Abstract
BackgroundRecent reports of transmission interruption of Onchocerca volvulus, the causing agent of river blindness, in former endemic foci in the Americas, and more recently in West and East Africa, raise the question whether elimination of this debilitating disease is underway after long-term treatment of the population at risk with ivermectin. The situation in Central Africa has not yet been clearly assessed.Methods and findingsEntomologic data from two former endemic river basins in North Cameroon were generated over a period of 43 and 48 months to follow-up transmission levels in areas under prolonged ivermectin control. Moreover, epidemiologic parameters of animal-borne Onchocerca spp. transmitted by the same local black fly vectors of the Simulium damnosum complex were recorded and their impact on O. volvulus transmission success evaluated. With mitochondrial DNA markers we unambiguously confirmed the presence of infective O. volvulus larvae in vectors from the Sudan savannah region (mean Annual Transmission Potential 2009–2012: 98, range 47–221), but not from the Adamawa highland region. Transmission rates of O. ochengi, a parasite of Zebu cattle, were high in both foci.Conclusions/significanceThe high cattle livestock density in conjunction with the high transmission rates of the bovine filaria O. ochengi prevents the transmission of O. volvulus on the Adamawa plateau, whereas transmission in a former hyperendemic focus was markedly reduced, but not completely interrupted after 25 years of ivermectin control. This study may be helpful to gauge the impact of the presence of animal-filariae for O. volvulus transmission in terms of the growing human and livestock populations in sub-Saharan countries.
Highlights
The interruption of transmission of Onchocerca volvulus, the causing agent of river blindness or onchocerciasis, has been confirmed for a growing number of endemic foci on the American continent [1,2,3] and in West [4] and East Africa [5,6,7]
Over the past decades the Fight against river blindness, a tropical disease caused by a nematode worm, has been relatively successful, and a number of countries have been reported to be free of parasite transmission
In North Cameroon, we checked the occurrence of infective stages of Onchocerca volvulus in the transmitting black fly populations for more than three years and were able to confirm that the transmission there is low, but not yet interrupted
Summary
The interruption of transmission of Onchocerca volvulus, the causing agent of river blindness or onchocerciasis, has been confirmed for a growing number of endemic foci on the American continent [1,2,3] and in West [4] and East Africa [5,6,7]. Filariae closely-related to O. volvulus might repopulate the human host [22] posing a potential risk of infection, or they might transfer genes to O. volvulus which negatively affect the effectiveness of ivermectin, presently the sole drug intervention in use [23] For this reason we combine microscopic differentiation of infective larvae with a PCR-based molecular approach which allows the detection of yet unknown filarial species and strains in addition to already known Onchocerca spp. Recent reports of transmission interruption of Onchocerca volvulus, the causing agent of river blindness, in former endemic foci in the Americas, and more recently in West and East Africa, raise the question whether elimination of this debilitating disease is underway after long-term treatment of the population at risk with ivermectin. The situation in Central Africa has not yet been clearly assessed.
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