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Lecture_6_Macroparasites1 - The Population Biology of...

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1 The Population Biology of Macroparasites Hookworms sink their teeth into the intestinal walls of more than a billion people every day to drink their blood The population biology of macroparasites 1. Biology of macroparasites 2. Distributions and aggregation 3. Effects on individual hosts 4. Basic model for macroparasite ecology What are macroparasites? Characteristics •Larger than microparasites • Longer generation times • Transmission usually occurs via release of infective stages (stages outside of host) • Persistent infections – with continual reinfection; can have cumulative, sub- lethal effects • Two major classes • Chelicerata (ticks, mites) •I n s e c t a (bot flies, lice, What are macroparasites? Some arthropods fleas) • Many are ectoparasites with special holdfast mechanisms What are macroparasites? Typically Helminths Cestodes - tapeworms Tt d li i t ti l Tapeworm Hymenolepis Trematodes - liver, intestinal and blood flukes Nematodes - round worms Blood fluke Schistosoma Hookworm Necator What are macroparasites? Host-parasite interaction • Egg output per female parasite • Pathogenic effects on the host • Ellicitation of host immune response • Parasite death rates …ALL DEPEND ON THE NUMBER OF PARASITES IN A GIVEN HOST
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2 Measures of infection • Prevalence – proportion of population affected (for micro- and macroparasites) • Intensity – mean number of worms per infected host (# parasites / # infected infected hosts) Also need to consider the frequency distribution of parasites in the population Most parasites are highly aggregated Population biology of Macroparasites Distributions and aggregation Most individuals harbor just a few parasites… and a few hosts harbor many parasites. Variation in exposure to parasite infective stages Differences in susceptibility The 80/20 rule: 20% of the hosts harbor 80% of the parasites 40 45 50 As a generalization. . 80% of the parasites are Population biology of Macroparasites f hosts 0 5 10 15 20 25 30 35 024681 0 Large proportion of worm population in small proportion of host population concentrated in 20% of hosts Number of worms Number o A relatively small number of hosts are in the ‘tail’ of the distribution Distributions in statistical terms ases) Random Aggregated Value (number of worms per host) Frequency (# of ca M1 M2 s 2 s 2 Distributions in statistical terms • Random distributions: – variance = mean (S 2 = M) – For count data: Poisson distribution Variance to mean ratio, or s 2 /m, is one way to characterize the degree of aggregation • Aggregated distributions:
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Lecture_6_Macroparasites1 - The Population Biology of...

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