Rising temperatures fueled by climate change may allow pathogen-infected ticks attached to birds migrating from tropical to cooler locations to survive at their destinations, researchers reported this week in Frontiers in Cellular and Infection Microbiology.
The University of Southern Mississippi–led study team used the mitochondrial 12S rRNA gene to assess the prevalence of exotic tick parasites on 14,929 migratory songbirds at six stopover sites along the northern Gulf of Mexico in autumn and seven sites in spring.
The birds (residents and short-distance and long-distance migrants) were netted, sampled, and banded from up to four sites in southwest Louisiana and three in southern Alabama. The team used forceps to remove the mostly immature ticks from Central and South America and store them in ethanol.
So far, exotic, neotropical, bird-parasitizing ticks have had limited success in establishing themselves in North America, the researchers noted.
"Due to the energetic cost of migration, most migratory birds must stop en route at stopover sites to rest and restore their energy reserves in unfamiliar habitats," they wrote. "Birds may pick up new or drop off existing ectoparasites, such as ticks, during stopover, acting as a long-distance dispersal mechanism for ectoparasites and their pathogens."
Dispersal distances of 260 to 3,100 miles
The most common parasitized birds were the hooded warblers (31%) and swamp sparrows (16%). Overall, tick parasitism among the 14,929 birds was low. The researchers documented 359 bird-parasitizing ticks belonging to the genera Amblyomma, Haemaphysalis, and Ixodes attached to 163 birds of 28 songbird species.
The most common tick genera and species were Amblyomma and Amblyomma longirostre, respectively, but no pathogenic (disease-causing) microbes were seen in Ixodes ticks. RNA sequencing revealed microbes in all ticks, most of which were seen in late spring, especially in April, when over 65% of all ticks were obtained.
The most abundant phylum was Proteobacteria, microbial profiles showed. The most common pathogens were Rickettsia and endosymbiont Francisella, Candidatus Midichloria, and Spiroplasma.
Permutation multivariate analysis of variance (ANOVA) demonstrated that the relative abundance of Francisella and Rickettsia drives microbial patterns among tick genera. The percentage of positive correlations in microbe-microbe interactions was higher among microbial community members.
Francisella help ticks function, and Rickettsia may also help ticks survive the significant energy expenditure of migration, the authors said. They added that some Rickettsia can cause human diseases such as spotted fevers, but it's unknown whether invasive tick species are likely to spread these diseases to people.
Mapping the distributions of tick-carrying bird species during spring migrations highlighted geographic hot spots where songbirds could shed pathogen-carrying ticks on the way to or upon arrival at their breeding grounds, the latter showing average dispersal distances of 421 to 5,003 kilometers (262 to 3,109 miles). Short-distance migrants carried more ticks than their long-distance counterparts.
Ticks may establish in new animal hosts
The researchers said the findings provide insight into the mechanisms of tick dispersal, underscoring the crucial nature of understanding bird migration patterns in predicting the introduction and establishment of potentially invasive ticks.
