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

Abstract

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 s.l.), 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 ingestion 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.

Onchocerca ochengitransmission dynamics and the correlation ofO. ochengimicrofilaria density in cattle with the transmission potential

“…In an analysis of data on the relationship between microfilaria uptake by the vectors and the density of microfilariae in the skin in human onchocerciasis from Guatemala (S. ochraceum s.l. ), West and Central Africa (Savanna members of the S. damnosum complex), and South Venezuela (S. guianense), it was concluded that microfilaria uptake by the vector hosts was not strongly density-dependent in the three geographical areas and the ranges of dermal microfilaria load examined [3]. Our results in the cattle/O.…”

Transmission intensity and the patterns of Onchocerca volvulus infection in human communities.

“…When reported incubation period of biopsies was less than this, values were corrected according to standardized criteria 36 that had been successfully applied previously. 2 When mf burden was presented per skin snip (buttocks only), 31 an average weight of 2.0 mg/snip was used. 9 Because some investigators 30 gave only geometric means (iliac snips), their values are those presented by Dietz,9 who calculated arithmetic averages from the original data set at OCP headquarters.…”

“…In an attempt to further understanding of such processes, a number of researchers have explored the properties of simple and complex mathematical models of the transmission dynamics of human onchocerciasis. 1 Considerable progress has been made concerning the regulatory constraints operating within the vector, [2][3][4][5][6] with some progress being recently made in investigating those acting within the human host. 7,8 A previous mathematical model 9 used an empirical relationship between the observed mean microfilarial (mf) load per milligram of skin per person in the community and the intensity of transmission in such a community, measured by the annual transmission potential (ATP).…”

Uptake ofOnchocerca volvulus(Nematoda: Onchocercidae) bySimulium(Diptera: Simuliidae) Is Not Strongly Dependent on the Density of Skin Microfilariae in the Human Host

“…It would have been possible to transform our mf counts to those obtained after a 24-h incubation in saline and to numbers of mf/mg as described in Basáñ ez et al (1994). This would have made little difference to our results, which can be seen as follows.…”