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In Wisconsin white-footed mice (Peromyscus leucopus noveboracensis) and woodland deer mice (P. maniculatus gracilis) are difficult to distinguish. Recent climatic trends have facilitated encroachment of P. leucopus north into the range of P. maniculatus, necessitating unambiguous species identification as researchers begin to untangle the ecological implications of such community changes. Cranial and external measurements have been used by previous investigators to differentiate these species in other regions. However, because large geographic morphological variation occurs and most previous studies used measurements from dead specimens, definitive morphological characteristics need to be identified that can quickly and effectively classify live Wisconsin Peromyscus in the field. During the summer of 2010, we collected tissue samples and measured ear length, tail length, hindfoot length, and body weight of 84 P. maniculatus and 293 P. leucopus live-trapped in six Wisconsin counties. We used mDNA analysis to identify species. We developed discriminate function analysis (DFA) equations to identify characteristics that best distinguished species. Ear length correctly classified 97.9% of the samples with all but one P. leucopus <17 mm and all but seven P. maniculatus ≥17 mm. By adding body weight to the function, we were able to achieve 99.2% classification accuracy and with the addition of tail length were able to achieve 99.5% accuracy.

In response to hypoxic stress, many animals compensate for a reduced cellular O2 supply by suppressing total metabolism, thereby reducing O2 demand. For small endotherms that are native to high-altitude environments, this is not always a viable strategy, as the capacity for sustained aerobic thermogenesis is critical for survival during periods of prolonged cold stress. For example, survivorship studies of deer mice (Peromyscus maniculatus) have demonstrated that thermogenic capacity is under strong directional selection at high altitude. Here, we integrate measures of whole-organism thermogenic performance with measures of metabolic enzyme activities and genomic transcriptional profiles to examine the mechanistic underpinnings of adaptive variation in this complex trait in deer mice that are native to different elevations. We demonstrate that highland deer mice have an enhanced thermogenic capacity under hypoxia compared with lowland conspecifics and a closely related lowland species, Peromyscus leucopus. Our findings suggest that the enhanced thermogenic performance of highland deer mice is largely attributable to an increased capacity to oxidize lipids as a primary metabolic fuel source. This enhanced capacity for aerobic thermogenesis is associated with elevated activities of muscle metabolic enzymes that influence flux through fatty-acid oxidation and oxidative phosphorylation pathways in high-altitude deer mice and by concomitant changes in the expression of genes in these same pathways. Contrary to predictions derived from studies of humans at high altitude, our results suggest that selection to sustain prolonged thermogenesis under hypoxia promotes a shift in metabolic fuel use in favor of lipids over carbohydrates.

Lyme disease is common in the northeastern United States, but rare in the southeast, even though the tick vector is found in both regions. Infection prevalence of Lyme spirochetes in host-seeking ticks, an important component to the risk of Lyme disease, is also high in the northeast and northern midwest, but declines sharply in the south. As ticks must acquire Lyme spirochetes from infected vertebrate hosts, the role of wildlife species composition on Lyme disease risk has been a topic of lively academic discussion. We compared tick–vertebrate host interactions using standardized sampling methods among 8 sites scattered throughout the eastern US. Geographical trends in diversity of tick hosts are gradual and do not match the sharp decline in prevalence at southern sites, but tick–host associations show a clear shift from mammals in the north to reptiles in the south. Tick infection prevalence declines north to south largely because of high tick infestation of efficient spirochete reservoir hosts (rodents and shrews) in the north but not in the south. Minimal infestation of small mammals in the south results from strong selective attachment to lizards such as skinks (which are inefficient reservoirs for Lyme spirochetes) in the southern states. Selective host choice, along with latitudinal differences in tick host-seeking behavior and variations in tick densities, explains the geographic pattern of Lyme disease in the eastern US.

Ixodes scapularis ticks are expanding their range in parts of northeastern North America, bringing with them pathogens of public health concern. While rodents like the white-footed mouse, Peromyscus leucopus , are considered the primary reservoir of many emerging tick-borne pathogens, the contribution of birds, as alternative hosts and reservoirs, to local transmission cycles has not yet been firmly established. From 2016 to 2018, we collected host-seeking ticks and examined rodent and bird hosts for ticks at 48 sites in a park where blacklegged ticks are established in Quebec, Canada, in order to characterize the distribution of pathogens in ticks and mammalian and avian hosts. We found nearly one third of captured birds (n = 849) and 70% of small mammals (n = 694) were infested with I . scapularis . Five bird and three mammal species transmitted Borrelia burgdorferi to feeding larvae (n larvae tested = 2257) and we estimated that about one fifth of the B . burgdorferi -infected questing nymphs in the park acquired their infection from birds, the remaining being attributable to mice. Ground-foraging bird species were more parasitized than other birds, and species that inhabited open habitat were more frequently infested and were more likely to transmit B . burgdorferi to larval ticks feeding upon them. Female birds were more likely to transmit infection than males, without age differentiation, whereas in mice, adult males were more likely to transmit infection than juveniles and females. We also detected Borrelia miyamotoi in larvae collected from birds, and Anaplasma phagocytophilum from a larva collected from a white-footed mouse. This study highlights the importance of characterising the reservoir potential of alternative reservoir hosts and to quantify their contribution to transmission dynamics in different species assemblages. This information is key to identifying the most effective host-targeted risk mitigation actions.

