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

Abstract

The compatibility between sympatric and allopatric combinations of Onchocerca volvulus-anthropophilic species of Simulium was studied in the north-eastern focus of human onchocerciasis as well as in a densely populated locality of the Amazonas State in Venezuela. The objectives were to test the conjecture that local adaptation exists between the parasite and its vectors (the Onchocerca-Simulium complex hypothesis), and assess the possibility of the infection spreading from its present distributional range. For the homologous combination, O. volvulus-S. metallicum cytospecies E in Anzoátegui State (north-eastern focus), parasite yield was 45% in contrast to 1% for the heterologous, southern parasite-S. metallicum infection. This was significantly lower than the parasite yield (4-10%) expected after allowing for the effect of density-dependent limitation of infective larval output described in this paper for S. metallicum. The population of S. exiguum s.l. from southern Venezuela allowed no larval development beyond the L1 stage of either northern or southern parasites. Mechanisms for such refractoriness probably operate at the level of the thoracic muscles, not affecting microfilarial uptake or migration out of the bloodmeal. The parasite yield of southern 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 observed in southern S. oyapockense. These results, together with considerations of typical worm burdens in the human host, presence/absence of armed cibaria in the simuliids, parasite-induced vector mortality, and fly biting rates, suggest a lower potential for onchocerciasis to spread between the northern and southern endemic areas of Venezuela than that between Amazonian hyperendemic locations and settlements outside this focus with high densities of S. oyapockense s.l.

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

“…in northern Venezuela. 5 Other factors such as fly size, vector's immune response, and gut factors that limit parasite passage, among others, may also contribute to overall vector competence. 3 It is also conceivable that vector-related factors are not completely independent from each other.…”

“…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).…”

Neglected Tropical Diseases - Latin America and the Caribbean

“…Most importantly, O. volvulus is transmitted by several different species of black fly in LA (Fig. Vector competence is dramatically influenced by the presence or absence of an anatomical structure known as the cibarial armature (Duke 1970;Reid 1994;Basanez et al 2000). One of the most important variables in the different vectorial capacities of the various Simulium species that transmit the parasite is their vector competence or their intrinsic ability to support the development of the parasite to the infectious stage.…”

Biodiversity of Biting Flies: Implications for Humanity

“…Similar quandaries will continue to arise as land use changes and as the interface between civilization and raw nature expands. Persistent incursions into undeveloped areas of the tropics, for example, expose people to new and greater risks of vector-borne diseases (Basáñez et al 2000), while development projects (e.g., irrigation) increase the prevalence of diseases such as leishmaniasis (Lane 1993).…”