• Open Access

Consequences of immunopathology for pathogen virulence evolution and public health: malaria as a case study

Authors

  • Gráinne H. Long,

    1.  Department of Epidemiology and Public Health, London School of Hygiene and Tropical Medicine, London, UK
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  • Andrea L. Graham

    1.  Institutes of Evolution, Immunology and Infection Research, School of Biological Sciences, University of Edinburgh, Edinburgh, UK
    2.  Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ, USA
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Gráinne H. Long, University of Sheffield, Department of Animal and Plant Sciences, Western Bank, Sheffield, S10 2TN, UK. Tel.: +44 (0) 114 222 4670; fax: +44 (0) 114 222 0002; e-mail: grainne.long@lshtm.ac.uk

Abstract

Evolutionary theories explaining virulence—the fitness damage incurred by infected hosts—often focus on parasite strategies for within-host exploitation. However, much virulence can be caused by the host’s own immune response: for example, pro-inflammatory cytokines, although essential for killing malaria parasites, also damage host tissue. Here we argue that immune-mediated virulence, or ‘immunopathology,’ may affect malaria virulence evolution and should be considered in the design of medical interventions. Our argument is based on the ability of immunopathology to disrupt positive virulence-transmission relationships assumed under the trade-off theory of virulence evolution. During rodent malaria infections, experimental reduction of inflammation using reagents approved for field use decreases virulence but increases parasite transmission potential. Importantly, rodent malaria parasites exhibit genetic diversity in the propensity to induce inflammation and invest in transmission-stage parasites in the presence of pro-inflammatory cytokines. If immunopathology positively correlates with malaria parasite density, theory suggests it could select for relatively low malaria virulence. Medical interventions which decrease immunopathology may therefore inadvertently select for increased malaria virulence. The fitness consequences to parasites of variations in immunopathology must be better understood in order to predict trajectories of parasite virulence evolution in heterogeneous host populations and in response to medical interventions.

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