Spatial survey finds infective black flies 19 km from Benin breeding site

Black flies infected with the parasite associated with onchocerciasis were detected as far as 19 km from a breeding site in Benin, according to a spatial entomological survey conducted around historical breeding areas in Benin and Ghana during the 2023 and 2024 wet seasons. The study found that catches and biting rates generally declined with distance from the Bétérou breeding site, although local variation was substantial. At Asubende in Ghana, low catches limited analysis of distance-related trends, and no infective flies were detected by PCR.

The researchers selected 40 settlements within a 25-km radius of each of the two breeding sites and used a radial sampling design to examine the spatial distribution of Simulium damnosum sensu lato, the black fly group that includes the vectors of onchocerciasis. Human landing catches were conducted at the breeding sites and in settlements, while one Esperanza window trap was operated at each settlement. Morphological methods were used to identify the black flies, and catches were examined to assess parity and determine the presence of parasite material.

Parasite detection combined O-150 PCR pool screening with dissection, allowing the investigators to determine whether collected flies carried the parasite and to relate infection status to distance from the breeding site. Biting rates were calculated in relation to sampling effort, while generalized additive models were used to describe trends in entomological indices over distance. The study was observational and spatially descriptive: it characterized patterns in sampled locations rather than directly tracing individual flight paths or establishing the contribution of particular flies to transmission.

Across 656 sampling days, the survey collected 76,050 black flies, primarily around Bétérou. At that site, black fly counts and biting rates generally decreased with increasing distance from the breeding site, but the pattern was not uniform. Positive O-150 PCR pools were found at 31 of 41 sites examined around Bétérou, and infective flies were recorded up to 19 km from the breeding site. These observations show that the spatial extent of infection-associated flies was not confined to settlements immediately beside the breeding habitat.

The prevalence of infective flies at Bétérou was 0.11% in 2023 and 0.08% in 2024. Both values exceeded the World Health Organization infectivity prevalence threshold specified for interrupted transmission. The parous rate, defined in the study through dissection, remained relatively constant across distance from the breeding site. The abundance of the catches therefore changed with distance even though the proportion of parous flies did not show a corresponding distance-related trend.

Asubende showed similar broad patterns in the available data, but the number of black flies collected was too low to support a detailed analysis of trends with distance. No infective flies were detected there through PCR. The contrast in sampling yield means that the Asubende observations should not be interpreted as evidence that infection-associated flies were absent from every unsampled location or across the full 25-km study area.

The results identify a potential limitation in relying only on riverine sites for onchocerciasis vector surveillance. The abstract notes that black flies can disperse over hundreds of kilometres, so sampling confined to breeding sites or first-line villages may miss exposed communities farther away. In the surveyed Beninese landscape, the detection of infective flies at distances of up to 19 km directly demonstrated that infection-associated vectors occurred beyond the immediate breeding area. The findings therefore support broader, spatially extensive sampling for a more complete account of where infected vectors are present.

Broader sampling does not, by itself, establish the causes of the observed spatial patterns. The study did not directly measure long-distance flight paths, settlement-level disease outcomes or the epidemiological contribution of individual flies, and the descriptive models were restricted by the amount of data available at Asubende. The authors conclude that comprehensive spatial–temporal modelling is needed to investigate the dynamics underlying the observed distribution. For now, the measured result is clear at the surveyed sites: near Bétérou, black fly abundance and biting rates generally diminished with distance, yet parasite-positive and infective flies remained detectable well beyond the first settlements around the breeding site.

Subject of Research: Spatial patterns and parasite infection in onchocerciasis black fly vectors around breeding sites in Ghana and Benin

Article Title: Spatial patterns of black fly (Simulium damnosum s.l.) dispersal in Ghana and Benin and their implications for onchocerciasis surveillance

Article References: Kyomuhangi, I., Carnaghi, M., Boko-Collins, P., Soglongbe, B. B., Koukpo, Z. C., Otabil, K. B., Veriegh, F. B. D., Fronterre, C., Basáñez, M.-G., Cheke, R. A., & Hawkes, F. M. (2026). Spatial patterns of black fly (Simulium damnosum s.l.) dispersal in Ghana and Benin and their implications for onchocerciasis surveillance. Parasites & Vectors. https://doi.org/10.1186/s13071-026-07747-w

Image Credits: AI Generated

DOI: 10.1186/s13071-026-07747-w

Keywords: black flies, onchocerciasis, vector surveillance, spatial dispersal, Simulium damnosum, Ghana, Benin, parasite PCR, entomological monitoring, wet-season sampling, Spatial, patterns