We demonstrate that 6 distinct Peromyscus rodent species are permissive to experimental infection with Sin Nombre orthohantavirus (SNV). Viral RNA and SNV antibodies were detected in members of all 6 species. P. leucopus mice demonstrated markedly higher viral and antibody titers than P. maniculatus mice, the established primary hosts for SNV.

Leptospirosis is a global zoonotic disease affecting humans, wildlife, companion, and domestic animals. Incidental hosts can contract the disease directly or indirectly from asymptomatic reservoir hosts, most commonly small rodents. The Golden Syrian hamster is recognized as the dominant rodent model for acute leptospirosis because the animals are susceptible to many serovars and are used to maintain laboratory strains and test bacterin vaccine efficacy. However, hamsters are primarily used in survival-based studies, and investigations into host immune response and disease pathogenesis are limited. We found that Peromyscus leucopus white-footed deer mice are susceptible to acute leptospirosis, and thus might be an alternative rodent model. Furthermore, similar to hamsters, deer mice produce circulating foamy macrophages in response to Leptospira challenge. Deer mice exhibit differences in response to different serovars, clinical disease severity, kidney and liver lesions, and an overall sex effect, with male mice demonstrating more severe clinical signs and higher bacterial burden.

Exposure to environmental pollutants, such as polycyclic aromatic hydrocarbons (PAHs) found in coal tar mixtures and tobacco sources, is considered a significant risk factor for the development of heart disease in humans. The goal of this study was to determine the influence of PAHs present at a Superfund site on human coronary artery endothelial cell (HCAEC) phospholipase A sub(2) (PLA sub(2)) activity and apoptosis. Extremely high levels of 12 out of 15 EPA high-priority PAHs were present in both the streambed and floodplain sediments at a site where an urban creek and its adjacent floodplain were extensively contaminated by PAHs and other coal tar compounds. Nine of the 12 compounds and a coal tar mixture (SRM 1597A) activated group IVC PLA sub(2) in HCAECs, and activation of this enzyme was associated with histone fragmentation and poly (ADP) ribose polymerase (PARP) cleavage. Genetic silencing of group IVC PLA sub(2) inhibited both super(3)H-fatty acid release and histone fragmentation by PAHs and SRM 1597A, indicating that individual PAHs and a coal tar mixture induce apoptosis of HCAECs via a mechanism that involves group IVC PLA sub(2). Western blot analysis of aortas isolated from feral mice (Peromyscus leucopus) inhabiting the Superfund site showed increased PARP and caspase-3 cleavage when compared to reference mice. These data suggest that PAHs induce apoptosis of HCAECs via activation of group IVC PLA sub(2).

Moonlight structures activity patterns of many nocturnal species. Bright moonlight often limits the activity of nocturnal prey, but dense vegetation weakens this effect. Using 8 years of live-trapping data, we asked whether reintroduced megaherbivores (Bison bison) indirectly altered moonlight avoidance by small mammals in tallgrass prairies. In plots with bison, plants intercepted 20% less light, allowing more moonlight to reach ground level. During nights with no moonlight, Peromyscus maniculatus activity was similar in plots with and without bison. During nights with peak moonlight, P. maniculatus activity was four times greater in plots without bison compared to plots with bison. Conversely, Microtus ochrogaster activity was twice as great during full moons compared to new moons, but only in plots with bison. We also equipped a subset of traps with temperature sensors to estimate trap-entry time. Although M. ochrogaster was more active on bright nights, most activity occurred before moonrise or after moonset, avoiding periods of bright moonlight. We conclude that megaherbivores play an unappreciated but important indirect role in tallgrass prairies by inducing behavioral shifts in other animal species. Because overlap in activity patterns can predict the likelihood of predator–prey encounters, such activity shifts have important implications for trophic interactions throughout restored prairie food webs. Additional work to understand interspecific and intraspecific variation in response to moonlight may improve efforts to forecast changes in community assembly due to restoration and land-use change.

