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Glossary
ABR: Annual biting rate (number of vectors that take a blood meal
on one human host per year).
ATP: Annual transmission potential [average number of infectious
larvae (L3) transmitted to one human host per year].
Breakpoint: A parasite number or density below which infection
cannot persist. Breakpoints can be determined for each parasite stage
and also for the ATP; they represent ABR-specific, unstable equilibria,
resulting from facilitation processes.
Breakpoints guarantee the existence of a stable zero equilibrium
which is a prerequisite of stable elimination (if there are no
breakpoints and the ABR exceeds the TBR, the zero equilibrium is
unstable, that is, the reintroduction of few infections can lead
in the endemic state). The reasons for the existence of breakpoints
are best explained by the mating process: as transmission depends on
sexual reproduction of the parasite, an individual must harbor at least
one female and one male parasite to contribute to the transmission
of the infection. Averaged over the population, however, breakpoints
do not necessarily remain at two parasites per host and can still be
relevant for control measures if falling short of one parasite per host.
The mating process transforms the process of
microfilariae (Mf) production
by adult female worms into a facilitation process because mating (and hence reproduction) becomes increasingly facilitated at high parasite burdens.
Control: Reduction of the incidence of infection to a certain level
where the disease is no longer considered to be a public health problem.
Density-dependent processes:
Regulatory processes in the
vectorparasitehost relationship which depend in a nonlinear way
on the parasite density that is, the number of parasites per host
(see also
Facilitation and
Limitation).
Elimination: Local reduction to zero of the incidence of infection;
because infection can be imported from other areas that are still endemic,
permanent intervention is required to maintain elimination as a stable
state of the trivial equilibrium.
Equilibrium: A parasite density (e.g. adult parasites per host,
microfilariae per mg skin snip, L3 per fly or ATP) which is sufficiently
constant over a long period of time. The equilibrium solution of a
mathematical model
results from setting the derivatives equal to zero, such that there is
no longer a change in the variables. The term 'trivial equilibrium'
describes the zero equilibrium which is stable in the case of facilitation
and unstable in the case of limitation (provided ABR > TBR;
below the TBR the trivial equilibrium is stable).
Positive unstable equilibria are synonyms for breakpoints.
Eradication: Global reduction to zero of the incidence of infection;
once achieved, further interventions are not necessary
(Eradicability
is a term often used in a broader sense, without discriminating between the local and the global aspect).
Facilitation:
A positive feedback process in which a parasite (at any stage) promotes
the success of parasites at the same or another stage, regarding survival,
development, reproductivity, etc. Because any population is limited in space,
reproductivity, etc., a facilitation process must be associated with
at least one 'stronger' limitation process.
Facilitation processes increase transmission
thresholds and breakpoints and thereby positively influence ('facilitate')
the eradicability of an infection.
Limitation:
A negative feedback process in which a parasite (at any stage) compromises
the success of parasites at the same or another stage, regarding survival,
development, reproductivity, etc. Because any population is limited in space,
reproductivity, etc., limitation is the rule. However, it is possible
that the limited part of the process is not yet, or might not be, observable.
Limitation processes decrease transmission
thresholds and breakpoints and thereby negatively influence ('limit')
the eradicability of an infection.
L3: Third stage larva. This is the infectious parasite stage
which can be transmitted from a fly to a human host during a bloodmeal.
If the L3 survives the machanisms of host resistence or immunity,
it develops into an adult worm via the fourth stage larva (L4).
See also life cycle.
L4: Fourth larva stage, between the L3 and the adult parasite.
In onchocerciasis, it is assumed that the period of development
from the L3 into and adult parasite and thus, the lifespan of an L4,
is about one year.
See also life cycle.
PER: Parasite establishment rate, i.e. the number of adult
female O. volvulus which establish in a human host per year.
Superinfection: In the context of filarial diseases, this term
is used for describing the process of infection with a new parasite
while being already infected with one or more parasites
of the same species.
TBR:
Threshold biting rate (an ABR below which the infection
cannot persist). A more general term is transmission threshold.
Transmission threshold: A vector density below which infection
cannot persist (see also TBR). If there are too few vectors,
an adult parasite will die before any of its offspring are transmitted,
and, consequently, the infection does not get transmitted.
A key variable is the ABR. The threshold below which the infection
cannot persist, the threshold biting rate (TBR), is a measure
of the persistence
of the parasite population. The values of the TBR
vary considerably between parasite subspecies and depend on the
density-dependent processes operating in vectors and hosts and
on the heterogeneity in contact rates between vectors and hosts.
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