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Parasite/pathogen
Coinfections and virulence
| Question | Answer |
|---|---|
| Mutualist-parasite spectrum | - symbionts: individuals of one species lives on or in individuals of another species - (+) mutualism, commensalism, parasitism (-) - many species go up and down the spectrum, some are at both ends |
| Helicobacter pylori | - both ends of the spectrum - can cause ulcers and stomach cancer, but also protects against esophageal cancer |
| Amphibiosis | - natural partnerships that are helpful in some contexts and harmful in others |
| Virulence is a primitive character for a parasite 'conventional wisdom' | Evolution (coevolution) will necessarily lead to commensalism, mutualism or the extinction of the host and/or parasite Virulence is an indication of a recent association between a parasite and its host |
| Virulence decreases transmission duration | Parasite fitness determined by transmission: high transmission = high fitness Parasites depend on host for transmission ∴ low virulence increases transmission |
| Problems with conventional wisdom | Virulent, obligate human pathogens, e.g. Neisseria gonorrhoeae Long associations with humans, e.g. the agents of malaria and TB Is “long” not long enough, or do some pathogens evolve higher virulence? |
| Virulence-transmission trade off | Virulence provides fitness cost of host death + benefit of high replication ∴intermediate virulence maximizes total transmission/ fitness Level of intermediate virulence depends on ecology and the system |
| Transmission strategy affects optimal virulence | - direct transmission: requires active hosts, low virulence - vector borne: might prefer inactive hosts, high virulence ie. waterborne |
| Transmission strategy: vertical/horizontal | - vertical: infected individual reproduces infected progeny - horizontal: parasite moves from infected to uninfected, by direct contact or infectious particle |
| Community context | - In nature, rare to get just pairwise interactions - Intraspecific competition in coinfections of multiple strains of same species - Interspecific competition in coinfection of multiple species |
| Social behaviours | - classified social behaviours according to whether the consequences they entail for the actor and recipient are beneficial (increase direct fitness) or costly (decrease direct fitness) |
| Relatedness | Relatedness of parasite individuals within a host can affect virulence due to social behaviour |
| Prudence | - Individuals limit resource use for benefit of others |
| Public goods cooperation | - Individuals donate resources that are of benefit to others |
| Spiteful interactions | - costly to all |
| Intraspecific competition ie. malaria and hookworms | Mass de-worming programmes to treat hookworm: exacerbated intensity of malaria 2-3X Study of 4000 people in Indonesia with coinfection Competition for red blood cells. Strain effect: P. vivax less competitive with worms than P. falciparum |
| ie. bacteria and nematodes | Microbe-mediated host defense drives the evolution of reduced pathogen virulence |
| One-sided evolution ie. C. elegans-E. faecalis coevolution | - adaptive mutualism evolves - Coevolution + coinfections can shape interactions towards defensive symbiosis Allowing protective bacteria to colonise is an immune defense |
| How to control symbionts? | Remember: location important – switch from beneficial or harmful depending on where they are Bacteriocytes in insects (e.g. Stoll et al 2010) |
| Phage and mucus to control our symbionts | Phage 15X more likely to find bacterial host if stick to mucus Mucus is universal to all animals; phage universal to all mucus Phages the original immune system (one of many theories for origin of IS) - mutually beneficial relationship |
| Immune system evolution | Main function is to manage an animals relationship with its microbes Vertebrates have complex immune systems Rather than limiting microbes, vertebrate immune system evolved to support even more of them |
Created by:
reub8n
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