Different Competence as a Lyme Borreliosis Causative Agent Reservoir Found in Two Thrush Species: The Blackbird ( Turdus merula ) and the Song Thrush ( Turdus philomelos )

Owing to certain aspects of their biology and behavior, passerine birds are significantly involved in the dissemination of pathogens responsible for numerous zoonoses. This principally involves infectious diseases vectored by blood-sucking parasites, for example, hard ticks belonging to the Ixodidae family. The most prevalent tick-spread disease is Lyme borreliosis (Lyme disease) caused by Borreliella spp. bacteria (formerly Borrelia burgdorferi sensu lato complex). In terms of epidemiology, it is the most common among those involving birds as a pathogen reservoir in its enzootic cycle.

Passerine birds predominantly inhabit forests, and moreover often penetrate the lowest zones of these habitats, which are the most extensively tick inhabited. However, the relative contribution of any bird species in terms of hosting ticks may vary despite similar environmental conditions. One of the reasons for this is differences in species-typical behaviors (Marsot et al. 2012). Among several dozen passerine species investigated, thrushes (Trudidae) have been shown to carry substantially higher tick loads. Consequently, as evidenced in tissue samples and from the ticks feeding on them, these birds bear relatively high levels of Borreliella spp. infection (Gryczyńska et al. 2004, Gryczyńska and Welc-Falęciak 2016). This study investigates two passerine species, blackbird (Turdus merula) and song thrush (Turdus philomelos). Both are ground feeders that tend to nest in the lowest zones of their respective habitats (Marsot et al. 2012). When plotted against multipasserine bird communities, these species form relatively good Borreliella spp. infection reservoirs. The principal aim of this study was to identify the differences in Borreliella spp. reservoir competence between these two thrush species.

A total of 26 blackbirds and 20 song thrushes living in the same forest habitat (The Mazurian Lake region, northeast Poland, 53°47′N, 21°34′E) were captured using mist nets, ringed, and visually examined for the presence of ticks. All ectoparasites found were removed, and their species and life stages identified. The birds were then released back into their habitat. All the ticks collected were at least partially engorged, so all of them were tested for the presence of Borreliella spp. DNA. This was performed using the nested-PCR technique, based on two-stage amplification of the fragment of the fla gene coding for a bacterium flagellar protein. All laboratory procedures used in this study were as published previously (Wodecka et al. 2009).

All the ticks removed from the birds, a total of 157 individuals, belonged to the species Ixodes ricinus (19 larvae, 138 nymphs). No significant difference of tick prevalence was found, with 88.5% blackbirds and 70% of song thrushes infested (χ^{2 =} 2.45; p = 0.12). However, the mean number of ticks per bird was found to be significantly greater for blackbirds than for song thrushes, at 4.62 and 1.85, respectively (F = 7.03; p = 0.01) (Table 1). Subsequent screening for Borreliella spp. in ticks revealed that both tick infection prevalence (49.2% vs. 18.9%; χ² = 24.26; p < 0.001) and mean number of infected individuals engorging on birds (2.27 vs. 0.35; F = 6.07; p = 0.02) were higher in blackbirds (Table 1). The presence of infected larvae was recorded in a total of five individuals (26.3%): four blackbirds, and one song thrush. In addition, among the group of birds with at least four engorging ticks (blackbirds: n = 14; song thrushes: n = 4) four blackbirds (28.6%) were found to carry almost all (80–100%) the infected ticks, whereas there were no song thrushes with such extensively infected tick burdens.

The Borreliella spp. reservoir competence of particular bird species is evaluated based on its ability to infect pathogen-free ticks. Consequently, both the investigated thrush species presumably could participate in the pathogen enzootic cycle, although with different efficiencies. Compared with other passerine bird species, both blackbirds and song thrushes are known to host relatively high tick burdens (Taragel'ová et al. 2008, Lommano et al. 2014). Our results show a very high tick infestation level in both the species, which is in line with previous findings presented in studies conducted in Central Europe (Kipp et al. 2006, Dubska et al. 2009). The prevailing majority of specimens examined were attacked by at least one tick, so the infestation pattern seems to be similar for both species. However, the blackbirds were found to carry relatively more infected ticks than song thrushes, suggesting a greater reservoir competence among blackbirds. This is consistent with previous data concerning infected tick levels among the two thrush species [Poland 13.3% vs. 3.9% (Michalik et al. 2008); Switzerland 54.6% vs. 41% (Lommano et al. 2014); Germany 38.4% vs. 33.3% (Kipp et al. 2006)]. A study by Dubska et al. (2009) conducted in the Czech Republic clearly demonstrated that avian-associated Borrelia garinii and Borrelia valaisiana spirochetes were detected more frequently in ticks infesting blackbirds compared with those parasitizing song thrushes. On the contrary, there is no such evident species-specific difference in tick infection prevalence shown in studies conducted in Slovakia and the Czech Republic [ticks derived from blackbirds vs. song thrushes: 35.9% vs. 38.1% (Taragel'ová et al. 2008), and 28.6% vs. 30% (Hanincová et al. 2003)]. A reverse trend with a lower prevalence of Borreliella spp. in ticks feeding on blackbirds compared with song thrushes was described in Switzerland [27.6% vs. 41.3% (Humair et al. 1993)].

Nonetheless, this study confirms a greater efficiency in blackbirds infecting ticks, by the fact that some individuals of this species were found to convey the pathogen to almost all feeding ticks. This includes larvae, which are known as a stage showing a very low (near zero) infection rate. This difference in infection competence between the thrush species therefore supports the conjecture that certain species are particularly predisposed to act as a pathogen reservoir. Besides species-typical behavior facilitating interactions with ticks, it is worth noting that some aspects of birds' biology or physiology may result in differing Borreliella spp. sensitivity. Certain stress factors, for example, migratory restlessness, could also adversely affect thrushes' immune system and decrease their bactericidal responses, causing reactivation of latent Borreliella spp. (Gylfe et al. 2000). The greater reservoir competence of blackbirds suggests the differentiated dynamics of host–pathogen interactions among selected species, and consequently their potentially disparate role in the ecology of Lyme borreliosis.