Onchocerca-Simulium complexes. VI. Experimental studies on the transmission of Venezuelan and West African strains of Onchocerca volvulus by Simulium metallicum and S. exiguum in Venezuela

Duke, B.O.

Annals of Tropical Medicine and Parasitology 64(4): 421-431


ISSN/ISBN: 0003-4983
PMID: 5498785
Accession: 014591990

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When the Venezuelan species Simulium metallicum and S. exiguum fed on carriers of the West African forest and Sudan-savannah strains of Onchocerca volvulus, microfilariae were ingested in numbers similar to those taken in by S. damnosum, but there was no development to infective larvae. In carriers of the Venezuelan strain of O. volvulus, microfilariae concentrated around feeding S. metallicum and S. exiguum and resulted in high intakes of microfilariae from areas of skin with low densities of microfilariae. Development of microfilariae of the Venezuelan strain to the infective stage was observed in both S. metallicum and S. exiguum, but relatively few of the parasites reached maturity in the latter. S. metallicum appeared to be more efficient as an experimental vector and was the most numerous man-biting species. Wild S. metallicum were found carrying O. volvulus-type infective larvae and were regarded as the species mainly responsible for transmission in Venezuela. No wild infective S. exiguum were encountered. Intakes of around 20 microfilariae per fly resulted in high mortality but, as most of the human infections show low concentrations of microfilariae in the skin, it is unlikely that vectors will die in this way very often.