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High infection rates at low transmission potentials
One of the least understood processes in the transmission cycle
of filarial infections is the relationship between
the annual transmission potential
(ATP: no. of L3 transmitted to a human host per year)
and the parasite establishment rate
(PER: no. of adult parasites which establish in a host per year).
Whereas the ATP usually ranges from dozens
up to several thousands of L3 per host per year,
the PER usually amounts to a few parasites per host per year,
in onchocerciasis hardly exceeding values over five per year,
even at high ATPs.
We have investigated the relationship between
the ATP and the PER for 14 West African villages.
The PER increases rapidly with the ATP anda value of
0.3 parasites per year can already be possible
for ATPs of only ~10 L3 per man and year
(the value of 0.3 parasites per year seems low, but
it is an average over the whole population, with many
children who have low parasite burdens).
The PER is not necessarily proportional to the ATP,
and villages even exist where higher ATPs
are associated with lower PERs
(e.g. Guinea villages Gu863, Gu867, Gu910).
This shows that the establishment rate underlies
considerable variability and that it may substantially
be influenced by factors other than the ATP.
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Figure: Relationship between the parasite establishment rate (PER) and the annual transmission potential (ATP).
(A) Linear least-squares fit through zero on a log-transformed scale.
(B) Same data and same fit as in A, but shown on a linear scale to illustrate the degree of limitation.
(Source: Duerr HP et al., 2006)
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Comparing the estimates of all 14 villages suggests considerable limitation in the relationship between the ATP and the PER implying that
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the maximum PER is already reached for ATPs of some hundred L3 per person and year. The consequence for successfully controlled regions is that already a slight increase in the ATP may lead to a considerable increase in the PER.
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the average PER remains almost constant at higher ATPs, even if these differ by magnitudes. The consequence for the control of onchocerciasis is that lowering the ATP from very high to moderate values (say from 2000 to 500 L3 per person and year) may result in only a minor reduction in the infection of humans.
Related pages:
Limitation & control,
Eradicability,
Infection & immunosuppression.
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 in PubMed
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Responsible for this page:
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Dr. H.-P. Duerr
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Webmaster:
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Prof. Dr. M. Eichner
(last change of this page on
13 July 2009)
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Cooperation with:
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Prof. K. Dietz, Institut für Medizinische Biometrie (IMB), Tübingen
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Prof. M. Eichner
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Prof. Dr. H. Schulz-Key |
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Financial support by:
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Deutsche Forschungsgemeinschaft (DFG, DI 308/12-1)
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Eberhard-Karls-University Tübingen,
Tübingen University Hospital,
the Department for Medical Biometry (IMB),
and the authors of this page disclaim all liability for the content of any page referenced by hyper-link from this page
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