|
|
Facilitation
Facilitation is based on positive feedback
which is rarely observed in nature.
In filariases, it occurs, for example, if
vectors possess a cibarial armature,
a tooth-like chitin structure which lacerates ingested Mf.
At low microfilariae densities, this cibarial armature
substantially reduces the proportion of surviving microfilariae.
At high Mf densities, however, the cibarial armature
might be inefficient because it is masked
by a few Mf promoting the survival of the others
As a result, we obtain a profile as shown above:
the proportion of surviving microfilaria in the fly
increases with increasing microfilarial density in the human skin.
Another example of facilitation in filariasis is parasite-induced
immunosuppression.
Figure above: Fitting a facilitated and a non-regulated relationship
to the same data leads to an intersection between both curves.
This intersection is an unstable equilibrium, called
breakpoint, i.e. a parasite density below which infection
cannot persist.
The trivial equilibrium (the infection-free state) is
in this case stable. Without external influences,
the system tends towards the infection-free state state (gray arrow).
Facilitation processes 'facilitate' the eradicability of an infection,
because they destabilize the endemic state and
stabilize the infection-free state.
The endemic equilibrium must be provided by an associated
limitation process
leading to a second intersection between both curves (see figure below).
Starting from the infection-free state, this leads to equilibriae in
the following order:
Elimination (stable) - Breakpoint (unstable) - Endemic state (stable).
This figure has important implications for the control
of an infection: will the intervention be cancelled
while the parasite density still exceeds the value of the breakpoint,
then, the system will tend back to the stable equilibrium
of the endemic state (gray arrow pointing upwards).
If, on the other hand, the intervention will be continued
until the parasite density underruns the value of a breakpoint,
then, the infection will go extinct without further efforts,
because the systems tends by itself towards the stable
infection-free state (gray arrow pointing downwards).
See also
Persistence graphs.
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
Related pages:
Limitation,
Eradicability of onchocerciasis,
Limitation & Control.
|
|
You are the
th visitor of our pages.
|
|
|
|
|
