Uit: Frontiers in Cellular and Infection Microbiology.
Gunnar Hasle, University of Oslo, Blindern. Norway
Birds, particularly passerines, can be parasitized by Ixodid ticks, which may be infected with tick-borne pathogens, like Borrelia spp., Babesia spp., Anaplasma, Rickettsia/Coxiella, and tick-borne encephalitis virus. The prevalence of ticks on birds varies over years, season, locality and different bird species. The prevalence of ticks on different species depends mainly on the degree of feeding on the ground. In Europe, the Turdus spp., especially the blackbird, Turdus merula, appears to be most important for harboring ticks. Birds can easily cross barriers, like fences, mountains, glaciers, desserts and oceans, which would stop mammals, and they can move much faster than the wingless hosts. Birds can potentially transport tick-borne pathogens by transporting infected ticks, by being infected with tick-borne pathogens and transmit the pathogens to the ticks, and possibly act as hosts for transfer of pathogens between ticks through co-feeding. Knowledge of the bird migration routes and of the spatial distribution of tick species and tick-borne pathogens is crucial for understanding the possible impact of birds as spreaders of ticks and tick-borne pathogens. Successful colonization of new tick species or introduction of new tick-borne pathogens will depend on suitable climate, vegetation and hosts. Although it has never been demonstrated that a new tick species, or a new tick pathogen, actually has been established in a new locality after being seeded there by birds, evidence strongly suggests that this could occur.
There is ample evidence that birds, particularly passerines, can be parasitized by Ixodid ticks (Hoogstraal et al., 1961, 1963; Nuorteva and Hoogstraal, 1963; Anderson and Magnarelli, 1984; Mehl et al., 1984; Weisbrod and Johnson, 1989; Stafford et al., 1995; Olsen et al., 1995a; Nicholls and Callister, 1996; Smith et al., 1996; Ishiguro et al., 2000; Alekseev et al., 2001; Bjöersdorff et al., 2001; Scharf, 2004; Comstedt et al., 2006; Poupon et al., 2006; Ogden et al., 2008; Hasle et al. 2009). These ticks may be infected with tick-borne pathogens, like Borrelia spp. (Olsen et al., 1995a,b; Gylfe et al., 2000; Hanincova et al., 2003; Comstedt et al., 2006; Poupon et al., 2006; Ogden et al., 2008; Hasle et al. 2010; Kjelland et al., 2010; Franke et al., 2012; Socolovschi et al., 2012), Anaplasma spp. (Alekseev et al., 2001; Bjöersdorff et al., 2001; Daniels et al., 2002; Ogden et al., 2008; Franke et al., 2012), Babesia spp. (Hasle et al., 2011), Rickettsia/Coxiella (Elfving et al., 2010; Socolovschi et al., 2012) and Tick-borne encephalitis virus (TBEV) (Waldenström et al., 2007; Geller et al., 2013).
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Migrating bats can, like birds, cross barriers and move long distances in a short time, and could also have a potential of transporting ticks and tick-borne pathogens, like Bartonella, Borrelia spp. and members of the family Rickettsiales (Gill et al., 2008; Mühldorfer, 2013).
Although it has never been demonstrated that a new tick species, or a new tick pathogen, actually has been established in a new locality after being seeded there by birds, the evidence strongly suggests that this could happen. Hasle et al. (2009)
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Transmission of a tick-borne pathogen through co-feeding has been demonstrated for the tick-borne encephalitis-virus (TBEV) on Myodes glareolus and Apodemus sylvaticus (Labuda and Randolph, 1999), but no data exists for such transmission of TBEV on birds. In Waldenström et al.'s material (2007) they found three TBEV-positive I. ricinius, one nymph and two larvae, on one individual European robin, Erithacus rubecula, which strongly suggests that the bird either was viremic or that transmission through co-feeding did occur.
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I. persulcatus survives extreme winter temperatures in Siberia, and could possibly do that in Scandinavia as well. The findings of I. persulcatus and human cases with S-TBEV in Kokkola (N63°50′ E23°07′), Finland, several hundred kilometres from the known western distribution range of I. persulcatus (Jääskeläinen et al., 2006), suggests long distance transport by birds.
