Our investigation was designed to determine the frequency of Ehrlichia-infected ticks on migrating birds in Sweden and estimate the number of Ehrlichia-infected ticks being imported and exported by these birds. Ticks were collected from migratory passerine birds at a stopover site in southern Sweden (east coast of Öland), identified, and checked for the presence of Ehrlichia by polymerase chain reaction (PCR). To determine if different genomic species of Ehrlichia could be found in bird-borne ticks and indirectly in the birds, all Ehrlichia-positive PCR products were further subjected to DNA sequencing analysis.

A total of 3,054 birds (1,204 in the spring and 1,850 in the fall) of 56 species were caught and examined for ticks. One hundred sixty-five ticks, all I. ricinus, were collected from 73 birds of 18 species (Table), for a tick infestation frequency of 2.4% (73/3,054) and a relative density of 0.054 ticks per bird (165/3,054). Fifty-one larvae, 112 nymphs, and 2 adult females of I. ricinus were found (Table), suggesting that nymphs are more common than larvae on birds and that adult I. ricinus rarely feed on birds.

Ehrlichia DNA was detected in none of the 51 larvae, in 8.0% (9/112) of the nymphs, and in none of the adult ticks. Ehrlichia DNA was detected in 8.9% (7/79) of the nymphs in the spring and 6.1% (2/33) in autumn.

Ehrlichia 16SrRNA gene sequences in the nine positive nymphs were analyzed by DNA sequencing. The gene sequences (base positions 20-570 corresponding to base numbering for the U.S. human granulocytic ehrlichiosis (HGE) agent 16S rDNA; GenBank Accession Number U02521) were identical to E. phagocytophila genogroup sequences found in clinical cases of granulocytic ehrlichiosis in horses, dogs, cats, and humans in Sweden (1).

Every spring, approximately 100 million birds migrate through Sweden, importing large numbers of potentially infectious ectoparasites. If the frequency of infected ticks is assumed to be similar to that in this study (7 Ehrlichia-infected ticks on 1,204 birds), 581,395 Ehrlichia-infected ticks were imported into Sweden by birds during the spring migration of 1996. The bird species we found carrying Ehrlichia-infected ticks (Table) breed in FennoScandia and arrived from wintering grounds in continental Europe, North Africa, and sub-Saharan Africa. During the fall migration, the frequency was lower (2 Ehrlichia-infected ticks found on 1,850 birds). However, even if the same number of birds migrated in the fall as in the spring, an estimated 108,108 Ehrlicha-infected ticks were exported from Sweden that fall by the birds. These figures support previous studies indicating that migratory birds are an important component in the dispersal of ticks infected with medically important pathogens (8).

Only one species of tick (I. ricinus) was recorded in this study. This species is the main vector of B. burgdorferi sensu lato, E. phagocytophila, and tick-borne encephalitis virus, both in Sweden and the rest of western and central Europe. The facts that a large number of birds are infested with this tick and that the Ehrlichia 16S rRNA gene sequences found in these ticks are identical to the gene sequences found in HGE patients in Sweden, Slovenia, and the USA suggest that birds may play an important role in the dispersal of I. ricinus infected with Ehrlichia and may contribute to the distribution of granulocytic ehrlichiosis (9). We found no I. ricinus larvae infected with Ehrlichia, which may suggest that birds are incompetent reservoirs of Ehrlichia but act as carriers of infected ticks. In this way, avian transport of Ehrlichia might differ from that of B. burgdorferi sensu lato. In the latter disease, birds not only carry infected ticks of all stages but can also carry latent infections of Borrelia that can be reactivated by the stress of migration (10). However, the potential of a vertebrate to function as a tick host and a reservoir for Ehrlichia depends on many factors, including the host's density in the tick habitat, the degree of contact between the potential tick vector and the host, and its infectivity. Therefore, the question of whether birds act as reservoirs of Ehrlichia has not yet been conclusively answered, and additional studies of various bird host species should be conducted over longer time spans than our study. Moreover, the reservoir competence of birds should be studied by isolation or detection of Ehrlichia after natural or experimental infection.

The apparent increase of granulocytic ehrlichiosis in animals and humans during the last few decades may be the result of several factors, such as increased awareness of this disease and the distribution of reservoirs, vectors, and host animals. Whether passerine birds have affected this increase has not yet been conclusively demonstrated. However, our data suggest that birds are important in the dispersal of Ehrlichia.