Parasitism PDF

Parasitism Lesson Objectives Be able to describe basic parasite natural history: diversity, distribution, abundance Describe ecological and evolutionary advantages and disadvantages to various parasite life histories Know how and why parasites and hosts evolve defenses against one anoth...

Parasitism Lesson Objectives Be able to describe basic parasite natural history: diversity, distribution, abundance Describe ecological and evolutionary advantages and disadvantages to various parasite life histories Know how and why parasites and hosts evolve defenses against one another Describe how parasites affect host populations and communities Parasitism: the basics Parasites feed on the tissues or body fluids of their host organisms All parasites require a host for at least some part of their life cycle Some parasites, called pathogens, can cause disease (Disease is a product of a pathogen) Parasites harm their hosts, but they do not always kill them Parasites typically feed on only one or a few host species, but each host species can have multiple parasite species  Parasite and Pathogen Diversity Too many to name Apicomplexa (e.g., Helminths Viruses, bacteria, fungi Metazoa Protozoa tapeworms (cestodes) plasmodium, flukes (trematodes) cryptosporidium) roundworms (nematodes) Arthropod Amoebas Lice Flagellates (e.g., Fleas giardia, leishmania) Flies Arachnids (ticks, mites) Bed bugs Isopods (tongue-eating louse) Plants Mistletoe/Dodder Mollusks Marine snails Chordates Lampreys Vampire finch/bat/fish Brood parasitic birds  Nearly every metazoan organism has at least one parasite! Parasites are estimated to make up 40-60% of all species on the planet!! Parasite Natural History Macroparasites vs. Microparasites: based on size of the parasite Macroparasites are relatively large species (arthropods, worms, plants) Microparasites are too small to be seen with the naked eye (bacteria, protists, unicellular fungi) Parasite Natural History Ectoparasites vs. Endoparasites: based on where they reside on the host Ectoparasites live on the surface of their host Endoparasites live inside their host Parasite Natural History Parasite life cycles vary, A LOT Parasite Natural History Parasite life cycles vary, A LOT ….a whole lot… Virulence The harm done by a pathogen or parasite to its host It is highly variable: influenza chicken pox head lice ebola Virulence Resistance Tolerance Avoidance  Host Defenses Parasite Counter Defenses Immune response (e.g., Hide from immune system (e.g., surface macrophage production to engulf proteins that are not recognized as foreign) antigens) Biochemical defense (e.g., removal Break the system (e.g., robbing transferrins – of iron from blood) which are hiding the iron) Partnerships (e.g., symbiotic Attack the partners (e.g., infect fungus first, relationship with other organisms, then tree) like fungus that protects cacao trees from pathogen infection) Encapsulation (e.g., akin to an Build tolerance or resistance (e.g., evolve to immune response, but functions tolerate toxins or chemicals that cause more by entombing parasites or encapsulation) pathogens within the host tissues)  When populations to two interacting species Co-evolution evolve together, each in response to selection pressure imposed by the other Co-evolution A case study: European Rabbit and Myxoma Virus Red Queen Hypothesis “Run as fast as you can to stay in the same place.” Species must constantly evolve because every other species is constantly evolving. Rabbits quickly evolved resistance. New strains had to be sought out and introduced to maintain effects on rabbits. How do we measure disease spread? We changed the predator/prey models in at least 3 key ways: 1. The host population is divided into susceptible (S), infected (I), and recovered (R) individuals. 2. We need to keep track of host and parasite genotypes, since they can vary greatly in their resistance and virulence 3. Must account for the factors that influence spread, for example… Differences in the likelihood that hosts of different ages will come together Latent periods (in which hosts are infected but cannot spread the disease) Whether vertical transmission (mother to offspring) is possible CDC Parasite effects at the individual level Cons: negative fitness consequences including slower growth, lower fecundity, reduced longevity, reduced attractiveness, etc. Pros: co-infection dynamics can limit infection by more severe parasites, exposure can prime immune system and prevent autoimmune disorders Hygiene hypothesis – a lack of exposure to certain parasites may increase the likelihood of developing autoimmune disease or allergies  Parasite effects at the population level Cons: can reduce population size, can reduce population growth rates, can cause allee effects Pros: can reduce population size which reduces competition for resources, can remove sick individuals which can reduce transmission to rest of population Parasite effects at the community level Cons: can reduce competitive advantage of native hosts, can destabilize food webs if parasite-induced mortality on any one species is too great Pros: can regulate species populations facilitating competition and biodiversity, can stabilize food webs by keeping multiple species in check, can prevent community invasibility Parasite conservation Movement to recognize importance of parasites only really gained momentum about 20 years ago Currently very few parasites listed as threatened by IUCN Gaps in our understanding of the positive effects of parasites on individuals, populations, communities, and ecosystem are hurting that movement Hyperfocus on human parasites also hurts this movement