Ticks are fascinating creatures. Slightly alien, with a hard carapace and saw-toothed mouth parts, and all too many crawly legs – little nightmare creatures hiding in the backyard waiting to draw your blood. Sane people tend to detest them, but many scientists (me included) have taken a liking to them. Not only do ticks have an interesting biology – being parasites with distinct development stages, each stage associated with a blood meal from an (unwilling) host – they also are perfect little transport vessels for various pathogens that seek shelter and future transmission opportunities. The ability to use ticks as vectors seems to be a fitness bonanza, given the range of known tick borne pathogens. A specific bond exists between small rodents and tick borne diseases, but also larger mammals and birds can host zoonotic tick borne pathogens. And birds and zoonoses are what we love in our laboratory, so of course we have to study ticks on birds!
What does a tick on a bird tell you about the epidemiology of a tick borne disease? Actually quite a lot, if you consider both the biology of the bird and the tick. Some ticks have a broad range of hosts, such as Ixodes ricinus – the most common tick in Europe – while others have a more narrow host range, sometimes even restricted to a single host species. Although many tick species favors mammal hosts, there are those that are strict bird eaters (ornithophagus) – for instance, there is a tick specialized on Sand Martins. Thus knowing the species, and the development stage of the tick can tell you whether the bird is merely carrying infected ticks, or if it can be infected too – factors that influence the bird’s capacity to affect disease transmission over short or long temporal and spatial scales. Similarly, on the host side, knowing the breeding biology and migration ecology of a species allow you to say something on the current and future geographical range of the ticks and their pathogens.
Generally, as there are more bird lovers than tick lovers, the overwhelming majority of ecological data comes from the bird side, and very little from the tick side. We wanted to properly investigate birds as carriers of tick borne infections and carried out a very large sampling effort a few years ago. From roughly a thousand ticks, collected in a standardized way during a full year at Ottenby Bird Observatory, we extracted RNA and DNA for molecular screening for a smorgasbord of tick borne infections. These samples have been doing rounds in different Swedish labs, and the idea is to have an end-product where several pathogens are analyzed in parallel. We are not there yet, but a few weeks ago we published one article from this material, analyzing the prevalence of a particular bacterial species: Candidatus Neoehrlichia mikurensis. This novel bacterium has recently been linked to disease in immunocompromised patients, and exploratory studies have shown that it is widely distributed in Ixodes ricinus ticks and different rodent and mammal hosts in Europe. But was is the role of migratory birds?
Our first line of tick identification was digital photos. Each tick was photographed on both dorsal and ventral sides, just like flipping hamburgers on the grill, and the photos captured by a little USB-driven microscope with a camera. Although convenient in the field, the device didn’t really produce high quality pictures, making separation of similar looking species difficult. Ticks are hard to identify, and you need clear views of distinguishing characters (such as the shape of the mouth parts) to be certain of species identity. It is like when you renew your passport: there are specific guidelines for how your ears and nose should be portrayed. Our tick photo booth was more of the party pic variety, and we were therefore forced to set up molecular typing methods in the lab to complement the morphology-based identification. This turned out to be surprisingly difficult. A number of typing methods have been proposed, often targeting mitochondrial or ribosomal genes, but in most cases these protocols have not been evaluated on a large range of species, and their performance turned out to be variable. After much tinkering we settled with two methods that together worked well on our assemblage of bird borne ticks.
We searched 5365 birds of 65 species for ticks, and screened the resulting 1150 ticks for Neoehrlichia. Neo was found in a low prevalence in Ixodes ricinus ticks, roughly 2%, but not in other tick species, although the sample size for the latter was rather low. Furthermore, the bacterium was only detected in nymphs, and not in the numerous larvae, suggesting that the tick primarily got the infection via the first blood meal, and that it is less likely to be spread by vertical transmission (i.e. from mother to egg). Some birds carried more than one infected ticks, but we couldn’t find conclusive evidence whether they acquired the infection via a bacterimic host, by feeding close to each other on a non-bacterimic host, or whether the ticks had independently acquired the infection via earlier blood meals. Unfortunately, we did not have any blood samples from the birds, thus at present it is unknown whether birds can be competent hosts for Neoehrlichia or not. As regards which species that had infected ticks, there wasn’t any revolutionary patterns – the majority of birds were species that are heavily infested with ticks, such as ground-dwelling birds like Robins, Blackbirds and Wrens.
Neoehrlichia is a new kid on the block in tick borne disease research, and a lot of fundamental information is still missing. However, things are happening at the moment, and the knowledge gaps filled with speed. We are still hampered by the lack of culturing tools for this pathogen – hence the prefix Candidatus – as well as the lack of genomic data. The data at hand suggest very little genetic diversity, regardless of from where, and from what host the bug is detected. This may be artefactual, caused by phylogenies based on a gene with low phylogenetic resolution (i.e. “conserved”), or, more likely, suggests it to have a clonal population structure.
You can read the paper here:
Labbé Sandelin L, Tolf C, Larsson S, Wilhelmsson P, Salaneck E, Jaenson TGT, et al. (2015) Candidatus Neoehrlichia mikurensis in Ticks from Migrating Birds in Sweden. PLoS ONE 10(7): e0133250. doi:10.1371/journal.pone.0133250