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Age-intensity profiles and dispersion patterns

Waiting to receive treatment
(Source: WHO/TDR/image 98031329)
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:
  • Age-dependent exposure of hosts
  • Parasite-induced host mortality
  • Density-dependent parasite mortality
  • Density-dependent infection rates of hosts
  • Heterogeneity in exposure/susceptibility among hosts
  • "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.
Figure 1
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).
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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Responsible for this page: Dr. H.-P. Duerr
Webmaster: Prof. Dr. M. Eichner (last change of this page on 13 July 2009)
Cooperation with: Prof. K. Dietz, Institut für Medizinische Biometrie(IMB), Tübingen, Germany
Dr. M. Eichner
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