SUMMARYThis review examines the phenomenon of co-feeding transmission in tick-borne pathogens. This mode of transmission is critical for the epidemiology of several tick-borne viruses but its importance for Borrelia burgdorferi sensu lato, the causative agents of Lyme borreliosis, is still controversial. The molecular mechanisms and ecological factors that facilitate co-feeding transmission are therefore examined with particular emphasis on Borrelia pathogens. Comparison of climate, tick ecology and experimental infection work suggests that co-feeding transmission is more important in European than North American systems of Lyme borreliosis, which potentially explains why this topic has gained more traction in the former continent than the latter. While new theory shows that co-feeding transmission makes a modest contribution to Borrelia fitness, recent experimental work has revealed new ecological contexts where natural selection might favour co-feeding transmission. In particular, co-feeding transmission might confer a fitness advantage in the Darwinian competition among strains in mixed infections. Future studies should investigate the ecological conditions that favour the evolution of this fascinating mode of transmission in tick-borne pathogens.

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bMany vector-borne pathogens consist of multiple strains that circulate in both the vertebrate host and the arthropod vector. Characterization of the community of pathogen strains in the arthropod vector is therefore important for understanding the epidemiology of mixed vector-borne infections. Borrelia afzelii and B. garinii are two species of tick-borne bacteria that cause Lyme disease in humans. These two sympatric pathogens use the same tick, Ixodes ricinus, but are adapted to different classes of vertebrate hosts. Both Borrelia species consist of multiple strains that are classified using the highly polymorphic ospC gene. Vertebrate cross-immunity against the OspC antigen is predicted to structure the community of multiple-strain Borrelia pathogens. Borrelia isolates were cultured from field-collected I. ricinus ticks over a period spanning 11 years. The Borrelia species of each isolate was identified using a reverse line blot (RLB) assay. Deep sequencing was used to characterize the ospC communities of 190 B. afzelii isolates and 193 B. garinii isolates. Infections with multiple ospC strains were common in ticks, but vertebrate cross-immunity did not influence the strain structure in the tick vector. The pattern of genetic variation at the ospC locus suggested that vertebrate cross-immunity exerts strong selection against intermediately divergent ospC alleles. Deep sequencing found that more than 50% of our isolates contained exotic ospC alleles derived from other Borrelia species. Two alternative explanations for these exotic ospC alleles are cryptic coinfections that were not detected by the RLB assay or horizontal transfer of the ospC gene between Borrelia species. Many vector-borne pathogens consist of multiple genetically distinct strains (1-4). The adaptive arm of the vertebrate immune system plays a key role in generating and maintaining this diversity of pathogen strains (5-7). Genetic diversity is often the highest at loci coding for surface-exposed pathogen molecules that function during the invasion and infection of host tissues (8, 9). The study of these highly polymorphic pathogen molecules is important for understanding how cross-reactive acquired immunity can mediate indirect competition and superinfection in the vertebrate host (10, 11). In addition, these pathogen outer surface proteins are often used to characterize pathogen strains because they provide an upper estimate of pathogen strain richness.In vector-borne diseases, the community of pathogen strains can be studied in both the vertebrate host and the arthropod vector. The vertebrate immune system creates nonrandom associations between pathogen strains (1, 12) that are subsequently transmitted to the arthropod vector. Conversely, the study of mixed infections in the arthropod vector can provide information on the processes that structure the community of multiple pathogen strains in the vertebrate host (13,14). In addition, estimates of strain richness in the arthropod vector are important for understanding the frequency with which ...

