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Density-dependent processes in the transmission of human onchocerciasis: intensity of microfilariae in the skin and their uptake by the simuliid host

Published online by Cambridge University Press:  06 April 2009

M. G. Basáñez ,
M. Boussinesq ,
J. Prod'hon ,
H. Frontado ,
N. J. Villamizar ,
G. F. Medley  and
R. M. Anderson
M. G. Basáñez
Affiliation:
Instituto de Medicina Tropical, Universidad Central de Venezuela, Caracas, Venezuela Department of Biology, Parasite Epidemiology Research Group, Imperial College, Prince Consort Road, London SW7 2BB, UK
M. Boussinesq
Affiliation:
Antenne ORSTOM auprés du Centre Pasteur, BP 1274, Yaoundé, Cameroon
J. Prod'hon
Affiliation:
Antenne ORSTOM auprés du Centre Pasteur, BP 1274, Yaoundé, Cameroon
H. Frontado
Affiliation:
Centro Amazónico de Investigación y Control de Enfermedades Tropicales (CAICET), Puerto Ayacucho, Amazonas, Venezuela.
N. J. Villamizar
Affiliation:
Centro Amazónico de Investigación y Control de Enfermedades Tropicales (CAICET), Puerto Ayacucho, Amazonas, Venezuela.
G. F. Medley
Affiliation:
Department of Biology, Parasite Epidemiology Research Group, Imperial College, Prince Consort Road, London SW7 2BB, UK
R. M. Anderson
Affiliation:
Department of Biology, Parasite Epidemiology Research Group, Imperial College, Prince Consort Road, London SW7 2BB, UK
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Summary

The transmission success of Onchocerca volvulus is thought to be influenced by a variety of regulatory or density-dependent processes that act at various points in the two-host life-cycle. This paper examines one component of the life-cycle, namely, the ingestion of microfilariae by the simuliid vector, to assess the relationship between intake of larvae and the density of parasites in the skin of the human host. Analysis is based on data from three areas in which onchocerciasis is endemic and includes published information as well as new data collected in field studies. The three areas are: Guatemala (Simulium ochraceum sl.), West and Central Africa (savanna members of the S. damnosum complex), and South Venezuela (S. guianense). The data record experimental studies of parasite uptake by flies captured in the field and fed to repletion on locally infected subjects who harboured varying intensities of dermal microfilarial infection. Regression analyses of log transformed counts of parasite burdens ingested by the flies plotted against log transformed counts of microfilariae per mg of skin revealed little evidence for saturation in parasite uptake by the flies as the intensity in the human host increased. There was a positive and highly significant rank correlation between both variables for the three blackfly species. In an alternative analysis a model was fitted to data on prevalence of flies with ingested microfilariae (mff) versus dermal mean intensities. The model assumed an overdispersed distribution of the number of mff/fly and a given functional relationship between intake and skin load. The results of both approaches were consistent. It is concluded that parasite ingestiossssn by the vector host is not strongly density dependent in the three geographical areas and ranges of dermal loads examined. It therefore appears that this transmission process is of reduced importance as a regulatory mechanism in the dynamics of parasite population growth.

Keywords

Onchocerca volvulussimuliid hostsmicrofilarial intakemicrofilarial concentrationdensity dependence
Type
Research Article
Information
Parasitology , Volume 108 , Issue 1 , January 1994 , pp. 115 - 127
Copyright
Copyright © Cambridge University Press 1994

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