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      Investment in Constitutive Immune Function by North American Elk Experimentally Maintained at Two Different Population Densities

      1 , * , 1 , 2

      PLoS ONE

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          Natural selection favors individuals that respond with effective and appropriate immune responses to macro or microparasites. Animals living in populations close to ecological carrying capacity experience increased intraspecific competition, and as a result are often in poor nutritional condition. Nutritional condition, in turn, affects the amount of endogenous resources that are available for investment in immune function. Our objective was to understand the relationship between immune function and density dependence mediated by trade-offs between immune function, nutritional condition, and reproduction. To determine how immune function relates to density-dependent processes, we quantified bacteria killing ability, hemolytic-complement activity, and nutritional condition of North American elk ( Cervus elaphus) from populations maintained at experimentally high- and low-population densities. When compared with elk from the low-density population, those from the high-density population had higher bacteria killing ability and hemolytic-complement activity despite their lower nutritional condition. Similarly, when compared with adults, yearlings had higher bacteria killing ability, higher hemolytic-complement activity, and lower nutritional condition. Pregnancy status and lactational status did not change either measure of constitutive immunity. Density-dependent processes affected both nutritional condition and investment in constitutive immune function. Although the mechanism for how density affects immunity is ambiguous, we hypothesize two possibilities: (i) individuals in higher population densities and in poorer nutritional condition invested more into constitutive immune defenses, or (ii) had higher parasite loads causing higher induced immune responses. Those explanations are not mutually exclusive, and might be synergistic, but overall our results provide stronger support for the hypothesis that animals in poorer nutritional condition invest more in constitutive immune defenses then animals in better nutritional condition. This intriguing hypothesis should be investigated further within the larger framework of the cost and benefit structure of immune responses.

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          Most cited references 31

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          How should pathogen transmission be modelled?

           H. McCallum (2001)
          Host-pathogen models are essential for designing strategies for managing disease threats to humans, wild animals and domestic animals. The behaviour of these models is greatly affected by the way in which transmission between infected and susceptible hosts is modelled. Since host-pathogen models were first developed at the beginning of the 20th century, the 'mass action' assumption has almost always been used for transmission. Recently, however, it has been suggested that mass action has often been modelled wrongly. Alternative models of transmission are beginning to appear, as are empirical tests of transmission dynamics.
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            Population biology of infectious diseases: Part I.

            If the host population is taken to be a dynamic variable (rather than constant, as conventionally assumed), a wider understanding of the population biology of infectious diseases emerges. In this first part of a two-part article, mathematical models are developed, shown to fit data from laboratory experiments, and used to explore the evolutionary relations among transmission parameters. In the second part of the article, to be published in next week's issue, the models are extended to include indirectly transmitted infections, and the general implications for infectious diseases are considered.
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              The immune system in pregnancy: a unique complexity.

              Placental immune response and its tropism for specific viruses and pathogens affect the outcome of the pregnant woman's susceptibility to and severity of certain infectious diseases. The generalization of pregnancy as a condition of immune suppression or increased risk is misleading and prevents the determination of adequate guidelines for treating pregnant women during pandemics. There is a need to evaluate the interaction of each specific pathogen with the fetal/placental unit and its responses to design the adequate prophylaxis or therapy. The complexity of the immunology of pregnancy and the focus, for many years, on the concept of immunology of pregnancy as an organ transplantation have complicated the field and delayed the development of new guidelines with clinical implications that could help to answer these and other relevant questions. Our challenge as scientists and clinicians interested in the field of reproductive immunology is to evaluate many of the 'classical concepts' to define new approaches for a better understanding of the immunology of pregnancy that will benefit mothers and fetuses in different clinical scenarios.

                Author and article information

                Role: Academic Editor
                PLoS One
                PLoS ONE
                PLoS ONE
                Public Library of Science (San Francisco, CA USA )
                20 May 2015
                : 10
                : 5
                [1 ]Department of Natural Resources and Environmental Sciences, University of Nevada, Reno, Nevada, United States of America
                [2 ]Pacific Northwest Research Station, United States Forest Service, La Grande, Oregon, United States of America
                Sonoma State University, UNITED STATES
                Author notes

                Competing Interests: The authors have declared that no competing interests exist.

                Conceived and designed the experiments: CJD KMS. Performed the experiments: CJD KMS BLD. Analyzed the data: CJD. Wrote the paper: CJD KMS.


                This is an open access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication

                Page count
                Figures: 3, Tables: 0, Pages: 17
                This study was supported by a Hatch grant awarded by Nevada Agriculture Experimental Station and funding from the University of Nevada Reno, and US Forest Service grant awarded to KMS. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
                Research Article
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                All relevant data are within the paper and its Supporting Information files.



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