Tim R. Hofmeester

Few vertebrate species dominate the Borrelia burgdorferi s.l. Life cycle

Background. In the northern hemisphere, ticks of the Ixodidae family are vectors of diseases such as Lyme borreliosis, Rocky Mountain spotted fever and tick-borne encephalitis. Most of these ticks are generalists and have a three-host life cycle for which they are dependent on three different hosts for their blood meal. Finding out which host species contribute most in maintaining ticks and the pathogens they transmit, is imperative in understanding the drivers behind the dynamics of a disease. Methods. We performed a systematic review to identify the most important vertebrate host species for Ixodes ricinus and Borrelia burgdorferi s.l. as a well-studied model system for tick-borne diseases. We analyzed data from 66 publications and quantified the relative contribution for 15 host species. Review results. We found a positive correlation between host body mass and tick burdens for the different stages of I. ricinus. We show that nymphal burdens of host species are positively correlated with infection prevalence with B. burgdorferi s.l., which is again positively correlated with the realized reservoir competence of a host species for B. burgdorferi s.l. Our quantification method suggests that only a few host species, which are amongst the most widespread species in the environment (rodents, thrushes and deer), feed the majority of I. ricinus individuals and that rodents infect the majority of I. ricinus larvae with B. burgdorferi s.l. Discussion. We argue that small mammal-transmitted Borrelia spp. are maintained due to the high density of their reservoir hosts, while bird-transmitted Borrelia spp. are maintained due to the high infection prevalence of their reservoir hosts. Our findings suggest that Ixodes ricinus and Borrelia burgdorferi s.l. populations are maintained by a few widespread host species. The increase in distribution and abundance of these species, could be the cause for the increase in Lyme borreliosis incidence in Europe in recent decades.

Predicting Tick Presence by Environmental Risk Mapping

Public health statistics recorded an increasing trend in the incidence of tick bites and erythema migrans (EM) in the Netherlands. We investigated whether the disease incidence could be predicted by a spatially explicit categorization model, based on environmental factors and a training set of tick absence–presence data. Presence and absence of Ixodes ricinus were determined by the blanket-dragging method at numerous sites spread over the Netherlands. The probability of tick presence on a 1 km by 1 km square grid was estimated from the field data using a satellite-based methodology. Expert elicitation was conducted to provide a Bayesian prior per landscape type. We applied a linear model to test for a linear relationship between incidence of EM consultations by general practitioners in the Netherlands and the estimated probability of tick presence. Ticks were present at 252 distinct sampling coordinates and absent at 425. Tick presence was estimated for 54% of the total land cover. Our model has predictive power for tick presence in the Netherlands, tick-bite incidence per municipality correlated significantly with the average probability of tick presence per grid. The estimated intercept of the linear model was positive and significant. This indicates that a significant fraction of the tick-bite consultations could be attributed to the I. ricinus population outside the resident municipality.

New foci of Haemaphysalis punctata and Dermacentor reticulatus in the Netherlands

In 2014 Haemaphysalis punctata was found in several locations on the mainland of the Netherlands for the first time since 1897. In the same areas Dermacentor reticulatus and Ixodes ricinus were found. Haemaphysalis punctata and D. reticulatus were tested for presence of Babesia spp. and Rickettsia spp. by PCR. Babesia spp. and spotted fever Rickettsiae were not detected in any of the collected H. punctata, while several D. reticulatus (6%) collected from the same areas were found to be positive for R. raoultii, a causative agent of tick-borne lymphadenopathy. We discuss the role of free-ranging domestic animals in maintaining H. punctata and D. reticulatus populations in dune areas in the Netherlands.

Cascading effects of predator activity on tick-borne disease risk

Predators and competitors of vertebrates can in theory reduce the density of infected nymphs (DIN)—an often-used measure of tick-borne disease risk—by lowering the density of reservoir-competent hosts and/or the tick burden on reservoir-competent hosts. We investigated this possible indirect effect of predators by comparing data from 20 forest plots across the Netherlands that varied in predator abundance. In each plot, we measured the density of questing Ixodes ricinus nymphs (DON), DIN for three pathogens, rodent density, the tick burden on rodents and the activity of mammalian predators. We analysed whether rodent density and tick burden on rodents were correlated with predator activity, and how rodent density and tick burden predicted DON and DIN for the three pathogens. We found that larval burden on two rodent species decreased with activity of two predator species, while DON and DIN for all three pathogens increased with larval burden on rodents, as predicted. Path analyses supported an indirect negative correlation of activity of both predator species with DON and DIN. Our results suggest that predators can indeed lower the number of ticks feeding on reservoir-competent hosts, which implies that changes in predator abundance may have cascading effects on tick-borne disease risk.

Quantifying the Availability of Vertebrate Hosts to Ticks: A Camera-Trapping Approach

The availability of vertebrate hosts is a major determinant of the occurrence of ticks and tick-borne zoonoses in natural and anthropogenic ecosystems and thus drives disease risk for wildlife, livestock, and humans. However, it remains challenging to quantify the availability of vertebrate hosts in field settings, particularly for medium-sized to large-bodied mammals. Here, we present a method that uses camera traps to quantify the availability of warm-bodied vertebrates to ticks. The approach is to deploy camera traps at questing height at a representative sample of random points across the study area, measure the average photographic capture rate for vertebrate species, and then correct these rates for the effective detection distance. The resulting “passage rate” is a standardized measure of the frequency at which vertebrates approach questing ticks, which we show is proportional to contact rate. A field test across twenty 1-ha forest plots in the Netherlands indicated that this method effectively captures differences in wildlife assemblage composition between sites. Also, the relative abundances of three life stages of the sheep tick Ixodes ricinus from drag sampling were correlated with passage rates of deer, which agrees with the known association with this group of host species, suggesting that passage rate effectively reflects the availability of medium- to large-sized hosts to ticks. This method will facilitate quantitative studies of the relationship between densities of questing ticks and the availability of different vertebrate species—wild as well as domesticated species—in natural and anthropogenic settings.

The genetic diversity of Borrelia afzelii is not maintained by the diversity of the rodent hosts

BackgroundSmall mammals are essential in the enzootic cycle of many tick-borne pathogens (TBP). To understand their contribution to the genetic diversity of Borrelia afzelii, the most prevalent TBP in questing Ixodes ricinus, we compared the genetic variants of B. afzelii at three distinct genetic loci. We chose two plasmid loci, dbpA and ospC, and a chromosomal one, IGS.ResultsWhile the larvae that fed on shrews (Sorex sp.) tested negative for B. afzelii, those fed on bank voles (Myodes glareolus) and wood mice (Apodemus sylvaticus) showed high infection prevalences of 0.13 and 0.27, respectively. Despite the high genetic diversity within B. afzelii, there was no difference between wood mice and bank voles in the number and types of B. afzelii haplotypes they transmit.ConclusionsThe genetic diversity in B. afzelii cannot be explained by separate enzootic cycles in wood mice and bank voles.