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at the Department of Medical Biometry, University of Tübingen

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Uncertainties in the control of filariasis

Check: Ivermectin taken?
Check: Ivermectin taken?
(Source: WHO/TDR/image 98031362)
The eradicability of a filarial infection can be affected by density-dependent side effects of certain interventions. The figure below shows how the persistence graph (lower graphs, colors see persistence graph) is altered by administration of a microfilaricide. The pale-colored curve in each graph represents the persistence pattern without intervention. The full-colored curves represent the persistence patterns resulting from a microfilaricide-based intervention, whereby density-dependent characteristics of the microfilaricidal effect are shown in the upper graphs. In the three scenarios, the microfilaricide reduces the microfilariae (Mf) density by 95% at the population level but can deviate from this in a density-dependent manner by 2%.
Uncertainty analysis
Figure: (a) An effect which might result from ivermectin-facilitated immunity. As a result of this mechanism, the reduction in the Mf density improves in (a) from 95% to 97% during the course of therapy (from right to left in the upper graph). The corresponding persistence graph shows that under this assumption, the threshold biting rate (TBR, see also Glossary) increases from ~900 to ~18000 bites per person per year, suggesting that elimination might be easily achieved. Compared with the TBR of ~11000 in the case of no density dependence [scenario (b), 95% reduction independent of the Mf density], a mechanism such as ivermectin-facilitated immunity can considerably increase the prospects of elimination. In scenario (c), chosen with respect to symmetry to (a), the reduction in the Mf density declines from 95% to 93% during the course of therapy (from right to left in the upper graph), which shifts the TBR to ~8000 bites per person per year. The examples show that a slight variation in the density-dependent side effects of an intervention can considerably alter the prospects of elimination. (Source: Duerr et al., 2005)
Related pages: Persistence graphs, Eradicability of onchocerciasis, Limitation, Facilitation.

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: 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
Dr. M. Eichner
Financial support by: Deutsche Forschungsgemeinschaft (DFG, DI 308/12-1)
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