The sheep tick and the bacteria that cause Lyme disease in humans are maintained by just five host species, a paper in Environmental Research Letters has discovered. The study was the first to quantify the relative importance of host species for the different life stages of the tick.
“We have shown that apparently there are only a few host species that are important for ticks and B. burgdorferi,” Tim Hofmeester of Wageningen University in The Netherlands told environmentalresearchweb. “The big question is now, can we reduce tick densities or pathogen prevalence in ticks by targeting one of these host species, or do we need to target all five species in the three most important groups?”
Lyme disease in humans has become more common in Europe in recent decades. It’s caused by infection with B. burgdorferi through a bite from a sheep tick (Ixodes ricinus). The sheep tick picks up the bacteria by feeding on infected blood from another vertebrate – the tick uses three different hosts in its transition from larva to nymph to adult. Host species vary in their ability to infect sheep tick larvae with different genospecies of B. burgdorferi. B. afzelii is mainly transmitted by small mammals, for example, whilst B. garinii is mainly transmitted by birds.
“A lot of work has been published on the relationship between vertebrate hosts, the sheep tick (Ixodes ricinus) and Borrelia burgdorferi sensu lato [s.l. – “in a broad sense”],” said Hofmeester, who carried out the review as part of his PhD in conjunction with fellow doctoral student Elena Claudia Coipan of the Netherlands’ National Institute for Public Health and Environment (RIVM), and colleagues. “Many papers state that deer are important hosts for adults and small mammals for larvae. However, most papers do not use a reference for this. Also, there have been some very good descriptive reviews on the system, but a formal meta-analysis was missing.”
Hofmeester, Coipan and colleagues used a quantitative method to pinpoint the most important host species for the sheep tick and B. burgdorferi in Europe. They scoured the literature to build up a database on 15 host species that included tick burdens, host infection prevalence and infection prevalence in feeding ticks, before analysing the data. This approach provided new insight; the majority of sheep ticks feed on only a few host species.
Rodents fed the majority of sheep tick larvae, the team found, followed by thrushes at 5% then smaller birds. Thrushes generally feed on the ground, so it’s likely to be easy for ticks to find them.
For the nymph stage, there was more variation in host. Thrushes fed around 40% of nymphs in the database, with rodents in second place at 28%, small birds at 23% and ungulates (hoofed animals, a group that includes sheep, cattle, deer and horses) at 8%. Adult ticks mainly fed on ungulates (92%) and medium-sized mammals (5%).
“What I had not expected was that thrushes (Eurasian blackbirds Turdus merula and song thrush T. philomelos) would be so important as hosts for the nymphal stage,” said Hofmeester. “This is because they do not have a very high nymphal burden and also not very high densities, but apparently the combination of both is quite high.”
Rodents, thrushes and deer, the most important host groups feeding adult ticks and infecting larvae, have increased in distribution and abundance due to changes in land use and forest management, a factor that could be responsible for the rise in Lyme disease.
In another surprise, the team discovered that tick-borne pathogens have two different maintenance strategies – either infecting a high proportion of questing ticks, or a high proportion of reservoir hosts. Borrelia species transmitted by small mammals are maintained because their reservoir hosts occur in high densities, the team found, whilst Borrelia species transmitted by birds are maintained because the infection is highly prevalent in their reservoir hosts. “As far as I know, nobody has ever looked into this,” said Hofmeester, “although there were some researchers asking how it could be possible that some pathogen species occur in very low prevalence in questing ticks.” This finding might provide the answer, he believes.
“Another challenge would be to determine tick burden and reservoir competence for B. burgdorferi for some species on which we did not find any data but which are often found in forests and city parks, such as the wood pigeon (Columba palumbus),” added Hofmeester. There’s also a need for data on other widespread species, such as the Eurasian red squirrel (Sciurus vulgaris), European hedgehog (Erinaceus europaeus), wood pigeon, European pine marten (Martes martes) and European badger (Meles meles).
In Coipan’s view, the ultimate challenge is developing an anti-tick vaccine to reduce the tick population. This researcher was surprised by the scarcity of data at the genospecies level for B. burgdorferi s.l. She discovered that some genospecies of the bacteria are over-represented in clinical cases of Lyme disease compared to their prevalence in the environment. “[These] might have particular maintenance hosts/vectors and patterns in nature, deserving further investigation,” she said. “A key element is finding hosts and/or (alternative) vectors of B. burgdorferi s.l. genospecies that are less frequent in nature but, nevertheless, present in human Lyme disease cases, and understanding which are the mechanisms allowing their maintenance.”
Over the next 50 years, molecular methods will become more important for tick-borne pathogen research, the researchers believe. “Differences between genospecies and genotypes of B. burgdorferi s.l. and maybe also between genotypes of the different host species might be studied,” said Hofmeester. “Also the adaptation of animals to a more urbanized landscape might become important as more and more host species that are good reservoirs for B. burgdorferi , such as wood mice (Apodemus sylvaticus) and blackbirds, have adapted to living close to humans.” So in the future the highest risk of getting Lyme disease could be in gardens and city parks rather than in forests. Alternatively, advances in knowledge could have produced good control strategies for the sheep tick or tick-borne pathogens, reducing the prevalence of Lyme disease. “We need to understand to what extent the transmission of the pathogenic microorganisms [the bacteria] is influenced by the microbiota they are part of in the vectors,” said Coipan.
Hofmeester is continuing field work in the Netherlands to fill some of the gaps in knowledge pinpointed by the review. “I hope that other researchers will pick up this [quantitative meta-analysis] idea and use a similar approach for other parasites or pathogens,” he said. Ticks also carry diseases such as Rocky Mountain spotted fever and encephalitis.