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Age-intensity profiles and dispersion patterns
Parasite burdens in host populations usually
show a great variance and their distributions are skewed
(most of the hosts harbour few parasites while a few harbour much parasites).
Since the 1980s, parasitological dispersion patterns
have often been analysed with the intention to gain insight into
regulatory processes
in the host-parasite relationship.
Particularly, it has been suggested that parasite-induced
host mortality can be estimated from such cross-sectional data.
We have written an overview into this, considering the
following six processes:
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Age-dependent exposure of hosts
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Parasite-induced host mortality
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Density-dependent parasite mortality
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Density-dependent infection rates of hosts
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Heterogeneity in exposure/susceptibility among hosts
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"Clumped" infection
The parasite distributions produced by these process are simulated
dependent on the host's age (age-intensity-profile) and their
dispersion patterns are compared.
We point at the ambiguities in the interpretation of dispersion patterns
emerging from simultaneously operating processes.
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Figure: Age-intensity-profiles (AIPs) of parasite burdens
among hosts, under the influence of six modifying processes.
The age-dependent distributions of parasite burdens are
represented by the median (bold line), and by the quantiles
2.5, 25, 75 und 95% (thin lines, from bottom to top).
The 'nullmodell', i.e. the pure immigration-death process
without any additional assumptions, is shown on the left.
Graphs on the right hand side show:
Age-dependent exposure of hosts
shifts the AIP according to the function of exposure.
Parasite-induced host mortality
hardly modifies the AIP which is only altered if the parasite
causes an unrealistically high mortality in hosts.
Density-dependent parasite mortality
reduces the mean parasite burden and the variance in the AIP.
Density-dependent infection rates of hosts
alters the shape and the dispersion pattern of the AIP.
Heterogeneity in exposure/susceptibility among hosts
only increases the variance in the AIP, but not the
trend in the mean parasite burden by age.
"Clumped" infection
only increases the variance in the AIP, but not the
trend in the mean parasite burden by age.
(For the parameter values and the details of simulations see
Duerr HP et al., 2003).
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Related pages:
Density-dependence
Further reading:
Duerr, H. P., Dietz, K., Eichner, M., 2003.
On the interpretation of age-intensity profiles and dispersion patterns in parasitological surveys.
Parasitology 126, 87-101.
Abstract at PubMed |
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