Wild birds across the globe can carry the causative agent of avian borreliosis, Borrelia anserina, and that of human Lyme borreliosis, Borrelia burgdorferi sensu lato in the ticks attached to them. Currently, only limited proof exists for the presence of these pathogens in samples taken from living wild birds, carried by the birds as a reservoir, without symptoms. We investigated blood samples of large-bodied wild birds admitted to a bird hospital, where basic clinical symptoms were recorded. Thirty-six blood samples were collected from them, according to the DualDur kit methodology, and investigated using dark-field microscopy to search for spirochetes. Five of the samples were also investigated by immunofluorescence microscopy to identify Borrelia burgdorferi. Since the main reason for admittance to the hospital was a physical injury, no significant symptoms were observed regarding a current Borrelia infection. Out of the sixteen birds of prey, eleven (68%) were infected with spirochetes, and fifteen out of twenty storks (75%) were also infected with spirochetes, without major symptoms. All five samples investigated using immunofluorescence were confirmed to contain the human pathogenic Borrelia burgdorferi sensu lato. Thus, direct investigations of the blood of wild birds may show the asymptomatic prevalence of Borrelia in the wild bird population.

Dark-field microscopy has been increasingly applied in diagnostics in recent years, for example in Lyme borreliosis, due to the growing concerns about the diagnostic utility of standard serological tests. Although the diagnostic value of microscopic methods is still debated due to the lack of proper clinical testing and the definition of the diagnostic cutoff levels, it is important that the spirochetes causing the disease can be visualized from body fluids via dark-field investigation. Additionally, a specific identification of Borrelia is still not routinely possible, visual observation is usually fortified with additional PCR investigations. We are publishing the pictures from the first-time application of an enhanced dark-field illumination technique that improves the resolution and picture quality of digital recordings. The same illumination setup, combined with a novel live staining of Borrelia burgdorferi with cleaned, FITC-labelled anti-Borrelia antibodies, has produced remarkable recordings of the same spirochetes, in natural motion. All experiments were performed on remnant samples of DualDur kit laboratory investigations.