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Vector competence for Onchocerca volvulus in the Simulium (Notolepria) exiguum complex: cytoforms or
Sally Wetten1, Richard C Collins, Juan Carlos Vieira
1Department of Infectious Disease Epidemiology, Imperial College London, Faculty of Medicine (St. Mary's Campus), Norfolk Place, London W2 1PG, UK.
Acta Tropica
|July 10, 2007
Summary
The Cayapa form of Simulium exiguum black flies in Ecuador efficiently transmit Onchocerca volvulus. Other Simulium exiguum forms are less susceptible, indicating intrinsic differences in vector competence for onchocerciasis transmission.
Area of Science:
- Medical Entomology
- Parasitology
- Vector Biology
Background:
- Simulium exiguum is a South American black fly species complex.
- Its role in transmitting Onchocerca volvulus (onchocerciasis) varies geographically.
- This variation is linked to sibling forms with differing vector capacities.
Purpose of the Study:
- To investigate the relationship between Onchocerca volvulus microfilariae (mf) density in human skin and infective larval output in the Cayapa form of Simulium exiguum.
- To compare the vector competence of the Cayapa form with other Simulium exiguum cytoforms.
- To determine if intrinsic susceptibility or density-dependent factors drive differences in Onchocerca volvulus transmission.
Main Methods:
- Quantified Onchocerca volvulus L3 larvae in infective Simulium exiguum.
- Assessed microfilariae (mf) loads in human skin snips from 6 sites per patient.
- Compared infective larval output and fly infection proportions across different Simulium exiguum cytoforms, adjusting for mf density.
Main Results:
- A nonlinear relationship was observed for the Cayapa form: infective larvae increased rapidly with mf density, then saturated.
- The proportion of infective Cayapa flies reached 80% at >= 20 mf/mg skin.
- Non-Cayapa cytoforms showed significantly lower L3 loads and infection rates than the Cayapa form, even after accounting for density dependence.
Conclusions:
- The Cayapa cytoform of Simulium exiguum exhibits high intrinsic susceptibility to Onchocerca volvulus.
- Differences in vector competence among Simulium exiguum cytoforms are primarily due to intrinsic susceptibility, not density-dependent effects.
- This highlights the importance of cytoform-specific vector capacity in onchocerciasis epidemiology.
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