American Robins as Reservoir Hosts for Lyme Disease Spirochetes

To the Editor: We read with great interest the article by Richter et al. (1) reporting the competence of American robins as reservoir hosts for Lyme disease spirochetes. The article demonstrates that Turdus migratorius is a reservoir for Borrelia burgdorferi sensu lato under experimental conditions. However, we want to draw attention to certain statements in the article regarding current knowledge of the ecology of Lyme borreliosis in Europe.

First, in the discussion the statement "The standard of proof (implied xenodiagnosis)... has not previously been applied to candidate avian reservoirs" is inaccurate. In fact, this method was applied a few years ago to pheasants (Phasianus colchicus) in the United Kingdom (2) and to European blackbirds (Turdus merula) in Switzerland (3) to investigate their respective reservoir competence. Even though these articles were cited by Richter et al., the use of xenodiagnosis detailed in them was not mentioned.

A second statement in the introduction claims that "Although spirochetes have been isolated from naturally infected European blackbirds (T. merula) (Humair et al., 1998), a laboratory study failed to demonstrate reservoir competence of these birds (Matuschka & Spielman, 1992); the reason for this discrepancy remains unclear." However, the reservoir competence of European blackbirds has been clearly demonstrated by tick xenodiagnosis (3).

Another statement cited in the introduction that pheasants "...cannot contribute to transmission because larval ticks seem not to feed on them, either in the laboratory or in nature (Kurtenbach et al. 1998a, 1998b)" is incorrect; no such statement occurs in the two papers by Kurtenbach et al. (2,4). Moreover, Randolph and Craine have clearly demonstrated that pheasants contribute to Borrelia transmission to ticks (5).

Finally, in the statement "Although certain genospecies of the Lyme disease spirochetes are said to be more mouse-adapted than others (Humair et al. 1995), no experimental evidence is available to support this concept" the term concept is inappropriate. B. afzelii and B. burgdorferi have been found associated with both Muridae and Sciuridae in various ecologic situations (4,6-10). B. garinii and B. valaisiana have been observed preferentially associated with certain avian hosts and associated ticks in particular ecologic situations (3,4,11-13). All these published results (3,4,6-13) demonstrate that the relationships between genospecies of Borrelia and hosts observed in some areas of Europe have gone beyond concept and are facts. Because the explanation of a phenomenon is not immediately obvious one cannot assert that the phenomenon does not exist or that the evidence can be denied. Borrelia sensitivity to serum complement may explain the existence of a preferential relationship between host and Borrelia genospecies (14).

Lise Gern and Pierre-François Humair
Institut de Zoologie, University of Neuchâtel, Neuchâtel, Switzerland

References

1. Richter D, Spielman A, Komar N, Matuschka FR. Competence of American robins as reservoir hosts for Lyme disease spirochetes. Emerg Infect Dis 2000;6:133-8.

2. Kurtenbach K, Carey D, Hoodless AN, Nuttall PA, Randolph SE. Competence of pheasants as reservoirs for Lyme disease spirochetes. J Med Entomol 1998;35:77-81.

3. Humair PF, Postic D, Wallich R, Gern L. An avian reservoir (Turdus merula) of the Lyme disease spirochetes. Zentralbl Bakteriol 1998;287:521-38.

4. Kurtenbach K, Peacey M, Rijpkema SGT, Hoodless AN, Nuttall PA, Randolph SE. Differential transmission of the genospecies of Borrelia burgdorferi sensu lato by game birds and small rodents in England. Appl Environ Microbiol 1998;64:1169-74.

5. Randolph SE, Craine NG. General framework for comparative quantitative studies on transmission of tick-borne diseases using Lyme borreliosis in Europe as an example. J Med Entomol 1995;32:765-77.

6. Hovmark A, Jaenson TGT, Åsbrink E, Forsman A, Jansson E. First isolations of Borrelia burgdorferi from rodents collected in Northern Europe. Acta Pathol Microbiol Immunol Scand Sect B 1988;96:917-20.

7. Hu CM, Humair PF, Wallich R, Gern L. Apodemus sp. rodents, reservoir hosts for Borrelia afzelii in an endemic area in Switzerland. Zentralbl Bakt 1997;285:558-64.

8. Humair PF, Péter O, Wallich R, Gern L. Strain variation of Lyme disease spirochetes isolated from Ixodes ricinus ticks and rodents collected in two endemic areas in Switzerland. J Med Entomol 1995;32:433-8.

9. Humair PF, Gern L. Relationship between Borrelia burgdorferi sensu lato species, red squirrels (Sciurus vulgaris) and Ixodes ricinus in enzootic areas in Switzerland. Acta Trop 1998;69:213-27.

10. Humair PF, Rais O, Gern L. Transmission of Borrelia afzelii from Apodemus mice and Clethrionomys voles to Ixodes ricinus ticks: differential transmission pattern and overwintering maintenance. Parasitology 1999;118:33-42.

11. Hubálek Z, Anderson JF, Halouzka J, Hájek v. Borreliae in immature Ixodes ricinus (Acari: Ixodidae) ticks parasitizing birds in the Czech Republic. J Med Entomol 1996;33:766-71.

12. Olsén B, Duffy DC, Jaenson TGT, Gylfe Å, Bonnedahl J, Bergström S. Transhemispheric exchange of Lyme disease spirochetes by seabirds. J Clin Microbiol 1995;33:3270-4.

13. Olsén B, Jaenson TGT, Bergström S. Prevalence of Borrelia burgdorferi sensu lato-infected ticks on migrating birds. Appl Environ Microbiol 1995;61:3082-7.

14. Kurtenbach K, Sewell HS, Ogden NH, Randolph SE, Nuttall PA. Serum complement sensitivity as a key factor in Lyme disease ecology. Infect Immun 1998;66:1248-51.

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