Babesia microti and Borrelia burgdorferi, the respective causative agents of human babesiosis and Lyme disease, are maintained in their enzootic cycles by the blacklegged tick (Ixodes scapularis) and use the white-footed mouse (Peromyscus leucopus) as primary reservoir host. The geographic range of both pathogens has expanded in the United States, but the spread of babesiosis has lagged behind that of Lyme disease. Several studies have estimated the basic reproduction number (R0) for B. microti to be below the threshold for persistence (<1), a finding that is inconsistent with the persistence and geographic expansion of this pathogen. We tested the hypothesis that host coinfection with B. burgdorferi increases the likelihood of B. microti transmission and establishment in new areas. We fed I. scapularis larva on P. leucopus mice that had been infected in the laboratory with B. microti and/or B. burgdorferi. We observed that coinfection in mice increases the frequency of B. microti infected ticks. To identify the ecological variables that would increase the probability of B. microti establishment in the field, we integrated our laboratory data with field data on tick burden and feeding activity in an R0 model. Our model predicts that high prevalence of B. burgdorferi infected mice lowers the ecological threshold for B. microti establishment, especially at sites where larval burden on P. leucopus is lower and where larvae feed simultaneously or soon after nymphs infect mice, when most of the transmission enhancement due to coinfection occurs. Our studies suggest that B. burgdorferi contributes to the emergence and expansion of B. microti and provides a model to predict the ecological factors that are sufficient for emergence of B. microti in the wild.

Key Points Lyme disease first came to public attention in the 1970s as a result of an epidemic of oligoarthritis in children and adults living in the vicinity of the town of Lyme, Connecticut, USA. The observation that infected individuals had a skin lesion called erythema migrans was crucial to identifying the causative agent (the spirochaete Borrelia burgdorferi ) and its arthropod vector ( Ixodes scapularis ).Lyme disease is now known to be worldwide in distribution and is a major public health problem in the United States. B. burgdorferi is perpetuated in an enzootic cycle in which uninfected larval ticks acquire the spirochaete by feeding on an infected reservoir host, usually a small mammal (for example, the white-footed mouse, Peromyscus leucopus ). The infected larvae moult to become nymphs, which then transmit the bacterium to an uninfected animal with the next blood meal. The outcome of infection is variable and dependent on the mammalian host. Humans are an incidental, dead-end host. B. burgdorferi has an unusual genome consisting of a 1 Mb linear chromosome and numerous linear and circular plasmids. The plasmids are a primary repository for differentially expressed lipoproteins. The bacterium is an auxotroph for all amino acids, nucleotides and fatty acids; it also lacks genes encoding enzymes for the tricarboxylic acid cycle and oxidative phosphorylation. B. burgdorferi resembles a Gram-negative bacterium in that it contains both outer and inner membranes. However, the architecture and composition of its outer membrane differ markedly from those of Gram-negative bacteria. Most notably, decorating the surface of B. burgdorferi are differentially expressed outer-surface lipoproteins. As with all spirochaetes, the organelles of motility (the flagella) are located in the periplasmic space. With the onset of the nymphal blood meal, spirochaetes in flat nymphal ticks undergo extensive changes in gene and protein expression that enable transmission of B. burgdorferi to the mammalian host. Many of these transcriptional changes are regulated by the response regulatory protein 2 (Rrp2)–RpoN–RpoS and histidine kinase Hk1–Rrp1 pathways. Following transmission, borrelial virulence determinants (for example, OspC) act in concert with tick salivary components (SALPs) to enable the bacterium to establish a foothold at the bite site and subsequently disseminate. The presence of spirochaetes locally and in tissues following dissemination triggers innate immune pathogen-sensing mechanisms (for example, Toll-like receptors), which recruit circulating leukocytes and orchestrate the development of the adaptive response. Macrophages are thought to be crucial innate immune effectors for bacterial clearance. Clearance of organisms is dependent on the appearance of specific antibodies. Spirochaetes counter the humoral response of the host by downregulating target surface antigens and activating the recombinatorial surface lipoprotein system ( vls ) for antigenic variation. Justin Radolf and colleagues summarize our accumulated knowledge of the molecular biology and virulence of Borrelia burgdorferi , and its interactions with the arthropod vector and mammalian hosts. In little more than 30 years, Lyme disease, which is caused by the spirochaete Borrelia burgdorferi , has risen from relative obscurity to become a global public health problem and a prototype of an emerging infection. During this period, there has been an extraordinary accumulation of knowledge on the phylogenetic diversity, molecular biology, genetics and host interactions of B. burgdorferi . In this Review, we integrate this large body of information into a cohesive picture of the molecular and cellular events that transpire as Lyme disease spirochaetes transit between their arthropod and vertebrate hosts during the enzootic cycle.