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Abstract Host susceptibility may be critical for the spread of infectious disease, and understanding its basis is a goal of ecological immunology. Here, we employed a series of mechanistic tests to evaluate four factors commonly assumed to influence host susceptibility: parasite exposure, barriers to infection, immune responses, and body size. We tested these factors in an aquatic host–parasite system (Daphnia dentifera and the fungal parasite, Metschnikowia bicuspidata) using both laboratory-reared and field-collected hosts. We found support for each factor as a driver of infection. Elevated parasite exposure, which occurs through consumption of infectious fungal spores, increased a host’s probability of infection. The host’s gut epithelium functioned as a barrier to infection, but in the opposite manner from which we predicted: thinner anterior gut epithelia were more resistant to infectious spores than thick epithelia. This relationship may be mediated by structural attributes associated with epithelial cell height. Fungal spores that breached the host’s gut barrier elicited an intensity-dependent hemocyte response that decreased the probability of infection for some Daphnia. Although larger body sizes were associated with increased levels of spore ingestion, larger hosts also had lower frequencies of parasite attack, less penetrable gut barriers, and stronger hemocyte responses. After investigating which mechanisms underlie host susceptibility, we asked: do these four factors contribute equally or asymmetrically to the outcome of infection? An information-theoretic approach revealed that host immune defenses (barriers and immune responses) played the strongest roles in mediating infection outcomes. These two immunological traits may be valuable metrics for linking host susceptibility to the spread of infectious disease.
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Role of Multiple Infections on Immunological Variation in Wild Populations
ABSTRACT A central challenge in the fields of evolutionary immunology and disease ecology is to understand the causes and consequences of natural variation in host susceptibility to infectious diseases. As hosts progress from birth to death in the wild, they are exposed to a wide variety of microorganisms that influence their physical condition, immune system maturation, and susceptibility to concurrent and future infection. Thus, multiple exposures to the same or different microbes can be important environmental drivers of host immunological variation and immune priming. In this perspective, I discuss parasite infracommunity interactions and their imprint on host immunity in space and time. I further consider feedbacks from parasite community dynamics within individual hosts on the transmission of disease at higher levels of biological organization and highlight the promise of systems biology approaches, using flour beetles as an example, for studying the role of multiple infections on immunological variation in wild populations.
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- Award ID(s):
- 1753982
- PAR ID:
- 10135717
- Date Published:
- Journal Name:
- mSystems
- Volume:
- 4
- Issue:
- 3
- ISSN:
- 2379-5077
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1128/mSystems.00099-19
