Birds play an undetected role in the spread of tick-borne diseases as they can travel long distances and tend to split their time in different parts of the world – patterns that are changing due to climate change. When scientists know which species of birds can infect ticks with pathogens, they can predict where tick-borne diseases can occur and pose a health risk to humans.
A new study published in the journal Global Ecology and Biogeography used machine learning to identify species of birds that can transmit the Lyme disease bacterium (Borrelia burgdorferi) to feeding ticks. The team developed a model that identified birds known to spread Lyme disease with an accuracy of 80% and identified 21 new species that should be prioritized for monitoring.
Lead author Daniel Becker, a postdoctoral fellow at Indiana University, says, “We know that birds can infect ticks with the Lyme bacterium. So far, however, no one has systematically investigated the ecological and evolutionary factors that influence which bird species are most likely to harbor and spread Borrelia burgdorferi worldwide. We set out to fill this gap by identifying features of bird species that Lyme are most likely to pass on to feeding ticks. “
American Robin, Copyright Mark S Szantyr, from the Surfbirds Galleries
Senior author Barbara Han, a disease ecologist at the Cary Institute of Ecosystem Studies, says, “To predict and monitor species that can transmit tick-borne diseases to humans, we first need to know what traits make certain animals good pathogens . Here we used machine learning to assess bird species traits, coupled with Lyme infection data from ticks found in birds to predict bird species that have the potential to spread Lyme. “
In this study, the team searched published literature to find studies reporting Lyme infection from ticks that feed on birds. The global search found 102 studies, including data from ticks found on 183 species of birds. Of these, 91 carried ticks that tested positive for Borrelia burgdorferi. These bird species are considered “competent” reservoir species because they are known to infect feeding ticks with Borrelia burgdorferi. The marked species are diverse and extend across America, Africa, Asia and Oceania.
Next, machine learning was used to compare characteristics of competent bird species to 4,691 other bird species. The data included information on biographical characteristics such as diet composition, foraging location, height, lifespan, reproductive rate and young age, as well as geographical information such as migration distance, global distribution and maximum altitude. They also looked at the parent corticosterone – the stress hormone in birds – which can affect susceptibility to infection.
The model identified birds known to spread Lyme to ticks with 80% accuracy and revealed 21 new species to be prioritized for surveillance by sharing traits with known competent species. High-risk species tend to have low baseline corticosterone levels, breed and hibernate at high latitudes and low altitudes, are widespread and occur either in the extreme case of the continuum of the pace of life (species that breed early and die young or breed late and are lived longer) .
Species in the genus Turdus, commonly known as true thrushes, have been found to have a significantly higher likelihood of competence when compared to other taxa. This finding suggests that thrushes may be the riskiest species of bird for Lyme transmission. Passerines, or crouching birds, also tended to be more proficient, as did birds that primarily eat seeds and those that eat on the ground – behavior that would put them within the reach of ticks.
Identifying Lyme-competent bird species could have a direct impact on our health. Tick-borne diseases, especially Lyme disease, can be difficult to diagnose. Knowing where ticks and the diseases they transmit can help doctors prepare for diagnosis and treatment, and improve health outcomes for patients.
Due to climate change, the breeding areas of many birds are shifting north. When birds spread to higher latitudes, ticks and pathogens also spread. Some species of birds have fully or partially settled in cities and suburbs. Birds that can thrive in developed environments, especially those that hibernate in these new locations in close proximity to humans, increase residents’ risk of developing a tick-borne disease.
Becker says, “Birds don’t spread Lyme directly to humans, but they can move infected ticks to new places without Lyme having occurred in the past. A tick could fall from a bird into a garden or yard, where it could later bite and infect a person. If local doctors are unfamiliar with Lyme symptoms, proper diagnosis may be delayed. Knowing where ticks are spreading could improve the medical response to Lyme disease and other tick-borne diseases. “
Han summarizes: “These results remind us that pathogen competence differs widely, even in animals from the same family. Using machine learning techniques, we can analyze animal traits and predict risky species on a global scale – not just for Lyme but also for other tick-borne and zoonotic diseases that involve multiple host species. These predictions could provide vital information to guide early intervention, prevent disease overflow, and protect our health. “