related to the USA genotypes (Clade 1) including e.g., the Gray strain isolated from a patient in Massachusetts (AY693840), responsible for most cases of human babesiosis worldwide. The second
Ba. microti genotype, the ‘Munich’ type (AB071177) is widely distributed in Europe and belongs to Clade 3 [
78]. This genotype was originally thought to be non-zoonotic, however, it has recently been identified in six patients with mild and asymptomatic infections in Poland [
79] and in one from Spain [
80]. However, its pathological potential remains still uncertain [
77]. It should be noted, that
Ba microti sequences identified in our study, were identical to
Ba. microti genotype "Jena/Germany” that is considered as pathogenic to humans [
77]. In Poland, Welc-Falęciak et al. [
81] found the same genotype in two asymptomatic patients. Furthermore, among 1.3% of
Babesia infected
I. ricinus removed from humans in Poland, most showed a high similarity (> 99%) to the
Ba. microti strain Jena followed by
Ba. venatorum [
82]. Of note, that the sequences of
Ba. venatorum found in the two mentioned study as well as in our, were identical with those isolated from splenectomised patients in Italy and Austria [
83], and with an asymptomatic patient from Poland [
84].
Altogether, out of 121 Babesia-positive ticks, 19.8% were simultaneously Borrelia-positive. We found almost identical prevalences of co-infections with B. burgdorferi s.l. and Babesia spp. in host-seeking (1.1%) as well as in feeding ticks (1.0%). To the best of our
knowledge, this is the first report documenting the co-occurrence of both pathogens in ticks from companion animals in Europe. In Poland, coinfection rates of both pathogens in host-seeking ticks were described in the north (range; 0.3% to 0.6%) [
39,
85], in the east (1.6%) [
86], and in the northeast (2.8%) [
41]. In the present study,
Ba. canis prevailed in coinfections mostly with
B. afzelii and
B. garinii in host-seeking ticks, whereas
Ba. microti dominated predominantly with
B. afzelii, in ticks from the pets. The prevalence of 0.9% (n=9)
Ba. canis and
B. burgdorferi s.l. coinfections in host-seeking ticks in our study, is consistent with a previous Polish study, in which 1.0% of 104 ticks showed a coinfection of
Ba. canis with
B. afzelii [
87]. Recently, a fatal case of a dog co-infected with
Ba. canis and
B. burgdorferi s.l. was diagnosed in Romania [
88]. Hildebrand et al. [
89], found 1.6% of coinfections
Ba. microti followed by
Ba. divergens, mostly with
B. afzelii and
B. garinii in questing ticks in Middle Germany, whereas
Ba. microti and
B. afzelii prevailed in co-infected ticks from humans [
82].
Taking into account the mentioned studies, in host-seeking
I. ricinus,
Ba. microti seems to be the most prevalent species in co-infections with
B. afzelii and
B. garinii. Co-occurrence of these pathogens is of significant importance from a medical point of view. In humans, they may affect the clinical course of disease, especially in non-immunocompetent patients, and might be difficult to differentiate since both infections induce often nonspecific symptoms including fever, fatigue, and flu-like illness [
90,
91]. Jabłońska et al. [
92] described a symptomatic case of babesiosis and LB in a Polish immunocompetent patient after travelling to Canada and the USA. Furthermore, in a study of 24 tick-exposed individuals from southeastern Poland, a piroplasm 98.9% homologous with
Ba. divergens and
Ba. venatorum was detected in one person concurrently seropositive for
B. burgdorferi s.l. [
93]. In another Polish report, Pańczuk et al. [
94] also found a single
B. burgdorferi s.l. and
Ba. microti coinfection among foresters with significant levels of IgG anti-
B. burgdorferi antibodies.
Using our previous results regarding the presence of
B. miyamotoi spirochetes [
25] in the same group of host-seeking ticks, we found that 0.6% of 1,029 individuals yielded concurrently DNA of
B. burgdorferi s.l., with predominance of
B. afzelii and
B. garinii. Co-occurrence of both pathogens was also documented in 0.4% and 1.4% of adult
I. ricinus collected from vegetation in France [
95] and eastern Poland [
86], respectively.
In conclusion, the finding of a predominance of two pathogenic species of spirochetes, B. afzelii and B. garinii, and the detection in both groups of ticks of the sequences Ba. microti and Ba. venatorum, which are identical to human pathogenic strains, indicate that people using urban green spaces are at risk of contracting LB and/or babesiosis. The occurrence of double infections, even though with a low prevalence of 1.0%, is both clinically and epidemiologically significant and indicates that they pose a challenge for differential diagnosis in patients with acute febrile disease after contact with a tick. Furthermore, the finding of Ba. canis in both groups of I. ricinus suggests that this species could potentially be involved in the circulation of this piroplasm in areas of western central Poland, where the competent vector, Dermacentor reticulatus is absent or rare. If this is the case, urban areas may pose a potential risk of dogs acquiring babesiosis.