
Uncertainties in the control of filariasis
The
eradicability
of a filarial infection can be affected
by densitydependent 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 palecolored curve in each graph represents the persistence pattern
without intervention. The fullcolored curves represent
the persistence patterns resulting from a microfilaricidebased intervention,
whereby densitydependent 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 densitydependent manner by 2%.


Figure: (a) An effect which might result from ivermectinfacilitated 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 ivermectinfacilitated 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 densitydependent side effects of an intervention can considerably
alter the prospects of elimination. (Source: Duerr et al., 2005)




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