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1. Bartonella infections are prevalent in rodents despite efficient immune responses

   https://doi.org/10.1186/s13071-023-05918-7  

   Rodríguez-Pastor, Ruth; Hasik, Adam_Z; Knossow, Nadav; Bar-Shira, Enav; Shahar, Naama; Gutiérrez, Ricardo; Zaman, Luis; Harrus, Shimon; Lenski, Richard_E; Barrick, Jeffrey_E; et al (September 2023, Parasites & Vectors)

   Abstract BackgroundPathogens face strong selection from host immune responses, yet many host populations support pervasive pathogen populations. We investigated this puzzle in a model system ofBartonellaand rodents from Israel’s northwestern Negev Desert. We chose to study this system because, in this region, 75–100% of rodents are infected withBartonellaat any given time, despite an efficient immunological response. In this region,Bartonellaspecies circulate in three rodent species, and we tested the hypothesis that at least one of these hosts exhibits a waning immune response toBartonella, which allows reinfections. MethodsWe inoculated captive animals of all three rodent species with the sameBartonellastrain, and we quantified the bacterial dynamics andBartonella-specific immunoglobulin G antibody kinetics over a period of 139 days after the primary inoculation, and then for 60 days following reinoculation with the same strain. ResultsContrary to our hypothesis, we found a strong, long-lasting immunoglobulin G antibody response, with protective immunological memory in all three rodent species. That response prevented reinfection upon exposure of the rodents to the sameBartonellastrain. ConclusionsThis study constitutes an initial step toward understanding how the interplay between traits ofBartonellaand their hosts influences the epidemiological dynamics of these pathogens in nature. Graphical Abstract 

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2. Macro-parasites and micro-parasites co-exist in rodent communities but are associated with different community-level parameters

   https://doi.org/10.1016/j.ijppaw.2023.08.006  

   Salomon, Jordan; Sambado, Samantha B.; Crews, Arielle; Sidhu, Sukhman; Seredian, Eric; Almarinez, Adrienne; Grgich, Rachel; Swei, Andrea (August 2023, International Journal for Parasitology: Parasites and Wildlife)

   Wildlife species are often heavily parasitized by multiple infections simultaneously. Yet research on sylvatic transmission cycles, tend to focus on host interactions with a single parasite and neglects the influence of co- infections by other pathogens and parasites. Co-infections between macro-parasites and micro-parasites can alter mechanisms that regulate pathogenesis and are important for understanding disease emergence and dy- namics. Wildlife rodent hosts in the Lyme disease system are infected with macro-parasites (i.e., ticks and hel- minths) and micro-parasites (i.e., Borrelia spp.), however, there has not been a study that investigates the interaction of all three parasites (i.e., I. pacificus, Borrelia spp., and helminths) and how these co-infections impact prevalence of micro-parasites. We live-trapped rodents in ten sites in northern California to collect feces, blood, ear tissue, and attached ticks. These samples were used to test for infection status of Borrelia species (i.e., micro- parasite), and describe the burden of ticks and helminths (i.e., macro-parasites). We found that some rodent hosts were co-infected with all three parasites, however, the burden or presence of concurrent macro-parasites were not associated with Borrelia infections. For macro-parasites, we found that tick burdens were positively associ- ated with rodent Shannon diversity while negatively associated with predator diversity, whereas helminth burdens were not significantly associated with any host community metric. Ticks and tick-borne pathogens are associated with rodent host diversity, predator diversity, and abiotic factors. However, it is still unknown what factors helminths are associated with on the community level. Understanding the mechanisms that influence co- infections of multiple types of parasites within and across hosts is an increasingly critical component of characterizing zoonotic disease transmission and maintenance. 

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3. The cost of travel: How dispersal ability limits local adaptation in host–parasite interactions

   https://doi.org/10.1111/jeb.13754  

   Johnson, Pieter; Calhoun, Dana M.; Moss, Wynne E.; McDevitt‐Galles, Travis; Riepe, Tawni B.; Hallas, Joshua M.; Parchman, Thomas L.; Feldman, Chris R.; Achatz, Tyler J.; Tkach, Vasyl V.; et al (December 2020, Journal of Evolutionary Biology)

