Beware of hitchhiking ticks? Clarifying the variable roles of bird species in tick movement along migratory routes

Description

Ticks are blood-feeding parasites which act as major vectors for various pathogenic microorganisms affecting both animal and human health. Hard ticks are known to move passively (i.e. ‘hitchhike’) on migratory birds as they transit between breeding and non-breeding grounds. This potentially leads to exchange and establishment of non-endemic tick species in novel environments. However, it is not yet clear if all migratory bird taxa play an equal role in movement of specific tick species, especially outside of medically important tick taxa. To clarify these interactions, we performed a systematic literature review regarding primary data of ticks moving on migratory birds within the African–Western Palearctic flyways. In total, 35 studies were found which showed 123 bird species from 37 families and 12 orders connected to potential movement of 30 tick species representing six genera (Amblyomma, Dermacentor, Haemaphysalis, Hyalomma, Ixodes, Rhipicephalus). Most tick species did not show high abundance for any bird species, or, if they did, only on very few. Only Ixodes ricinus and Hyalomma marginatum were estimated to be carried at above average burdens by multiple bird species. This could indicate an increased likelihood of these species to be moved during migration. Specific tick species or whole genera were only found in certain migratory seasons as expected based on their geographic distributions. Even so, species found in both migratory seasons did not differ in their estimated abundances on birds between seasons. This result could suggest that tick abundance on migrating birds is not always a direct result of geographic distribution and may suggest an understudied importance of stopover sites towards potential tick introduction or turnover. Taken together, the results presented here provide guiding information for future analyses integrating individual level variation into the current understanding of tick movement with migratory birds. Data included here are the studies and the raw collected data in relation to the above mentioned project.

Steps to reproduce

For data acquisition, we searched the SCOPUS and Web of Science (WoS) databases utilizing a standardized search term. Both databases were accessed on February 21st 2023 using the search string “Tick AND Bird AND Migration NOT America”. To account for any studies which could have been missed in the original search query, we extracted all references from previously published reviews matching the criteria: (1) concerned hard ticks (Acari: Ixodidae) infesting birds, and (2) research focused on the African-Western Palearctic flyways (n=7) and added these references to the primary research articles found through the SCOPUS and WoS queries. This led to a total of 1024 studies which were then screened based on the following criteria: 1) removed duplicate publications, 2) provided primary data of hard ticks (Acari: Ixodidae) infesting birds, 3) research focused on African-Western Palearctic flyways, 4) study conducted during active migration period (March-May, August-October), 5) data on individual tick and bird species interactions provided, and 6) included the number of birds caught per species. This led to 35 studies being used for data collection. All screening was performed in the R-package “metagear” independently by two reviewers (LB, RER). Any disagreements regarding inclusion of a study were then discussed individually after processing. See Figure S1 for an overview of the literature search undertaken. All data is included collapsed_data_tickstudies_with_number_caught_birds.xlsx file. WorkSpace_260524.RData - contains the model outputs which are reported in the published manuscript found at DOI: 10.1111/jav.03275. Both models were run with a gamma error distribution with a log link function using the brms command from the “brms” package as implemented in R v. 4.4.0. Each model was run for 30,000 iterations with a warm up of 7,500 with adapt_delta =0.99 and max_treedepth = 15. We included a vaguely informative, normal distributed prior on the fixed effects with µ = 0 and Θ = 10. Priors for other parameters were left as defaults. Convergence of the model was determined through checking that Rhat = 1.0, that Bulk_ESS and Tail_ESS values exceeded 1000, and that proper mixing was observed in all trace plots.

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