Identification of the onchocerciasis vector in the Kakoi-Koda focus of the Democratic Republic of Congo

Abstract

Background: The objective of this study was to characterise the vector in a small hyper-endemic focus of onchocerciasis (the Kakoi-Koda focus) which has recently been discovered on the western slopes of the rift valley above Lake Albert.

Methodology/principal findings: Aquatic stages of blackflies were collected by hand from streams and rivers, and anthropophilic adult females were collected by human landing catches. Using a combination of morphotaxonomy and DNA barcoding, the blackflies collected biting humans within the focus were identified as Simulium dentulosum and Simulium vorax, which were also found breeding in local streams and rivers. Simulium damnosum s.l., Simulium neavei and Simulium albivirgulatum were not found (except for a single site in 2009 where crabs were carrying S. neavei). Anthropophilic specimens from the focus were screened for Onchocerca DNA using discriminant qualitative real-time triplex PCR. One specimen of S. vorax was positive for Onchocerca volvulus in the body, and out of 155 S. dentulosum, 30% and 11% were infected and infective (respectively).

Conclusions/significance: Simulium dentulosum currently appears to be the main vector of human onchocerciasis within the Kakoi-Koda focus, and S. vorax may be a secondary vector. It remains possible that S. neavei was the main (or only) vector in the past having now become rare as a result of the removal of tree-cover and land-use changes. Simulium vorax has previously been shown to support the development of O. volvulus in the laboratory, but this is the first time that S. dentulosum has been implicated as a probable vector of onchocerciasis, and this raises the possibility that other blackfly species which are not generally considered to be anthropophilic vectors might become vectors under suitable conditions. Because S. dentulosum is not a vector in endemic areas surrounding the Kakoi-Koda focus, it is probable that the Kakoi-Koda focus is significantly isolated.

Fig 1. Relief map showing location of the Kakoi-Koda focus and other nearby foci.

1 = Kakoi-Koda focus; 2 = North-Eastern DRC Oncho Endemic Area; 3 = Itwara focus; 4 = Mpamba-Nkusi focus; 5 = Wambabya-Rwamarongo focus; 6 = Budongo focus; 7 = Nyagak-Bondo focus; 8 = Maracha-Terego focus; 9 = West Nile focus. Location of onchocerciasis foci according to Katabarwa et al. [29] and Makenga Bof et al. [8]. (Produced in QGIS: [50]). The RASTER images used to produce Fig 1 are Landsat-7 and Landsat-8 images, courtesy of the U.S. Geological Survey (https://www.usgs.gov/). The basic shapefiles for administrative areas can be found on the Référentiel Géographique Commun for the DRC (www.rgc.cd).

Fig 2. Overview map of historical and recent surveys for Simulium spp.

A. Prospection sites in the Ituri highlands (IT), around the Mount Hoyo Reserve (MH) and some main rivers. B. Kakoi-Koda focus with sampled streams (Aw: Awoo, Ku: Kuda, Lb: Lebu, Ky: Kakoi, Ko: Koda) (produced in QGIS [50]).

Fig 3. Phylogenetic tree based on the mitochondrial sequences (COI, 658 bp) of specimens identified as Simulium dentulosum and Simulium vorax from Kakoi-Koda Focus.

The analysis was performed with Bayesian inference under the GTR + I + G model of sequence evolution. The green and red colours indicate specimens identified as “infected” by either O. volvulus and O. ochengi and “infective” for O. volvulus, respectively. The sequenced S. vorax specimen was infected with O. volvulus. Posterior probabilities are indicated at the nodes. GenBank accession numbers of sequences are indicated between brackets.