Onchocerca-Simulium complexes in Venezuela: can human onchocerciasis spread outside its present endemic areas?

The compatibility between sympatric and allopatric combinations of Onchocer ca volvulus-anthropophilic species of Simulium was studied in the north-eas tern focus of human onchocerciasis as well as in a densely populated locali ty of the Amazonas State in Venezuela. The objectives were to test the conj ecture that local adaptation exists between the parasite and its vectors (t he Onchocerca-Simulium complex hypothesis:), and assess the possibility of the infection spreading from its present distributional range. For the homo logous combination, O. volvulus-S. metallicum cytospecies E in Anzoategui S tate (north-eastern focus), parasite yield was 45 % in contrast to 1 % for the heterologous, southern parasite-S. metallicum infection. This was signi ficantly lower than the parasite yield (4-10%) expected after allowing for the effect of density-dependent limitation of infective larval output descr ibed in this paper for S. metallicum. The population of S. exiguum s.l. fro m southern Venezuela allowed no larval development beyond the L1 stage of e ither northern or southern parasites. Mechanisms for such refractoriness pr obably operate at the level of the thoracic muscles, not affecting microfil arial uptake or migration out of the bloodmeal. The parasite yield of south ern O. volvulus in S. oyapockense s.l. flies biting man at Puerto Ayacucho (Amazonas) was about 1%, in agreement with the figures recorded for highly compatible sympatric combinations such as O. volvulus-S. ochraceum s.l. in Guatemala. No infective larval development of the northern parasite was obs erved in southern S. oyapockense. These results, together with consideratio ns of typical worm burdens in the human host, presence/absence of armed cib aria in the simuliids, parasite-induced vector mortality, and fly biting ra tes, suggest a lower potential for onchocerciasis to spread between the nor thern and southern endemic areas of Venezuela than that between Amazonian h yperendemic locations and settlements outside this focus with high densitie s of S. oyapockense s.l.