   Abstract Classical theory suggests that parasites will exhibit higher fitness in sympatric relative to allopatric host populations (local adaptation). However, evidence for local adaptation in natural host–parasite systems is often equivocal, emphasizing the need for infection experiments conducted over realistic geographic scales and comparisons among species with varied life history traits. Here, we used infection experiments to test how two trematode (flatworm) species (Paralechriorchis syntomenteraandRibeiroia ondatrae) with differing dispersal abilities varied in the strength of local adaptation to their amphibian hosts. Both parasites have complex life cycles involving sequential transmission among aquatic snails, larval amphibians and vertebrate definitive hosts that control dispersal across the landscape. By experimentally pairing 26 host‐by‐parasite population infection combinations from across the western USA with analyses of host and parasite spatial genetic structure, we found that increasing geographic distance—and corresponding increases in host population genetic distance—reduced infection success forP. syntomentera, which is dispersed by snake definitive hosts. For the avian‐dispersedR. ondatrae, in contrast, the geographic distance between the parasite and host populations had no influence on infection success. Differences in local adaptation corresponded to parasite genetic structure; although populations ofP. syntomenteraexhibited ~10% mtDNA sequence divergence, those ofR. ondatraewere nearly identical (<0.5%), even across a 900 km range. Taken together, these results offer empirical evidence that high levels of dispersal can limit opportunities for parasites to adapt to local host populations. 

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4. A legacy of competitive exclusion: Host demography and amplified disease

   https://doi.org/10.1093/icb/icaf035  

   Suh, Daniel_C; Schroeder, Katie; Landolt, Emily_F; Tejada, Jenavier; Strauss, Alexander_T (May 2025, Integrative And Comparative Biology)

   Synopsis Dilution effects arise when increases in species diversity reduce disease risk, and amplification effects arise when the opposite occurs. Despite ample evidence for both phenomena, the mechanisms driving dilution and amplification effects and how they are mediated by environmental factors remain poorly understood. Mechanisms involving demographic rates or stage structure of hosts are particularly lacking in the diversity–disease literature. In Midwestern lakes, Metschnikowia bicuspidata parasites infect Daphnia dentifera focal hosts in autumn, with epidemics beginning when water is warm (∼25°C) and peaking when lakes have cooled (∼15°C). Epidemics are smaller in lakes with more Ceriodaphnia dubia alternative hosts, which serve as key diluters of disease. However, it is unclear whether seasonal changes in temperature affect their ability to alter host population dynamics and reduce disease. We conducted a mesocosm experiment to test how temperature (15, 20, or 25°C) mediated the effects of these key alternative hosts on density, stage structure, and disease dynamics in focal host populations. The experiment yielded several surprising results. First, focal hosts rapidly outcompeted alternative hosts at all temperatures. By the time parasites were added, alternative hosts had been almost completely excluded. Second, despite diluting disease in the field, initial presence of these alternative hosts amplified infection prevalence in the experiment. Third, this amplification arose as a legacy effect, lasting generations after alternative hosts were gone. Our explanation for this legacy amplification effect centers on focal host stage structure and demography. Competition with alternative hosts resulted in focal host populations that were more adult-biased when parasites were added, at all 3 temperatures. Additionally, host densities in these treatments increased more rapidly in the subsequent 10 days, consistent with reduced background death rates. Since adults consume more parasites than juveniles, and since exposed hosts must survive 10 days before producing infectious spores, these initial differences in stage structure and population growth seem to have set disease dynamics along amplified trajectories. These results highlight the need for a broader understanding of the mechanisms that can amplify or dilute disease, including altered host stage structure and mortality of exposed hosts in diverse communities. 

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5. Drivers and consequences of bacteriophage host range

   https://doi.org/10.1093/femsre/fuad038  

   Holtappels, Dominique; Alfenas-Zerbini, Poliane; Koskella, Britt (July 2023, FEMS Microbiology Reviews)

   Abstract Bacteriophages are obligate parasites of bacteria characterized by the breadth of hosts that they can infect. This “host range” depends on the genotypes and morphologies of the phage and the bacterial host, but also on the environment in which they are interacting. Understanding phage host range is critical to predicting the impacts of these parasites in their natural host communities and their utility as therapeutic agents, but is also key to predicting how phages evolve and in doing so drive evolutionary change in their host populations, including through movement of genes among unrelated bacterial genomes. Here, we explore the drivers of phage infection and host range from the molecular underpinnings of the phage–host interaction to the ecological context in which they occur. We further evaluate the importance of intrinsic, transient, and environmental drivers shaping phage infection and replication, and discuss how each influences host range over evolutionary time. The host range of phages has great consequences in phage-based application strategies, as well as natural community dynamics, and we therefore highlight both recent developments and key open questions in the field as phage-based therapeutics come back into focus. 

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