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Density-dependent processes in onchocerciasis
Transmit onchocerciasis: blackflies
(Source: WHO/TDR/image 9103222)
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Among filaria infections,
density-dependent processes have been extensively
investigated in onchocerciasis. These processes can occur in both,
in the intermediate (vector) as well as in the definite (human) host.
We discriminate between limitation and facilitation:
Limitation
is based on a negative feedback - the corresponding relationship
exhibits a saturating profile and approaches a limiting value.
Facilitation
is based on a positive feedback and the corresponding relationship
initially follows an exponential growth. This process, however,
does not proceed indefinately, but is in most cases followed
by a limitation process. The corresponding relationship
exhibits then a sigmoid profile.
As density-dependent processes strongly influence the
eradicability
of an infection,
their investigation is a prerequisite for sound predictions
into the prospects of success of interventions.
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(Source: Duerr et al., 2005)
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Figure: Parasite life cycle of a filarial infection,
using the example of Onchocerca volvulus,
and three stages for which density-dependent processes
have been suggested.
(a) Limitation in the establishment of adult parasites
is suggested by the observation that, in hyperendemic transmission,
the parasite burdens in human hosts are independent of,
rather than proportional to, the annual transmission potential (ATP).
This might be caused by mechanisms of protective immunity
in humans against infectious larvae (L3).
The saturating relationship becomes sigmoid if facilitation
(owing to parasite-induced immunosuppression)
in the establishment of adult parasites is assumed.
(b) Limitation in the microfilarial density
can result from either a reduction
in the life expectancy of microfilariae (Mf)
(as a consequence of protective immunity)
or from density-dependent Mf production
(crowding effects which reduce the fecundity
of adult female parasites).
(c) Both types of density dependence and nonregulation have been demonstrated
in vectors and strongly depend on whether or not the vectors have a cibarial
armature and/or form a peritrophic membrane. Limitation prevails in most vectors;
that is, the probability of successful development into an L3 decreases
if many Mf are ingested during the blood meal.
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Related pages:
Limitation,
Facilitation,
Eradicability,
Limitation & Control.
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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Prof. 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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