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Background: To predict the risk of tick-borne disease, it is critical to understand the ecological factors that determine the abundance of ticks. In Europe, the sheep tick (Ixodes ricinus) transmits a number of important diseases including Lyme borreliosis. The aim of this long-term study was to determine the abiotic and biotic factors driving the annual abundance of I. ricinus at a location in Switzerland where Lyme borreliosis is endemic. Methods: Over a 15-year period (2004 to 2018), we monitored the abundance of I. ricinus ticks on a monthly basis at three different elevations on Chaumont Mountain in Neuchâtel, Switzerland. We collected climate variables in the field and from nearby weather stations. We obtained data on beech tree seed production from the literature, as the abundance of Ixodes nymphs can increase dramatically two years after a masting event. We used AIC-based model selection to determine which ecological variables drive annual variation in tick density. Results: We found that elevation site, year, seed production by beech trees two years prior, and mean annual relative humidity together explained 73.2% of the variation in our annual estimates of nymph density. According to the parameter estimates of our models, (i) the annual density of nymphs almost doubled over the 15-year study period, (ii) changing the beech tree seed production index from very poor mast (1) to full mast (5) increased the abundance of nymphs by 86.2% two years later, and (iii) increasing the field-collected mean annual relative humidity from 50.0 to 75.0% decreased the abundance of nymphs by 46.4% in the same year. Climate variables collected in the field were better predictors of tick abundance than those from nearby weather stations indicating the importance of the microhabitat. Conclusions: From a public health perspective, the increase in nymph abundance is likely to have increased the risk of tick-borne disease in this region of Switzerland. Public health officials in Europe should be aware that seed production by deciduous trees is a critical driver of the abundance of I. ricinus, and hence the risk of tick-borne disease.

The sex‐determining mechanism has important demographic and genetic consequences by virtue of its effect on the population sex ratio. Here we investigate the effect of temperature dependent sex determination (TSD) on the primary sex ratio of the harpacticoid copepod, Tigriopus californicus. At the two experimental temperatures (15° and 22°C) used in this study, the primary sex ratio is almost always biased in favour of males. Higher temperatures induce masculinization and the change in sex ratio is not caused by differential mortality of the sexes. The mean level of TSD in the population is small (proportion of males increases by ~5% between 15° and 22°C) because only one‐third of the families actually exhibit a significant sex‐ratio response while the rest of the population is insensitive to temperature. A comparison of the primary sex ratio and the level of TSD between two locations reveals few differences among populations. Finally, individuals still exhibited TSD after having been maintained under constant temperature conditions in the lab for several generations. In addition the proportion of temperature‐sensitive individuals remained unchanged. This suggests that the observed level of TSD is not an artefact of testing field‐captured individuals in a novel laboratory environment. At this point the adaptive significance of temperature‐dependent sex determination in T. californicus remains unknown. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society, 2002, 76, 511–520.

bMixed infections have important consequences for the ecology and evolution of host-parasite interactions. In vector-borne diseases, interactions between pathogens occur in both the vertebrate host and the arthropod vector. Spirochete bacteria belonging to the Borrelia burgdorferi sensu lato genospecies complex are transmitted by Ixodes ticks and cause Lyme borreliosis in humans. In Europe, there is a high diversity of Borrelia pathogens, and the main tick vector, Ixodes ricinus, is often infected with multiple Borrelia genospecies. In the present study, we characterized the pairwise interactions between five B. burgdorferi sensu lato genospecies in a large data set of I. ricinus ticks collected from the same field site in Switzerland. We measured two types of pairwise interactions: (i) co-occurrence, whether double infections occurred more or less often than expected, and (ii) spirochete load additivity, whether the total spirochete load in double infections was greater or less than the sum of the single infections. Mixed infections of Borrelia genospecies specialized on different vertebrate reservoir hosts occurred less frequently than expected (negative co-occurrence) and had joint spirochete loads that were lower than the additive expectation (inhibition). In contrast, mixed infections of genospecies that share the same reservoir hosts were more common than expected (positive cooccurrence) and had joint spirochete loads that were similar to or greater than the additive expectation (facilitation). Our study suggests that the vertebrate host plays an important role in structuring the community of B. burgdorferi sensu lato genospecies inside the tick vector.