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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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 reproduction in the host leads to greater suppression of the host population R0 S fL your book S L Critical threshold susceptible host density where parasite population maintains itself R0 1 ST 1 fL your book L Conditions for parasite persistence R0 1 parasite will establish and increase in number and prevalence within a host population R0 1 parasitic infection will die out What are determinants of R0 1 Density of susceptibles host individuals S 2 Transmission rate of the parasite 3 Fraction of hosts that survive long enough to become infectious themselves f 4 Average time over which an infected host remains infectious L Predictions about parasite life histories 1 Diseases that are very infectious high unlikely to kill host large f or have long periods of infectiousness high L will have a high R0 and be able to persist in small host populations low ST 2 Diseases that have low infectivity low are likely to kill host quickly highly pathogenic low f or have short periods of infectiousness low L will have small R0 and can only persist in large populations of hosts high ST 1 IB 153 Parasitism and disease 10 12 2006 What s going on Case 1 protozoans in invertebrates long lasting infections in a host individual large L because of short lived immune response so parasite can persist in small populations low ST Case 2 viral or bacterial infections induce a strong immune reaction which makes the infection transient within individual hosts low L thus requires a large population of susceptibles to persist high ST For example for measles susceptible population must be 500 000 Isn t it bad for a parasite to kill its host Won t a parasite gradually evolve into a mutualist The answers are in R0 S fL If f and L all varied independently the a reduction in virulence low f would inevitably lead to a higher R0 and parasites would evolve into mutualists But These parameters often covary in a specific way virulence is a consequence of the production of transmission stages cell damage toxic metabolic wastes So lower means lower virulence high f also lower virulence means fasater recovery so lower means lower L A parasite can have a high R0 if it kills quickly small f but produces many infective stage high Or it can have an equally high R0 if produces fewer infective stages low but is less virulent large f More than one way to skin a cat evolution can push virulence in different directions In Myxoma intermediate virulence strains are the most fit highest R0 2


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