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Density-dependence & eradicability
Density-dependent process
within the host-parasite relationship
determine the location of transmission thresholds and breakpoints.
Using mathematical models, these quantities can be computed
under different assumptions on whether or not such a process is involved.
In general, we find that
limitation processes
decrease transmission
thresholds and breakpoints
(see also persistence graph)
and thereby negatively influence ('limit')
the eradicability of an infection. In contrast,
facilitation processes
increase transmission thresholds and breakpoints and thereby positively
influence ('facilitate') the eradicability of an infection.
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Figure:
Persistence graphs
resulting from density-dependent processes,
operating at different stages of the parasitic life cycle.
The pale-colored curve in each graph represents the persistence pattern
that results if there is no density dependence
(except for the limited relationship between the PER and the ATP).
The threshold biting rate (TBR) lies at ~900 bites per person per year (bpy),
and breakpoints result only from the mating process.
Parameter values refer to West African savannah onchocerciasis.
Adult: Number of adult female parasites establishing per year in a host.
Mf: Microfilarial density in the skin dependent on the number
of adult female parasites in the human host.
L3: Number of L3 developing in a fly from microfilariae
(Mf) ingested during preceding blood meals.
(a) Persistence graph resulting from facilitated parasite establishment:
the transmission threshold increases to a TBR of ~1700 bpy,
and breakpoints are shifted considerably towards higher parasite burdens,
indicating facilitated eradicability.
(b) Limitation in the Mf density complicates control measures
because the transmission threshold decreases to a TBR of ~400 bpy and,
accordingly, the breakpoints are shifted towards lower parasite burdens.
(c) Limitation in flies also worsens the prospects of elimination
by decreasing the transmission threshold to a TBR of ~500 bpy.
(d) Persistence graph under the assumption that
the density-dependent processes in (a), (b) and (c) operate simultaneously.
In this example, the parameter values chosen for the three processes
yield a TBR that is still more pessimistic compared with the case of
nonregulation in all three processes that is, the effects
of the two limitation processes outbalance the facilitation process.
(Source: Duerr HP et al., 2005)
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Related pages:
Limitation,
Facilitation,
Overview into eradicability,
Limitation & control,
Mathematical model.
Further reading:
Duerr HP, Dietz K, Eichner M, 2005.
Determinants of the eradicability of filarial infections: a conceptual approach.
Trends in Parasitology 21: 88-96.
Abstract at PubMed
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Responsible for this page:
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Dr. H.-P. Duerr
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Webmaster:
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Prof. Dr. M. Eichner
(last change of this page on
13 July 2009)
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Cooperation with:
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Prof. K. Dietz, Institut für Medizinische Biometrie(IMB), Tübingen
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Dr. M. Eichner
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Financial support by:
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Deutsche Forschungsgemeinschaft (DFG, DI 308/12-1)
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Disclaimer:
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Eberhard-Karls-University Tübingen,
Tübingen University Hospital,
the Department for Medical Biometry (IMB),
and the authors of this page disclaim all liability for the content of any page referenced by hyper-link from this page
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