To define the role of birds as reservoirs and disseminators of Borrelia spirochetes, we characterized tick infestation and reservoir competence of migratory passerine birds in Sweden. A total of 1,120 immature Ixodes ricinus ticks were removed from 13,260 birds and assayed by quantitative polymerase chain reaction (PCR) for Borrelia, followed by DNA sequencing for species and genotype identification. Distributions of ticks on birds were aggregated, presumably because of varying encounters with ticks along migratory routes. Lyme borreliosis spirochetes were detected in 160 (1.4%) ticks. Borrelia garinii was the most common species in PCR-positive samples and included genotypes associated with human infections. Infestation prevalence with infected ticks was 5 times greater among ground-foraging birds than other bird species, but the 2 groups were equally competent in transmitting Borrelia. Migratory passerine birds host epidemiologically important vector ticks and Borrelia species and vary in effectiveness as reservoirs on the basis of their feeding behavior.
Recent outbreaks of West Nile virus infection or avian influenza indicate that birds participate in the ecology of zoonotic infections, an important cause of illness and death in humans and animals. The emergence of these threats underscores the need for understanding the maintenance of bird-associated infections in nature, which is prerequisite for disease prevention.
Migratory birds are known to carry several microbial agents of human disease, including viruses, chlamydiae, and enterobacteria. Evidence of the last 2 decades indicates that birds in North America and Eurasia host vectorborne pathogens, such as Anaplasma species and Lyme borreliosis (LB) spirochetes. LB is the most common vectorborne zoonosis in temperate regions of the Northern Hemisphere and is transmitted to humans by Ixodes ticks. Borrelia spirochetes infect naive Ixodes larvae when they feed on a reservoir host and are transmitted back to the reservoir population by infected nymphs. Rodent species, such as the white-footed mouse (Peromyscus leucopus) in the northeastern United States and Apodemus and Clethrionomys species in continental Europe, are common hosts of both immature ticks and LB spirochetes. However, recent field vaccination and biodiversity studies suggest that alternative hosts play a greater role than expected in the natural cycle of LB.
In comparison with studies of mammals as LB reservoirs, few studies have been conducted on the role of birds as hosts of Borrelia. The natural cycle of LB spirochetes, in particular Borrelia garinii, involves seabirds in northern Europe and game birds in the United Kingdom, which are the most studied models. However, the relationship between migratory passerine birds and Borrelia is less understood. Although experimental studies on avian infection have been conducted, less is known about reservoir competence of natural bird populations, especially those that could transmit ticks that frequently bite humans.
Information that would allow comparison of the reservoir importance of bird and other vertebrate populations is not available or is controversial. Although 1 modeling study found that the frequency of LB cases was positively correlated with species diversity of ground-dwelling birds, other studies have found the contribution of birds in hosting and infecting ticks to be low. Another uncertainty is epidemiologic implications of LB group spirochetes associated with birds. For example, birds in Europe are reservoirs of B. valaisiana, which has not been associated with disease.
In the present study, we characterized tick infestation and Borrelia transmission from migratory passerine birds captured in southern Sweden to further define their importance as reservoirs and disseminators of these spirochetes. We found that these birds are hosts of epidemiologically important vector ticks and Borrelia species. However, exposure of birds to ticks, which depends on feeding habits, determines their effectiveness as Borrelia reservoirs.
Emerging Infectious Diseases
July 18, 2006
Original web page at Emerging Infectious Diseases