Berkeley INTEGBI 153 - Parasitism and disease (2 pages)

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Parasitism and disease



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Parasitism and disease

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Pages:
2
School:
University of California, Berkeley
Course:
Integbi 153 - Ecology

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IB 153 Parasitism and disease Host parasite population dynamics 10 12 2006 Simplest A M 1981 model directly transmitting parasite between invertebrate hosts with no immune response Persistent Infection Model Anderson and May models Some terms H total of hosts X Y X susceptible hosts Y infected or infectious hosts a per capita host birth rate b rate of host mortality due to factors other than disease r a b host intrinsic rate of increase disease induced mortality rate pathogenicity transmission coefficient dX dt a X Y bX XY Y dY dt XY b Y dH dt rH Y Assumptions Per capita birth rate a is same for infected and uninfected individuals Sources of mortality are additive in their effect Does not consider intensity of infection only prevalence No vertical transmission All parameters are constants No density dependence in host population growth grows exponentially in the absence of parasites rate of host recovery from infection There are lots of variants on this simple model that incorporate such complexities as free living stages immune responses and alternate hosts Some general predictions 1 High rates of host reproduction reduce the impact of parasites on host population density 2 Higher rates of transmission cause greater reduction in host population density If too low parasite goes extinct 3 Maximum degree of host population depression occurs at intermediate pathogenicity If it is r then host population escapes control but if it is very high hosts are killed before the parasite can be effectively transmitted so parasite declines eventually going extinct 4 Aggregation or clumping of parasites tends to reduce their impact on host populations Since most parasites are in a few individual hosts the parasite population drops when these individuals die and control of the host population is reduced On the other hand a random distribution of parasites can cause too strong a suppression of the host population leading to destabilization and parasite extinction 5 Direct



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