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Assessments of the transmission of Onchocerca volvulus by Simulium sanctipauli in the Upper Denkyira District, Ghana, and the intermittent disappearance of the vector

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Following studies on the transmission of Onchocerca volvulus (Leuckart) by Simulium sanctipauli Vajime & Dunbar (Diptera, Simuliidae) in Upper Denkyira District in Ghana in 2001 and 2002 (Kutin et al., Med Vet Ent 18:167–173, 2004), further assessments were carried out in 2006 and 2013/2014 to determine whether transmission parameters had changed since community-directed ivermectin treatment (CDTI) began in 1999. There were no marked changes of the transmission intensities in 2006. Only slight, but non-significant, reductions were observed in infection rates of parous flies with larval stages (L1–L3) of O. volvulus from 44.1 % (of 1672 parous flies) in 2001/2002 to 42.1 % (506) in 2006 and from 6.5 to 5.9 % of flies carrying infective larvae in their heads. This suggested that there was an ongoing transmission in the area and the parasite reservoir in the human population was still high. Unexpectedly, further assessments conducted in October 2013 and March and October 2014 revealed that the vector S. sanctipauli had apparently disappeared and transmission had ceased, probably as a result of intensified gold mining activities along the rivers Ofin and Pra. The water of both rivers was extremely turbid, heavily loaded with suspended solids, probably preventing the development of blackfly larvae. Some breeding and biting of Simulium yahense Vajime & Dunbar was observed in a small tributary of the Pra, the Okumayemfuo, which is not affected by gold mining. However, the infection rate of flies was low, only 3.7 % of 163 parous flies were infected with first stage (L1) larvae of O. volvulus.

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We thank the chiefs, opinion leaders, residents, and vector collectors of the study communities for their cooperation, and we are grateful to the District Health Officers for their support. Professor Rolf Horstmann and Dr. Christian Timmann, Bernhard Nocht Institute for Tropical Medicine (BNITM), Hamburg, Germany provided the 1996/1998 REMO data, and we thank Mr. Odame Asiedu, Ghana Health Services, for the REMO data for 2009. We thank Priscilla Adjei-Kusi, KCCR, for assisting the fieldwork 2014. The support of the Kumasi Centre for Collaborative Research in Tropical Medicine is appreciated. Dr. Jürgen Teckentrup and Dr. Norbert Benthaus, Institute of Hygiene and Environment, Hamburg, are thanked for the water analyses from the rivers Ofin and Pra. The visits of Rolf Garms to Ghana were made possible by the support of the German Senior Expert Service, Bonn, in 2006 and 2013. We are grateful to Professor Robert A. Cheke for the comments and Dr. Bari Howell for the proof-reading of the manuscript. The work was funded by the Bernhard Nocht Institute for Tropical Medicine. The funders had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.

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Garms, R., Badu, K., Owusu-Dabo, E. et al. Assessments of the transmission of Onchocerca volvulus by Simulium sanctipauli in the Upper Denkyira District, Ghana, and the intermittent disappearance of the vector. Parasitol Res 114, 1129–1137 (2015).

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