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Limitation and Control
A main determinant of the
eradicability
of filarial infections is the relationship
between the annual transmission potential (
(ATP,
the annual number of infectious larvae
transmitted by vectors to a human host) and the parasite establishment rate
(PER,
number of adult female parasites successfully establishing in a host per year).
In contrast to the ATP, which can roughly be estimated from fly-catching experiments,
the PER is not experimentally accessible.
Thus, the PER is a major source of uncertainty within the transmission cycle,
challenging our efforts to predict either the outcome of interventions
or the risk of the infection re-establishing itself in a population.
Relationships between the ATP and the PER can be classified
into two categories, non-regulated and regulated.
-
In a non-regulated relationship,
the PER grows linearly with the ATP, implying that a constant proportion of L3
successfully develop into the adult stage irrespective of the ATP.
The linear relationship yields an optimistic prediction of the outcome
of a control program because any effort to decrease the ATP proportionally
diminishes the establishment of new parasites.
-
In a regulated relationship of the
limitation type,
on the other hand, the PER is proportional
to the ATP only for low values of ATP. As ATP increases,
PER approaches a limiting value.
The figure below outlines the consequences for a control program:
to achieve a given reduction in PER, control efforts must increase
the greater the degree of limitation is. These considerations also
apply to the problem of re-infection in a post-control situation:
the likelihood and intensity of re-infection is higher, the greater the degree of limitation is.
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Figure: (AC) Control success achievable by a certain control effort
starting from a pre-control situation, illustrated by three relationships
between the parasite establishment rate (PER) and the annual transmission potential (ATP).
All three scenarios assume that the control program reduces the ATP by 90%.
The corresponding reductions in the PER are 90% (A, non-regulation),
70% (B, weak limitation) and 20% (C, strong limitation).
(D) Corresponding effects in the recrudescence during a post-control situation,
comparing the relationships in B and C and assuming that
the ATP temporarily returns to 10% of the pre-control level.
The PER returns to 30% (weak limitation) or to 80% (strong limitation) of its pre-control level.
(Source: Duerr et al., 2006)
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Related pages:
Relationship between PER and ATP,
Limitation,
Facilitation,
Eradicability.
Further reading:
Duerr HP, Leary CC, Eichner M, 2006.
High infection rates at low transmission potentials in West African onchocerciasis.
International Journal for Parasitology 36: 1367-1372.
Abstract at PubMed
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